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

Bone morphogenetic protein 6 and oxidized low-density lipoprotein synergistically recruit osteogenic differentiation in endothelial cells

AIMS

Vascular calcification contributes to mortality and morbidity in atherosclerosis, chronic kidney disease, and diabetes. Vascular calcific lesions contain osteoblast- and chondroblast-like cells, suggesting a process of endochondral or membranous ossification thought to result from the phenotypic plasticity of vascular cells. Bone morphogenetic protein (BMP) signalling potentiates atherosclerotic calcification, whereas BMP inhibition attenuates vascular inflammation and calcification in atherogenic mice. We hypothesized endothelial cells (ECs) may undergo osteogenic differentiation in response to BMP signalling and pro-atherogenic stimuli.

METHODS AND RESULTS

Among various BMP ligands tested, BMP6 and BMP9 elicited the most potent signalling in bovine aortic endothelial cells (BAEC), however, only BMP6 induced osteogenic differentiation. BMP6 and oxidized low-density lipoprotein (oxLDL) independently and synergistically induced osteogenic differentiation and mineralization, in a manner consistent with endothelial-to-mesenchymal transition. Treatment of ECs with BMP6 or oxLDL individually induced osteogenic and chondrogenic transcription factors Runx2 and Msx2, whereas treatment with BMP6 and oxLDL synergistically up-regulated Osterix and Osteopontin. Production of H2O2 was necessary for oxLDL-induced regulation of Runx2, Msx2, and Osterix in BAEC, and H2O2 was sufficient by itself to up-regulate these genes. Mineralization of ECs in response to BMP6 or oxLDL was abrogated by scavenging reactive oxygen species or inhibiting BMP type I receptor kinases. Similar synergistic effects of BMP and oxLDL upon osteogenic and chondrogenic transcription and phenotypic plasticity in human aortic endothelial cells were observed.

CONCLUSION

These findings provide a potential mechanism for the observed interactions of BMP signalling, oxidative stress, and inflammation in recruiting vascular calcification associated with atherosclerosis.

Mitogen-activated protein kinase 14 is a novel negative regulatory switch for the vascular smooth muscle cell contractile gene program

OBJECTIVE

Several studies have shown through chemical inhibitors that p38 mitogen-activated protein kinase (MAPK) promotes vascular smooth muscle cell (VSMC) differentiation. Here, we evaluate the effects of knocking down a dominant p38MAPK isoform on VSMC differentiation.

METHODS AND RESULTS

Knockdown of p38MAPKα (MAPK14) in human coronary artery SMCs unexpectedly increases VSMC differentiation genes, such as miR145, ACTA2, CNN1, LMOD1, and TAGLN, with little change in the expression of serum response factor (SRF) and 2 SRF cofactors, myocardin (MYOCD) and myocardin-related transcription factor A (MKL1). A variety of chemical and biological inhibitors demonstrate a critical role for a RhoA-MKL1-SRF-dependent pathway in mediating these effects. MAPK14 knockdown promotes MKL1 nuclear localization and VSMC marker expression, an effect partially reversed with Y27632; in contrast, MAP2K6 (MKK6) blocks MKL1 nuclear import and VSMC marker expression. Immunostaining and Western blotting of injured mouse carotid arteries reveal elevated MAPK14 (both total and phosphorylated) and reduced VSMC marker expression.

CONCLUSIONS

Reduced MAPK14 expression evokes unanticipated increases in VSMC contractile genes, suggesting an unrecognized negative regulatory role for MAPK14 signaling in VSMC differentiation.

Deconstructing fibrosis research: do pro-fibrotic signals point the way for chronic dermal wound regeneration?

Chronic wounds are characterized by inadequate matrix synthesis, no re-epithelialization, infection and ultimately no wound resolution. In contrast, fibrosis is characterized by overproduction of matrix and excess matrix contraction. As research in the fields of chronic wounds and fibrosis surges forward, important parallels can now be drawn between the dysfunctions in fibrotic diseases and the needs of chronic wounds. These parallels exist at both the macroscopic level and at the molecular level. Thus in finding the individual factors responsible for the progression of fibrotic diseases, we may identify new therapeutic targets for the resolution of chronic wounds. The aim of this review is to discuss how recent advances in fibrosis research have found a home in the treatment of chronic wounds and to highlight the benefits that can be obtained for chronic wound treatments by employing a translational approach to molecules identified in fibrosis research.

The pericyte: cellular regulator of microvascular blood flow

The vascular system - through its development, response to injury, and remodeling during disease - constitutes one of the key organ systems sustaining normal human physiology; conversely, its dysregulation also underlies multiple pathophysiologic processes. Regulation of vascular endothelial cell function requires the integration of complex signals via multiple cell types, including arterial smooth muscle, capillary and post-capillary pericytes, and other perivascular cells such as glial and immune cells. Here, we focus on the pericyte and its roles in microvascular remodeling, reviewing current concepts in microvascular pathophysiology and offering new insights into the specific roles that pericyte-dependent signaling pathways may play in modulating endothelial growth and microvascular tone during pathologic angiogenesis and essential hypertension.

Increased Levels of Transforming Growth Factor-Beta1 and -Beta2 in the Aqueous Humor of Patients With Neovascular Glaucoma

BACKGROUND AND OBJECTIVE

To measure the concentrations of transforming growth factor-betal and beta2 (TGF-beta1 and TGF-beta2) in the aqueous humor of patients with neovascular glaucoma (NVG).

PATIENTS AND METHODS

Patients were divided into four groups: NVG secondary to central retinal vein occlusion (group 1), NVG secondary to proliferative diabetic retinopathy (group 2), central retinal vein occlusion without rubeosis (group 3), and senile cataract (group 4). The total TGF-beta 1 and TGF-beta2 concentrations in the aqueous humor of the four groups were measured by enzyme linked immunosorbent assay.

RESULTS

The mean concentrations of total TGF-betal were 600.7 +/-436.7 microg/mL in group 1, 802.0 +/-359.5 pg/mL in group 2, and undetectable in groups 3 and group 4 (P < .05). The mean concentrations of total TGF-beta2 were 6,307.9+/- 2,206.2 microg/mL in group 1, 5,908.0+/-2,033.2 microg/mL in group 2, 899.7+/- 425.6 microg/mL in group 3, and 385.7 +/-89.9 microg/mL in group 4. The total TGF-betal and TGF-beta2 concentrations in groups 1 and 2 were significantly higher than those in groups 3 and 4, whereas the total TGF-beta2 concentration in group 3 was significantly higher than that in group 4 (P < .05). There was no significant difference in the TGF-betal or TGF-beta2 concentrations between groups 1 and 2 (P> .05).

CONCLUSIONS

The abnormally high concentrations of TGF-betal and TGF-beta2 in the aqueous humor of patients with NVG may explain some aspects of the pathogenesis of NVG and the high failure rate of filtering operations in NVG.

Smooth muscle alpha-actin expression in endothelial cells derived from CD34+ human cord blood cells

Human fetal cord blood contains subsets of mononuclear cells with the potential to form both hematological and endothelial cells. Vascular progenitor cells, which can produce all three elements of mature blood vessels, including smooth muscle, have been identified in animals. We hypothesized that similar multipotential progenitor cells exist in humans and used the expression of alpha-smooth muscle actin (alpha-SMA) to identify such cells in fetal cord blood. Mononuclear cell preparations were isolated from human umbilical cord blood and CD34(+) and CD133(+) cells obtained by magnetic bead separation. Isolated cells were cultured on fibronectin-coated dishes with medium containing vascular endothelial growth factor, basic fibroblast growth factor, and insulin-like growth factor. mRNA was extracted, and the expression of alpha-SMA and a number of endothelial cell markers (VEGFR-2, vWF, eNOS, VE-Cadhein, PECAM-1 and Tie-2) was determined by reverse transcriptase-PCR techniques. Human umbilical vein endothelial cells (HUVECs) were used as positive controls. Freshly isolated CD34(+) and CD133(+) cells expressed all endothelial cell markers, but did not express alpha-SMA. HUVECs expressed alpha-SMA. Following 4 weeks of culture, CD34(+) isolates produced morphologically endothelial-like cells that expressed both endothelial cell markers and alpha-SMA. CD133(+) cells failed to produce morphological endothelial-like cells but expressed a range of endothelial markers. However, they did not express alpha-SMA. Following culture in an endothelial cell-promoting environment, CD34(+), but not CD133(+), isolates produced endothelial-like cells that expressed alpha-SMA. Human fetal cord blood contains a population of cells that may differentiate toward both an endothelial and a smooth muscle phenotype in culture.

Growth factor-induced morphological, physiological and molecular characteristics in cerebral endothelial cells

The capacity of vascular endothelial cells to modulate their phenotype in response to changes in environmental conditions is one of the most important characteristics of this cell type. Since different growth factors may play an important signalling role in this adaptive process we have investigated the effect of endothelial cell growth factor (ECGF) on morphological, physiological and molecular characteristics of cerebral endothelial cells (CECs). CECs grown in the presence of ECGF and its cofactor heparin exhibit an epithelial-like morphology (type I CECs). Upon removal of growth factors, CECs develop an elongated spindle-like shape (type II CECs) which is accompanied by the reorganization of actin filaments and the induction of alpha-actin expression. Since one of the most important functions of CECs is the creation of a selective diffusion barrier between the blood and the central nervous system (CNS), we have studied the expression of junction-related proteins in both cell types. We have found that removal of growth factors from endothelial cultures leads to the downregulation of cadherin and occludin protein levels. The loss of junctional proteins was accompanied by a significant increase in the migratory activity and an altered protease activity profile of the cells. TGF-beta1 suppressed endothelial migration in all experiments. Our data provide evidence to suggest that particular endothelial functions are largely controlled by the presence of growth factors. The differences in adhesiveness and migration may play a role in important physiological and pathological processes of endothelial cells such as vasculogenesis or tumor progression.

Induction of smooth muscle alpha-actin in vascular smooth muscle cells by arginine vasopressin is mediated by c-Jun amino-terminal kinases and p38 mitogen-activated protein kinase

Exposure of vascular smooth muscle cells to arginine vasopressin (AVP) increases smooth muscle alpha-actin (SM-alpha-actin) expression through activation of the SM- alpha-actin promoter. The goal of this study was to determine the role of the mitogen-activated protein kinase (MAP kinase) family in regulation of SM-alpha-actin expression. AVP activated all three MAP kinase family members: ERKs, JNKs, and p38 MAP kinase. Inhibition of JNKs or p38 decreased AVP-stimulated SM-alpha-actin promoter activity, whereas inhibition of ERKs had no effect. A 150-base pair region of the promoter containing two CArG boxes was sufficient to mediate regulation by vasoconstrictors. Mutations in either CArG box decreased AVP-stimulated promoter activity. Electrophoretic mobility shift assays using oligonucleotides corresponding to either CArG box resulted in a complex of similar mobility whose intensity was increased by AVP. Antibodies against serum response factor (SRF) completely super-shifted this complex, indicating that SRF binds to both CArG boxes. Overexpression of SRF increased basal promoter activity, but activity was still stimulated by AVP. AVP stimulation rapidly increased SRF phosphorylation. These data indicate that both JNKs and p38 participate in regulation of SM- alpha-actin expression. SRF, which binds to two critical CArG boxes in the promoter, represents a potential target of these kinases.

Anastomotic tissue response associated with expanded polytetrafluoroethylene access grafts constructed by using nonpenetrating clips

PURPOSE

The gross, light microscopic, and scanning microscopic appearance of arterial and venous anastomoses in expanded polytetrafluoroethylene (ePTFE) access grafts constructed with nonpenetrating clips were compared with that of those constructed with polypropylene suture. We hypothesized that clip-constructed anastomoses would provide controlled approximation of native vessel intimal and medial components with the ePTFE grafts. We further hypothesized that anastomotic healing with clips would involve primarily an intimal cellular response, as compared with suture-constructed anastomoses in which cells within the media and adventitia walls participate.

METHODS

Femoral artery to femoral vein arteriovenous (AV) grafts were constructed in five dogs using 4-mm internal diameter ePTFE graft material. Each animal received one AV graft with anastomoses constructed by using polypropylene sutures in one leg and one AV graft with anastomoses constructed with Vascular Closure System clips in the contralateral leg. Animals were given aspirin for the duration of the study, and grafts were explanted at 5 weeks. At the time of explantation, graft segments were grossly evaluated and then underwent light and scanning electron microscopic analysis.

RESULTS

At the time of explantation, all access grafts were patent. Joining the ePTFE grafts to the native vessels with clips resulted in minimal vessel wall damage. The lumenal contours of the discontinuous approximation were smooth and without gross endothelial disruption. These observations are in contrast to the lumenal compromise and endothelial disturbance associated with the sutured anastomoses. Furthermore, hemostasis was achieved immediately in the clipped grafts, decreasing the incidence of perianastomic hematoma. Finally, cellular reconstitution occurred at the anastomotic cleft in both the sutured and the clipped junctions. The neointima exhibited an endothelial cell lining on the lumenal surface and the presence of alpha-smooth muscle cell actin positive cells within the subendothelial layer.

CONCLUSION

Vascular Closure System clips are a viable alternative to suture for the approximation of ePTFE AV access grafts to native blood vessels. The use of the clips resulted in a more streamlined anastomosis, with decreased vessel wall damage, immediate hemostasis, and a trend toward shorter procedure times.

Cardiac microvascular endothelial cells express alpha-smooth muscle actin and show low NOS III activity

We established a culture system of porcine coronary microvascular endothelial cells (MVEC) with high cellular yield and purity >98%. Endothelial origin was confirmed by immunostaining, immunoblotting and fluorescence-activated cell sorter (FACS) analysis using low-density lipoprotein uptake, CD31, von Willebrand factor, and the lectin Dolichos biflorus agglutinin. MVEC were positive for alpha-smooth muscle actin in culture and in myocardium, as confirmed by FACS. Of the primary MVEC, approximately 30% expressed nitric oxide synthase (NOS) III in numbers decreasing from the first passage (6 +/- 1%) to the second passage (4 +/- 1%; P < 0.001 vs. primary isolates), whereas approximately 100% of aortic endothelial cells (AEC) expressed NOS III. In AEC, NOS III activity (pmol citrulline. mg protein-1. min-1) was 80 +/- 10 and was nearly abolished in the absence of calcium (5 +/- 1, P < 0.001). In primary MVEC, however, NOS III activity in the presence and absence of calcium was 20 +/- 4 and 25 +/- 5, respectively. We conclude that cardiac MVEC, in contrast to AEC, contain alpha-smooth muscle actin, show low-grade NOS III activity, and provide a suitable in vitro system for the study of endothelial pathophysiology.

Functional morphology of the trabecular meshwork in primate eyes

The trabecular meshwork forms most of the resistance to aqueous humor outflow needed for maintenance of a pressure gradient between intraocular pressure of approximately 17 mmHg and venous pressure of approximately 10 mmHg. The composition of the extracellular material in the subendothelial or cribriform layer seems to be mainly responsible for outflow resistance. The aqueous humor pathways through the subendothelial layer can be influenced by ciliary muscle contraction and presumably also by contractile elements recently found both in trabecular meshwork and scleral spur. Pharmacologically induced disconnection of inner wall and cribriform cells leads to wash out of extracellular material through breaks of the endothelial lining of Schlemm's canal and to increase of outflow facility. In glaucomatous eyes the resistance to aqueous humor outflow is increased due to an increase in different forms of extracellular material deposited within the cribriform layer. The amount of this newly developed extracellular material is correlated with loss of axons in the optic nerve, indicating that a common factor is responsible for both changes. To investigate the effect of various factors on the biology of trabecular cells monolayer cultures derived from cribriform and corneoscleral trabecular meshwork have been established. The two cell lines can be differentiated because cribriform cells in vivo as in vitro stain for alphabeta-crystallin whereas the corneoscleral cells remain unstained. The effect of TGFbeta, a growth factor increased in aqueous humor of glaucomatous eyes and glycocorticoids on trabecular meshwork cells show typical changes in formation of extracellular matrix components and of stress proteins. Dexamethasone and oxidative damage also lead to increase of trabecular meshwork inducible glucocorticoid response (TIGR) protein. A mutation of the TIGR-gene family has recently been found in families with juvenile and chronic simple glaucoma. Future research has to clarify the significance of these genetic factors for the pathophysiology of glaucoma and the role of trabecular cell activity in this respect.

Differential expression of osteopontin, PC4, and CEC5, a novel mRNA species, during in vitro angiogenesis

The formation of new capillaries from preexisting blood vessels, a process termed angiogenesis, plays a key role in many physiological and pathological conditions such as wound healing, embryogenesis, and tumor growth. The aim of this study was to identify changes in endothelial cell gene expression specifically associated with angiogenesis. Using an in vitro model and the differential display strategy, we compared gene expression patterns of rat microvascular endothelial cells cultured in two (2D) and three-dimensional (3D) culture. In 2D culture, the cells express actin and proliferate, whereas in 3D culture actin expression is downregulated, and the cells are mitotically quiescent and reorganize into vascular tubes. We identified three differentially expressed genes, osteopontin, PC4, and CEC5, a novel mRNA species, with homology to calmodulin-dependent protein kinases. The expression patterns were confirmed by Northern blot analysis. In conclusion, the analysis of gene expression in endothelial cells in 3D and 3D culture allows the identification of genes differentially expressed during angiogenesis. These genes or proteins may serve as targets for therapeutic modulation of angiogenesis.

Phenotype and proliferation characteristics of cultured spindle-shaped cells obtained from normal human skin and lesions of dermatofibroma, Kaposi's sarcoma, and dermatofibrosarcoma protuberans: a comparison with fibroblast and endothelial cells of the dermis

Normal human dermis contains mesenchymal cells that are generally referred to as fibroblasts. However the relationships between fibroblasts and endothelial cells with respect to the types of spindle-shaped cells that are present in cultures obtained from tumor bearing-skin is unclear. To explore the potential heterogeneity amongst dermal-derived cells that grow in culture with a spindle-shaped morphology, we compared the immunophenotype and growth characteristics of several types of cells. Besides dermal fibroblasts and microvascular endothelial cells derived from normal adult skin, we also studied large vessel-derived endothelial cells, and spindle-shaped cells derived from three different tumor-bearing dermal-based neoplasms. Kaposi's sarcoma (KS), dermatofibroma (DF), and dermatofibrosarcoma protuberans (DFSP). A broad panel of eight different antibodies were used to immunophenotype the multi-passaged cultured cells. Spindle-shaped cells from all three neoplasms could be distinguished from the normal skin derived fibroblasts by their constitutive expression of factor XIIIa, and the gamma-interferon induced expression of VCAM-1. All seven types of cultured cells stained positive for s-actin and proline-4-hydroxylase, and none of the cells expressed CD34. Both large and small-vessel derived endothelial cells expressed factor VIII, ELAM-1, and VCAM-1. Using two different types of growth media, significant differences were also observed amongst these cultured cell types. Spindle-shaped cells from DFSP did not grow in DMEM containing 10% fetal bovine serum (DMEM-FBS); but they proliferated in KS cell growth medium (KSGM). Spindle-shaped cells from DF grew best in KSGM, but not in DMEM-FBS. KS tumor cells grew well in KSGM, but not in DMEM-FBS. Fibroblasts proliferated in DMEM-FBS, but failed to grow in KSGM; and even when pre-treated with conditioned medium from a transformed KS cell line (i.e. SLK cells), no fibroblast proliferation could be induced in KSGM. These results indicate that KS cell line (i.e. SLK cells), no fibroblast proliferation could be induced in KSGM. These results indicate that even though dermal-derived cells can have an identical spindle-shape by light microscopy, significant heterogeneity can be defined amongst such cells from normal and tumor-bearing human skin. Having established culture conditions to propagate these different cell types and phenotypic criteria to distinguish them from one another, will provide new research opportunities to explore the function and ontogeny of the diverse mesenchymal cells that take on a spindle-shaped morphology in culture.

Transfected muscle and non-muscle actins are differentially sorted by cultured smooth muscle and non-muscle cells

We have analyzed by immunolabeling the fate of exogenous epitope-tagged actin isoforms introduced into cultured smooth muscle and non-muscle (i.e. endothelial and epithelial) cells by transfecting the corresponding cDNAs in transient expression assays. Exogenous muscle actins did not produce obvious shape changes in transfected cells. In smooth muscle cells, transfected striated and smooth muscle actins were preferentially recruited into stress fibers. In non-muscle cells, exogenous striated muscle actins were rarely incorporated into stress fibers but remained scattered within the cytoplasm and frequently appeared organized in long crystal-like inclusions. Transfected smooth muscle actins were incorporated into stress fibers of epithelial cells but not of endothelial cells. Exogenous non-muscle actins induced alterations of cell architecture and shape. All cell types transfected by non-muscle actin cDNAs showed an irregular shape and a poorly developed network of stress fibers. beta- and gamma-cytoplasmic actins transfected into muscle and non-muscle cells were dispersed throughout the cytoplasm, often accumulated at the cell periphery and rarely incorporated into stress fibers. These results show that isoactins are differently sorted: not only muscle and non-muscle actins are differentially distributed within the cell but also, according to the cell type, striated and smooth muscle actins can be discriminated for. Our observations support the assumption of isoactin functional diversity.

Transforming growth factor-beta: vasculogenesis, angiogenesis, and vessel wall integrity

Genetic studies have recently revealed a role for transforming growth factor-beta-1 (TGF-beta 1) and its receptors (TGF-beta Rs I and II as well as endoglin) in embryonic vascular assembly and in the establishment and maintenance of vessel wall integrity. The purpose of this review is threefold: first, to reassess previous studies on TGF-beta and endothelium in the light of these recent findings; second, to describe some of the well-established as well as controversial issues concerning TGF-beta and its regulatory role in angiogenesis; and third, to explore the notion of "context' with respect to TGF-beta and endothelial cell function. Although the focus of this review will be on the endothelium, other vascular wall cells are also likely to be important in the pathogenesis of the vascular lesions revealed by genetic studies.

pp60c-src is required for the induction of a quiescent mesangial cell phenotype

The tyrosine kinase c-src associates with growth factor receptors, focal contacts and cytoskeletal proteins and is involved in signaling events. The aim of this study was to investigate the role of src in the regulation of mesangial cell (MC) proliferation and differentiation in three-dimensional (3D) culture in collagen gels. Using retroviral gene transfer we have overexpressed wild-type c-src, a kinase-negative c-src mutant (c-src295) and transforming v-src in MC. The MC differentiation in 3D culture was characterized by the formation of a nonproliferating multicellular network in control cells and in cells expressing wild-type c-src. Immunoblotting demonstrated a rapid down-regulation of the alpha-smooth muscle actin expression. The kinase-negative MC (c-src295) failed to differentiate, maintained a significant proliferative rate, and the alpha-smooth muscle actin expression remained stable during 3D culture. MC transformed with v-src showed a high level of tyrosine phosphorylation and proliferation in 3D culture. Analyses of proteins involved in cell cycle regulation demonstrated dephosphorylation of the retinoblastoma protein (Rb) during 3D culture in control and c-src transfected cells. Expression of v-src resulted in sustained Rb phosphorylation. Zymographic analysis of plasminogen activator (u-PA) revealed an inhibition of u-PA secretion in MC transfected with c-src295. These results indicate that c-src exerts regulatory effects on MC proliferation, cytoskeletal organization, matrix proteases and differentiation. Targeted manipulation of the c-src kinase may be useful in modulating MC behavior in vivo.

Establishment of bone morphogenetic protein 2 responsive chondrogenic cell line

A clonal cell line named RMD-1 was established from the skeletal muscle of a 20-day fetal rat. RMD-1 represents a morphologically homogeneous population of undifferentiated mesenchymal cells, expressing alpha-smooth muscle actin and type I collagen, but no cartilage-associated genes. When cultured in agarose gel containing 100 ng/ml of recombinant human bone morphogenetic protein 2 (rhBMP-2; BMP-2), RMD-1 cells formed colonies and showed chondrocyte-like features as assessed by their ultrastructure, metachromatic staining with toluidine blue, and the production of large hydrodynamic-size proteoglycans. RMD-1 cells also differentiated into chondrocytes when the cells were plated at high density (over 2.5 x 10(5) cells/cm2) on type I collagen and incubated in medium containing 0.5% fetal bovine serum and 100 ng/ml of BMP-2. This chondrogenic differentiation was evidenced by a distinct morphological change into spherical cells, an increase in the levels of sulfated glycosaminoglycans, a decrease in type I collagen mRNA and the expression of cartilage-associated genes, including type II collagen, type IX collagen, aggrecan and alkaline phosphatase. In the presence of ascorbic acid and 10% serum, RMD-1 cells increased in size and expressed type X collagen as well as high alkaline phosphatase activity, then induced matrix mineralization. Thus, RMD-1 is a unique cell line that can differentiate from undifferentiated mesenchymal cells into hypertrophic chondrocytes.

Modulation of transforming growth factor beta receptor levels on microvascular endothelial cells during in vitro angiogenesis

Microvascular endothelial cells (RFCs) cultured in two-dimensional (2D) cultures proliferate rapidly and exhibit an undifferentiated phenotype. Addition of transforming growth factor beta1 (TGFbeta1) increases fibronectin expression and inhibits proliferation. RFCs cultured in three-dimensional (3D) type I collagen gels proliferate slowly and are refractory to the anti-proliferative effects of TGF beta1. TGF beta1 promotes tube formation in 3D cultures. TGF beta1 increases fibronectin expression and urokinase plasminogen activator (uPA) activity and plasminogen activator inhibitor-1 (PAI-1) levels in 3D cultures. Since the TGF beta type I and II receptors have been reported to regulate different activities induced by TGF beta1, we compared the TGF beta receptor profiles on cells in 2D and 3D cultures. RFCs in 3D cultures exhibited a significant loss of cell surface type II receptor compared with cells in 2D cultures. The inhibitory effect of TGF beta1 on proliferation is suppressed in transfected 2D cultures expressing a truncated form of the type II receptor, while its stimulatory effect on fibronectin production is reduced in both 2D and 3D transfected cultures expressing a truncated form of the type I receptor. These data suggest that the type II receptor mediates the antiproliferative effect of TGF beta1 while the type I receptor mediates the matrix response of RFCs to TGF beta1 and demonstrate that changes in the matrix environment can modulate the surface expression of TGF beta receptors, altering the responsiveness of RFCs to TGF beta1.

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.

Actin cytoskeletal isoforms in human endothelial cells in vitro: alteration with cell passage

The microfilamentous actin component of the cytoskeleton is crucial to endothelial angiogenesis and vascular permeability. Differences in actin cytoskeletal profiles in cultured human endothelial cells were explored: when first isolated, both primary human umbilical vein endothelial cells (HUVEC) and primary human placental microvascular endothelial cells (HPMEC) expressed F-actin, but not beta-actin or alpha-smooth muscle actin. A similar endothelial actin profile was observed in cryo-sections of freshly delivered term umbilical cord and placenta. In subsequent cell culture, although the actin cytoskeleton of HUVEC remained unchanged, the actin profiles of HPMEC altered after the second passage with the induction of alpha-smooth muscle actin expression, which was intercellularly heterogeneous and increased to 20% at P4. This behaviour occurred in HPMEC monolayers cultured on a variety of extracellular matrices. Comparisons with a spontaneously immortalized human microvascular cell-line, HGTEN 21, revealed that in prolonged passage, both alpha-smooth muscle actin and beta-actin were expressed, whereas HPMEC at P4 showed a lower level of beta-actin expression. Therefore, in comparison with large vessels, microvascular cells are more likely to dedifferentiate. This may reflect the ability of microvascular cells to remodel according to changing requirements for new vessel formation. In conclusion, passage of human microvascular endothelial cells, but not of larger vessel endothelial cells, alters the expression of actin isoforms. This may be important in relation to comparisons of in vitro and in vivo vascular permeability; higher passage microvascular endothelial cells should thus be used with caution in such studies.

A retinoic acid-induced clonal cell line derived from multipotential P19 embryonal carcinoma cells expresses smooth muscle characteristics

Despite intense interest in understanding the differentiation of vascular smooth muscle, very little is known about the cellular and molecular mechanisms that control differentiation of this cell type. Progress in this field has been hampered by the lack of an inducible in vitro system for study of the early steps of smooth muscle differentiation. In this study, we describe a model system in which multipotential mouse P19 embryonal carcinoma cells (P19s) can be induced to express multiple characteristics of differentiated smooth muscle. Treatment of P19s with retinoic acid was associated with profound changes in cell morphology and with the appearance at high frequency of smooth muscle alpha-actin-positive cells that were absent or present at extremely low frequency in parental P19s. A clonal line derived from retinoic acid-treated P19s (9E11G) stably expressed multiple characteristics of differentiated smooth muscle, including smooth muscle-specific isoforms of alpha-actin and myosin heavy chain, as well as functional responses to the contractile agonists phenylephrine, angiotensin II, ATP, bradykinin, histamine, platelet-derived growth factor (PDGF)-AA, and PDGF-BB. Additionally, 9E11G cells expressed transcripts for MHox, a muscle homeobox gene expressed in smooth, cardiac, and skeletal muscles, but not the skeletal muscle-specific regulatory factors, MyoD and myogenin. Results demonstrate that retinoic acid treatment of multipotential P19 cells is associated with formation of cell lines that stably express multiple properties of differentiated smooth muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

Benign lymphangioendothelioma

We describe a 40-year-old white man with a red-brown, indurated plaque on the proximal aspect of his right thigh. The lesion had been present since birth, and the patient had a 20-year clinical history of recurrent cellulitis in the same area. The histopathologic features of the lesion included permeation of the dermis by flattened, endothelium-lined channels without cellular atypia, hemorrhage, or inflammation. The endothelial cells were stained intensely with monoclonal antibody anti-CD34 (clone MY10). In addition, antibodies to factor VIII antigen, HLA-DR, smooth muscle actin, ICAM-1, and the lectin Ulex europaeus labeled the luminal cells. The basement membrane of the channels stained with anti-type IV collagen and laminin. Desmin-positive cells were abundant adjacent to the channels. Factor XIIIa stained both mononuclear cells and occasional dendritic cells in the perivascular area. Ki-67 immunolabeling could not be demonstrated on fresh or frozen tissue. Electron microscopy revealed the presence of both tight junctions and a well-formed, continuous basement membrane but the absence of Weibel-Palade bodies.

Fibronectin and the basement membrane components laminin and collagen type IV influence the phenotypic properties of subcultured rat aortic smooth muscle cells differently

A substrate of the extracellular matrix protein fibronectin has previously been found to promote the modulation of freshly isolated rat aortic smooth muscle cells from a contractile to a synthetic phenotype early in primary culture. In contrast, substrates of the basement membrane proteins laminin and collagen type IV were found to retain the cells in a contractile phenotype. Here, we have studied whether rat aortic smooth muscle cells tht have already adopted a synthetic phenotype are also affected differently by these proteins. For this sake, subcultured cells were detached with trypsin, seeded on substrates of either fibronectin or laminin plus collagen type IV, and incubated in a serum-free medium for one to three days. RNA blot and immunoblot analyses indicated that cells grown on laminin plus collagen type IV expressed smooth muscle alpha-actin transcripts and protein at higher levels than cells grown on fibronectin. Moreover, immunocytochemical and electron-microscopic analyses revealed that cells positively stained for smooth muscle alpha-actin and cells with a cytoplasm dominated by large microfilament bundles were more numerous on laminin plus collagen type IV than on fibronectin. Finally, thymidine autoradiography showed that the DNA synthetic response to stimulation with platelet-derived growth factor or serum was weaker in cells grown on laminin plus collagen type IV than in cells grown on fibronectin. These findings confirm the notion that a substrate of laminin and collagen type IV stimulates the in vitro expression of differentiated smooth muscle traits at a higher level than does a substrate of fibronectin.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of platelet-derived growth factor receptor expression in microvascular endothelial cells during in vitro angiogenesis

Microvascular endothelial cells in vivo exhibit a plastic phenotype, forming a nonproliferative, differentiated capillary network, while retaining their ability to respond to injury by proliferation, migration and neovascularization. The presence of PDGF receptors and PDGF responsiveness in microvascular endothelial cells and the significance of PDGF isoforms in the control of endothelial cell growth and differentiation remain controversial. Since culture of microvascular endothelial cells in a three-dimensional (3D) system induced cell differentiation and angiogenesis and inhibited proliferation, the present study investigates the role of different extracellular matrix environments in inducing different microvascular endothelial cell phenotypes on microvascular endothelial cell PDGF receptor expression and PDGF responsiveness. In conventional two-dimensional (2D) culture, microvascular endothelial cells expressed both PDGF receptor alpha and beta chains. Suramin treatment demonstrated continuous downregulation of the alpha receptor surface expression. PDGF BB and, to a lesser extent, PDGF AB were mitogenic in 2D-culture, PDGF AA failed to induce any proliferative response despite inducing receptor autophosphorylation. During in vitro angiogenesis induced by 3D-culture, both PDGF receptors were rapidly downregulated. Assessment of cell proliferation showed quiescent cells and PDGF unresponsiveness. We conclude that the induction of a differentiated phenotype during in vitro angiogenesis (tube formation) driven in part by the spatial organization of the surrounding matrix is associated with a downregulation of PDGF receptors. Identification of the molecular cell-matrix interactions involved in this receptor regulation may allow for targeted manipulation of cell growth in vivo and lead to novel therapeutic applications for PDGF.

Extracellular matrix components affect the pattern of protein synthesis of endothelial cells responding to hyperthermia

The biosynthetic profile of endothelial cells responding to hyperthermia is altered by extracellular matrix components. The extracellular matrix components influence the quantitative expression of members of the HSP70 family and HSP90. The expression of several HSP70 mRNA species, which are strictly stress inducible, are modulated by extracellular matrix components. Both laminin and collagen type IV decrease the amount of HSP70 protein and mRNA expressed by endothelial cells exposed to hyperthermia relative to control cultures attached to virgin plastic. In contrast, both laminin and collagen type IV increased the amount of HSP90 mRNA constitutively expressed by endothelial cells at 37 degrees C. When endothelial cells were exposed to elevated temperatures, these two extracellular matrix proteins decrease the amount of HSP90 mRNA relative to control cultures attached to virgin plastic. Our observations are consistent with the proposal that the extracellular matrix components regulate gene expression and cell behavior in regard to thermotolerance.

In vitro expression of the alpha-smooth muscle actin isoform by rat lung mesenchymal cells: regulation by culture condition and transforming growth factor-beta

alpha-Smooth muscle actin (alpha SM actin)-containing cells recently have been demonstrated in intraalveolar lesions in both rat and human tissues following lung injury. In order to develop model systems for the study of such cells, we examined cultured lung cell lines for this phenotype. The adult rat lung fibroblast-like "RL" cell lines were found to express alpha SM actin mRNA and protein and to organize this actin into stress fiber-like structures. Immunocytochemical staining of subclones of the RL87 line demonstrated the presence in the cultures of at least four cell phenotypes, one that fails to express alpha SM actin and three distinct morphologic types that do express alpha SM actin. The proportion of cellular actin that is the alpha-isoform was modulated by the culture conditions. RL cells growing at low density expressed minimal alpha SM actin. On reaching confluent densities, however, alpha SM actin increased to at least 20% of the total actin content. This effect, combined with the observation that the most immunoreactive cells were those that displayed overlapping cell processes in culture, suggests that cell-cell contact may be involved in actin isoform regulation in these cells. Similar to the response of some smooth muscle cell lines, alpha SM actin expression in RL cells also was promoted by conditions, e.g., maintenance in low serum medium, which minimize cell division. alpha SM actin expression was modulated in RL cells by the growth factor transforming growth factor-beta. Addition of this cytokine to growing cells substantially elevated the proportion of alpha SM actin protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Spatial organization of the extracellular matrix modulates the expression of PDGF-receptor subunits in mesangial cells

The aim of this study was to test the hypothesis that changes in the extracellular matrix environment regulate rat mesangial cell growth by modulation of the expression of both PDGF-receptor alpha- and beta-subunits. We investigated the mitogenic effects of the PDGF isoforms AA, AB and BB in conventional two-dimensional (2D) culture on laminin, fibronectin, type I, IV and V collagen and in the different spatial organization of matrix in type I collagen gels in three-dimensional culture (3D). In 2D culture PDGF BB was a potent mitogen, AB elicited an intermediate response while AA had no effect on cell proliferation. Extracellular matrix did not modify the PDGF responsiveness in 2D-culture. The different effects of the three PDGF isoforms were due to differential expression and isoform specific association of the PDGF-receptor subunits. Specifically, the beta-receptor was strongly expressed, whereas the alpha-receptor was only barely detectable on the cell surface. Metabolic labeling revealed synthesis and intracellular accumulation of the complete alpha-receptor protein, and treatment with suramin increased its surface expression, suggesting continuous receptor down-regulation by endogenous PDGF. Morphological and ultrastructural analysis in 3D culture revealed a change in mesangial cell phenotype, forming a branching network of multicellular structures. Assessment of proliferation in 3D culture showed quiescent cells and PDGF unresponsiveness. Investigation of the PDGF beta-receptors revealed a rapid down-regulation in 3D culture; both receptor subunits were not detectable on the cell surface. We conclude that 3D culture promotes the induction of a different mesangial cell phenotype.(ABSTRACT TRUNCATED AT 250 WORDS)

Actin is a surface component of calf pulmonary artery endothelial cells in culture

An angiogenin binding protein isolated previously from endothelial cells has been shown to be a member of the actin family. Calf pulmonary artery endothelial (CPAE) cells were investigated for the presence of surface actin by immunoblotting of isolated surface proteins and by immunofluorescence. CPAE cell surface proteins were isolated by selective apical biotinylation and recovery of biotinylated proteins by avidin affinity chromatography. Immunoblotting with a specific smooth muscle alpha-actin antibody detected the presence of this type of actin among the isolated cell surface proteins. Immunofluorescence confirmed that smooth muscle alpha-actin is localized at the surface of nonpermeabilized CPAE cells. Exposure of CPAE cells to angiogenin prior to cell surface immunostaining diminished the signal. When CPAE and rat aortic smooth muscle cells were made permeable before staining, stress fibers could be recognized by the antibody in smooth muscle cells but not CPAE cells. The results indicate that a smooth muscle type of alpha-actin is localized specifically on the surface of cultured CPAE cells where it might interact with angiogenin and other actin binding proteins present in the extracellular environment.

Histamine-modulated transdifferentiation of dermal microvascular endothelial cells

Homeostatic and inflammatory functions of skin microvessels are tightly regulated by vasoactive amines. Following stimulation with histamine, dermal microvascular endothelial cells (MEC) undergo a rapid change in phenotype (transdifferentiation) and subsequently exhibit an enhanced rate of growth. To elucidate mechanisms regulating MEC transdifferentiation, this study investigated the functional relationships among vimentin, Ca2+, and protein kinase C (PKC) in histamine-modulated dermal MEC in vitro. Distribution of vimentin and PKC in foreskin-derived MEC cultivated in a modified Iscove's medium was assessed with immunocytochemistry. Calcium ion kinetics in histamine-treated MEC were analyzed using the Ca2+ probe Fluo-3 in conjunction with interactive laser cytometry. Histamine, acting through H-1 receptors, produces a rapid (less than 100 ms) and differential elevation of free calcium in each of three cytological compartments defined by the vimentin cytoskeleton in epithelial MEC. A distinctive compartmentalized and nonuniform distribution of PKC precisely coincides with that observed for free-Ca2+ released in response to histamine. The studies reveal that histamine modulation of the MEC phenotype is associated with a rapid patterned reorganization of the vimentin skeleton. It is hypothesized that histamine induces vimentin post-translational modifications by activating a spatially localized interaction among cytoplasmic free Ca2+, PKC, and the vimentin matrix. The results further suggest that vimentin, in addition to its structural role, may participate in signal transduction and gene regulation processes in effecting MEC transdifferentiation.

Reversible expression of sm alpha-actin protein and sm alpha-actin mRNA in cloned cerebral endothelial cells

The expression of smooth muscle (sm) alpha-actin was studied in cloned capillary cerebral endothelial cells of two phenotypes. Type I cells were cultured in medium containing 10% FCS, heparin and ECGS (or alpha-ECGF) and stained positive for a specific endothelial cell marker (Bandeiraea simplicifolia). Depletion of heparin and ECGS resulted in a smooth muscle-like appearance after 2-3 days. Cells of this phenotype, (type II) stained positive for the endothelial cell marker and for sm alpha-actin. In contrast to type I cells, type II cells expressed sm alpha-actin protein and mRNA as evidenced by Immunoblots and Northern blots. This phenotypic switch was shown to be reversible and so was the expression of sm alpha-actin.

Cytoskeleton in TFG-beta- and bFGF-modulated endothelial monolayer repair

Endothelial cell proliferation and migration in vitro is depressed by transforming growth factor beta (TFG-beta) and enhanced by basic fibroblast growth factor (bFGF) treatment. This study examines interactions between cytoskeletal changes and cell proliferation in regenerating endothelial monolayers treated with bFGF, TFG-beta, and both factors. As previously described by others, monolayer regeneration is enhanced by bFGF and reduced by TFG-beta. Endothelial cell morphology is altered by TFG-beta treatment. Cells lose their cobblestone appearance and assume a pleomorphic shape. Actin microfilament staining is modified in both intact and regenerating TFG-beta-treated monolayers as well. There is a loss of dense peripheral band staining and an enhancement in staining intensity of cytoplasmic stress fibers. No such alterations are seen in bFGF-treated cultures. Cell proliferation at the wound edge, as indicated by bromodeoxyuridine incorporation, is inhibited by TGF-beta. Although monolayer repair is modulated by growth factor treatment, centrosome reorientation and microtubule staining patterns are not altered by either factor. Thus these factors appear to have effects on a mechanism(s) other than centrosome reorientation which may be involved in repair of denuded endothelial monolayers.

Vascular cell responses to a hybrid transforming growth factor-beta molecule

Functional biological assays were performed using a hybrid molecule of Transforming Growth Factor-Beta (TGF-5 beta) where nine amino acids near the cleavage site of TGF-beta 1 were substituted with nine amino acids located in the identical position of TGF-beta 2. Bovine aortic endothelial and smooth muscle cells as well as rat epididymal fat pad microvascular endothelia were studied in three distinct bioassays examining proliferation, migration and angiogenesis. The data suggested TGF-5 beta elicited results that do not differ significantly from the TGF-beta 1 isoform, while TGF-beta 2 expressed unique characteristics. We have also shown that these amino acid substitutions to TGF-beta 1 do not, in fact, alter the biological functions of the growth factor.

Microvessel endothelial cell transdifferentiation: phenotypic characterization

Human dermal microvessel endothelial cells (MEC) have two basic functions: maintenance of tissue homeostasis and facilitation of inflammatory responses. The former requires that the endothelium expresses traits of an epithelium, while inflammatory reactions are associated with intimal disruption. Acute inflammation transiently alters endothelium, whereas chronic inflammation may result in vessel reorganization and MEC mesenchymalization. Foreskin MEC in vitro undergo a similar epithelial-mesenchymal modulation. In the presence of cAMP, cultivated dermal MEC exhibit the structural and functional characteristics of an epithelium. MEC grown in cAMP-deficient medium initially have a "transitional" configuration and are subsequently transformed into mesenchymal cells. If cAMP is replaced by histamine, MEC maintain a stable intermediate transitional configuration. Transitional MEC refed cAMP-supplemented medium revert to an epithelial phenotype, whereas parallel cultures fed cAMP-deficient medium are transformed into mesenchymal cells. Phenotypic modulation can be induced without cell division and thus provides a unique example of direct transdifferentiation. Our data furthermore suggest that this transdifferentiation results in the acquisition of properties usually attributed to cells of the reticuloendothelial system.

Chondrocytes inhibit endothelial sprout formation in vitro: evidence for involvement of a transforming growth factor-beta

Using a quantitative in vitro model of spontaneous endothelial sprout formation, we have attempted to define physiological inhibitors of angiogenesis from hyaline cartilage, a tissue whose antiangiogenic properties have been well described. The model consists of embedding bovine microvascular endothelial cell aggregates into fibrin or collagen gels, which results in the formation of radially growing sprouts. When chondrocytes derived from the permanent cartilagenous region of the chick embryo sternum are cocultured with the endothelial cell aggregates, sprout formation is markedly inhibited. Addition of anti-TGF-beta antibodies to the cocultures significantly reduced the inhibitory effect of chondrocytes on sprout formation. Chondrocyte-conditioned medium or exogenously added TGF-beta 1 have a similar albeit transient inhibitory effect. Depletion of TGF-beta from chondrocyte conditioned medium with anti-TGF-beta antibodies and solid-phase protein-A significantly decreases the inhibition of sprout formation. These results demonstrate that a chondrocyte-derived TGF-beta-like molecule inhibits capillary sprout formation in vitro and suggest that the antiangiogenic properties of cartilage may at least in part, be mediated by TGF-beta.

Vascular cell responses to TGF-beta 3 mimic those of TGF-beta 1 in vitro

The vascular cell responses to the type 3 isoform of transforming growth factor-beta (TGF-beta 3) were studied using bovine aortic endothelial (BAECs) and smooth muscle cells (BASMCs) as well as rat epididymal fat pad microvascular endothelia (RFCs). Four distinct bioassays indicated that TGF-beta 3 elicits results that do not differ significantly from those of the TGF-beta 1 isoform in all three cell populations. Inhibition of proliferation by TGF-beta 3 at a 5-day time point ranged from 85% on BAECs, to 55% and 53% on RFCs and BASMCs, respectively. The effects of TGF-beta 3 and TGF-beta 1 on cell migration were also found to be similar; migration of large vessel endothelial cells was inhibited 35%, while migration of smooth muscle cells was enhanced 30%. TGF-beta 1 and TGF-beta 3 also had equivalent effects on neovascularization while a 10-fold higher concentration of TGF-beta 2 was required to elicit a similar response. Experimentation to decipher cell surface binding by the different isoforms revealed that iodinated TGF-beta 1 bound to the surface of all three vascular cell types can be competed off in similar fashion by either TGF-beta 1 or TGF-beta 3; however, competition with TGF-beta 2 produced unique binding profiles dependent upon the cell type examined. In summary, both the TGF-beta 1 and TGF-beta 3 isoforms of the transforming growth factor-beta family evoke comparable responses in proliferation, migration, angiogenic and cell surface binding assays using three distinct vascular cell types, while the biofunctions of TGF-beta 2 on these cells are distinct.

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

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

Transforming growth factor-beta 1 modulates basic fibroblast growth factor-induced proteolytic and angiogenic properties of endothelial cells in vitro

Tightly controlled proteolytic degradation of the extracellular matrix by invading microvascular endothelial cells is believed to be a necessary component of the angiogenic process. We have previously demonstrated the induction of plasminogen activators (PAs) in bovine microvascular endothelial (BME) cells by three agents that induce angiogenesis in vitro: basic FGF (bFGF), PMA, and sodium orthovanadate. Surprisingly, we find that these agents also induce plasminogen activator inhibitor-1 (PAI-1) activity and mRNA in BME cells. We also find that transforming growth factor-beta 1 (TGF-beta 1), which in vitro modulates a number of endothelial cell functions relevant to angiogenesis, also increases both PAI-1 and urokinase-type PA (u-PA) mRNA. Thus, production of both proteases and protease inhibitors is increased by angiogenic agents and TGF-beta 1. However, the kinetics and amplitude of PAI-1 and u-PA mRNA induction by these agents are strikingly different. We have used the ratio of u-PA:PAI-1 mRNA levels as an indicator of proteolytic balance. This ratio is tilted towards enhanced proteolysis in response to bFGF, towards antiproteolysis in response to TGF-beta 1, and is similar to that in untreated cultures when the two agents are added simultaneously. Using an in vitro angiogenesis assay in three-dimensional fibrin gels, we find that TGF-beta 1 inhibits the bFGF-induced formation of tube-like structures, resulting in the formation of solid endothelial cell cords within the superficial parts of the gel. These results suggest that a net positive proteolytic balance is required for capillary lumen formation. A novel perspective is provided on the relationship between extracellular matrix invasion, lumen formation, and net proteolytic balance, thereby reflecting the interplay between angiogenesis-modulating cytokines such as bFGF and TGF-beta 1.

Covalent surface immobilization of Arg-Gly-Asp- and Tyr-Ile-Gly-Ser-Arg-containing peptides to obtain well-defined cell-adhesive substrates

The synthetic peptides Gly-Arg-Gly-Asp-Tyr and Gly-Tyr-Ile-Gly-Ser-Arg-Tyr, which contain Arg-Gly-Asp (RGD) and Tyr-Ile-Gly-Ser-Arg (YIGSR), the ligands for two important classes of cell adhesion receptors, were covalently coupled to a nonadhesive modified glass surface by the N-terminal Gly. The N-terminal Gly served as a spacer, and the C-terminal Y served as a site for radioiodination. These modified substrates supported the adhesion and spreading of cultured human foreskin fibroblasts (HFFs) independently of adsorbed proteins and, it was demonstrated that a covalently immobilized YIGSR-containing peptide has biological activity. The surface concentration of grafted peptide on the glass was measured by 125I radio-labeling and was 12.1 pmol/cm2. HFFs spread on both immobilized peptide substrates, but at much slower rates on grafted YIGSR glass surfaces than on the RGD-containing substrates. Cells formed focal contacts on the RGD-derivatized substrates in the presence or absence of serum. Focal contacts formed on the YIGSR-grafted surfaces only when serum was present in the medium and had morphologies different from those observed on the RGD-containing substrates. Serum influenced the organization of microfilaments and the extent of spreading of adherent cells, although adsorption of adhesion proteins was minimal on all substrates. This derivatization method produced chemically stable substrates which may be useful in studying receptor-mediated cell adhesion, as the quantity of peptide available at the surface may be precisely measured and controlled.