Platelet-derived growth factor: its potential roles in wound healing, atherosclerosis, neoplasia, and growth and development

Platelet-derived growth factor (PDGF) has been found to be derived not only from platelets that have been induced to release their contents, but also from a number of transformed cells (including cells transformed by both DNA and RNA viruses, spontaneous transformation, and cells from various human tumours), from activated macrophages, from embryonic rat aortic smooth muscle cells, and from rat aortic smooth muscle cells derived from experimentally induced intimal proliferative lesions. The studies discussed demonstrate the potential role of platelets and macrophages in atherosclerosis and in wound repair, and indicate the ability of anti-PDGF IgG to inhibit proliferative responses in vitro. With the demonstration that the transforming protein derived from the oncogene from the simian sarcoma virus is highly homologous with PDGF, it was possible to show that a number of transformed cells secrete PDGF and show markedly decreased binding of PDGF. The same is true for embryonic rat aortic smooth muscle cells and for cells from experimentally induced proliferative lesions in the rat carotid artery. All these findings point to the role of PDGF in the formation of these lesions and can be correlated with the capacity of the cells noted above, as well as injured endothelial cells, to secrete PDGF or PDGF-like molecules. The biological significance of these observations is discussed and a model for atherogenesis is proposed.

Activation of metalloproteinases and their association with integrins: an auxiliary apoptotic pathway in human endothelial cells

Anchorage of cells to the extracellular matrix and integrin-mediated signals play crucial roles in cell survival. We have previously shown that during growth factor deprivation-induced apoptosis in human umbilical vein endothelial cells (HUVECs), key molecules in focal adhesions and adherens junctions are cleaved by caspases. In this study we provide evidence for a selective upregulation of cell-associated matrix metalloproteinases (MMPs). We observe a physical association of MMP2 with beta1 and alphav integrins, which increased three- to fourfold during apoptosis and is dependent upon integrin beta1 levels and activation state. Both enforced activation of beta1 integrin by a specific antibody and inhibition of MMPs protect HUVECs from apoptosis. We hypothesize that, prior to the commitment to apoptosis, 'inside-out' signals initiated by the apoptotic stimulus alter cell shape together with the activation states and/or the availability of integrins, which promote matrix-degrading activity around dying cells. This 'auxiliary' apoptotic pathway may interrupt ECM-mediated survival signaling, and thus accelerate the efficient execution of the cell death program.

MMP9 production by human monocyte-derived macrophages is decreased on polymerized type I collagen

The production of matrix metalloproteinases (MMPs), such as MMP9, by macrophages may be a critical factor in the rupture of unstable atherosclerotic plaques and aortic aneurysms. Therefore, we studied the role of matrix and soluble cytokines in the regulation of monocyte/macrophage expression of MMP9. Although freshly isolated monocytes synthesize little MMP9, cells cultured on tissue-culture plastic differentiate into macrophages and synthesize maximal amounts of MMP9. Differentiated macrophages cultured on plastic are unresponsive to further stimulation by interleukin 1beta, tumor necrosis factor alpha, or platelet-derived growth factor BB. In contrast, monocytes cultured on polymerized collagen synthesize much less MMP9 than cells cultured on plastic and demonstrate a more than three-fold increase in MMP9 synthesis in response to interleukin 1beta, tumor necrosis factor alpha, and platelet-derived growth factor BB. To determine whether the physical state of the collagen was critical for the decrease in basal synthesis of MMP9, monocytes were cultured in suspension for 5 days to allow differentiation and then seeded onto monomer or polymerized collagen. Synthesis of MMP9 was significantly decreased in cells on polymerized collagen and modestly increased in macrophages seeded on monomer collagen. These results suggest that MMP9 synthesis by macrophages in the vessel wall may be under negative control by native, polymerized collagen and that disruption of this native conformation could increase MMP9 production. In addition, cells in contact with the collagen matrix are potentially more responsive to soluble mediators such as platelet-derived growth factor, interleukin 1beta, and tumor necrosis factor alpha.

ADAM15 overexpression in NIH3T3 cells enhances cell-cell interactions

ADAM15 is a member of the family of metalloprotease-disintegrins that have been shown to interact with integrins in an RGD- and non-RGD-dependent manner. In the present study, we examined the effects of ADAM15 overexpression on cell-matrix and cell-cell interactions in NIH3T3 cells. Tetracycline-regulated ADAM15 overexpression in NIH3T3 cells leads to an inhibition of migration on a fibronectin-coated filter in a Boyden chamber assay and in a scratch wound model. The effects of ADAM15 overexpression on cell migration are not due to changes in matrix attachment or to the lack of extracellular signal-regulated kinase signaling response to PDGF or fibronectin. However, a decrease in monolayer permeability with ADAM15 overexpression and altered cell morphology suggest a possible increase in cell-cell interaction. Analysis of adhesion of NIH3T3 cells to a polyclonal population of cells retrovirally transduced to overexpress ADAM15 demonstrates a 45% increase in cell adhesion, compared with enhanced green fluorescent protein-expressing control cells. In addition, we demonstrate localization of HA-epitope-tagged ADAM15 to cell-cell contacts in an epithelial cell line that forms extensive cell-cell contact structures. Thus, overexpression of ADAM15 in NIH3T3 cells appears to enhance cell-cell interactions, as suggested by decreased cell migration, altered cell morphology at the wound edge, decreased monolayer permeability, and increased cell adhesion to monolayers of cells expressing ADAM15 by retroviral transduction.

Platelet-derived growth factor B-chain of hematopoietic origin is not necessary for granulation tissue formation and its absence enhances vascularization

The hypothesis that wound repair is augmented by delivery of platelet-derived growth factor (PDGF) from platelets and macrophages is an attractive extrapolation from the known activities of PDGF in cell culture and in vivo. To test this hypothesis in mice, we prepared hematopoietic chimeras, in which the hematopoietic system of a normal adult mouse was replaced by the hematopoietic system of a PDGF B-chain -/- or +/+ donor. We initiated local granulation tissue formation either by implanting small surgical sponges to elicit a foreign body granulation tissue response, or by ligating the left common carotid to form an organized thrombus. We found that the absence of hematopoietic PDGF B-chain did not decrease the extent of granulation tissue or vascular lesion formation, and that the vascularization of both lesions increased by approximately 100%. We conclude that PDGF B-chain from cells of hematopoietic origin, including platelets and macrophages, is not important for granulation tissue formation, and that it reduces vascularization of granulation issue, probably through disabling of the short-range chemotactic gradients of PDGF that are important for recruiting pericytes/smooth muscle cells to the endothelium of new vessels.

Tumor necrosis factor-alpha-converting enzyme (ADAM17) mediates the cleavage and shedding of fractalkine (CX3CL1)

Fractalkine (CX3CL1) is an unusual member of the chemokine family that is synthesized with its chemokine domain at the end of a mucin-rich, transmembrane stalk. This membrane-bound localization allows fractalkine to function as an adhesion molecule for cells bearing its receptor, CX3CR1. In addition, fractalkine can be proteolytically released from the cell surface, generating a soluble molecule that functions as a chemoattractant similar to the other members of the chemokine family. In this study, we have examined the mechanisms that regulate the conversion between these two functionally distinct forms of fractalkine. We demonstrate that under normal conditions fractalkine is synthesized as an intracellular precursor that is rapidly transported to the cell surface where it becomes a target for metalloproteinase-dependent cleavage that causes the release of a fragment containing the majority of the fractalkine extracellular domain. We show that the cleavage of fractalkine can be markedly enhanced by stimulating cells with phorbol 12-myristate 13-acetate (PMA), and we identify tumor necrosis factor-alpha converting enzyme (TACE; ADAM17) as the protease responsible for this PMA-induced fractalkine release. In addition, we provide data showing that TACE-mediated fractalkine cleavage occurs at a site distinct from the dibasic juxtamembrane motif that had been suggested previously based on protein sequence homologies. The identification of TACE as a major protease responsible for the conversion of fractalkine from a membrane-bound adhesion molecule to a soluble chemoattractant will provide new information for understanding the physiological function of this chemokine.

Modulation of smooth muscle proliferation in rat carotid artery by platelet-derived mediators and fibroblast growth factor-2

Endothelial denuding injury to the rat carotid artery stimulates smooth muscle cell proliferation in the tunica media. Fibroblast growth factor-2 (FGF2) is responsible for a significant portion of this proliferation but other factors may contribute, particularly those released from adherent platelets. We therefore tested the effects of a range of platelet-derived factors. After filament injury, which minimises FGF2 release, the proliferation rate in thrombocytopaenic rats was decreased by 74% (P < 0.02). After balloon injury, antibody neutralisation of platelet-derived growth factor (PDGF) caused a 27% decrease in proliferation (P < 0.05), while inhibition of histamine H(1) receptors caused a 53% increase (P < 0.05). When filament injury was performed 1 h after FGF2 injection, the proliferation rate increased from 2.3+/-0.7 to 32.8+/-2.7% (P < 0.001), while filament injury alone caused a proliferation rate of only 18.3+/-2.9% (P < 0.01 versus filament plus FGF2). These data suggest that platelet-derived factors interact with FGF2 that is adsorbed to the vessel wall in the control of smooth muscle cell proliferation, and that the net effect of platelets is to stimulate smooth muscle cell proliferation. PDGF, but no other platelet agonist tested, contributes to that stimulation.

Basis of hematopoietic defects in platelet-derived growth factor (PDGF)-B and PDGF beta-receptor null mice

Platelet-derived growth factor (PDGF)-B and PDGF beta-receptor (PDGFR beta) deficiency in mice is embryonic lethal and results in cardiovascular, renal, placental, and hematologic disorders. The hematologic disorders are described, and a correlation with hepatic hypocellularity is demonstrated. To explore possible causes, the colony-forming activity of fetal liver cells in vitro was assessed, and hematopoietic chimeras were demonstrated by the transplantation of mutant fetal liver cells into lethally irradiated recipients. It was found that mutant colony formation is equivalent to that of wild-type controls. Hematopoietic chimeras reconstituted with PDGF-B(-/-), PDGFR beta(-/-), or wild-type fetal liver cells show complete engraftment (greater than 98%) with donor granulocytes, monocytes, B cells, and T cells and display none of the cardiovascular or hematologic abnormalities seen in mutants. In mouse embryos, PDGF-B is expressed by vascular endothelial cells and megakaryocytes. After birth, expression is seen in macrophages and neurons. This study demonstrates that hematopoietic PDGF-B or PDGFR beta expression is not required for hematopoiesis or integrity of the cardiovascular system. It is argued that metabolic stress arising from mutant defects in the placenta, heart, or blood vessels may lead to impaired liver growth and decreased production of blood cells. The chimera models in this study will serve as valuable tools to test the role of PDGF in inflammatory and immune responses. (Blood. 2001;97:1990-1998)

Fibrillar collagen specifically regulates human vascular smooth muscle cell genes involved in cellular responses and the pericellular matrix environment

Proliferation and alpha(v)beta(3) integrin-dependent migration of vascular smooth muscle cells are suppressed on polymerized type I collagen. To identify genes specifically regulated in human smooth muscle cells by polymerized collagen, we used the suppressive subtraction hybridization technique. Compared with smooth muscle cells cultured on monomer collagen, polymerized collagen suppresses the following: (1) a number of other extracellular matrix proteins, including fibronectin, thrombospondin-1, tenascin-C, and cysteine-rich protein 61; (2) actin binding proteins including alpha-actinin; (3) signaling molecules; (4) protein synthesis-associated proteins; and (5) genes with unknown functions. Some of the identified genes, including cysteine-rich protein 61, show unique kinetics of mRNA regulation by monomer or polymerized collagen distinct from growth factors, suggesting extracellular matrix-specific gene modulation. Moreover, in vivo balloon catheter-mediated injury to the rat carotid artery induces many of the genes that are suppressed by polymerized collagen. Protein levels of thrombospondin-1 and fibronectin are also suppressed by polymerized collagen. Thrombospondin-1-mediated smooth muscle cell migration on vitronectin is significantly inhibited after culture on polymerized collagen for 24 hours, which is associated with decreased alpha-actinin accumulation at focal adhesions. Thus, polymerized type I collagen dynamically regulates gene expression, pericellular accumulation of extracellular matrix molecules, and the response to a given matrix molecule.

xIAP induces cell-cycle arrest and activates nuclear factor-kappaB : new survival pathways disabled by caspase-mediated cleavage during apoptosis of human endothelial cells

Survival of human vascular endothelial cells depends on their ability to activate the transcription factor nuclear factor-kappaB (NF-kappaB), a regulator of antiapoptotic genes, such as the X chromosome-linked inhibitor of apoptosis protein (xIAP). In the present study, we demonstrated expression of xIAP in the endothelial lining of normal human arteries and veins and elevated levels in highly malignant human endothelial tumors. Using retroviral infection of human endothelial cells, we identified two novel survival mechanisms mediated by xIAP in endothelial cells. First, xIAP can activate the transcription factor NF-kappaB, a known survival factor for human endothelial cells. This positive feedback loop induced by xIAP is mediated via phosphorylation and sustained degradation of inhibitor (I) kappaBalpha. Second, xIAP can inhibit cell proliferation via downregulation of cyclins A and D1 and induction of the cyclin-dependent kinase inhibitors p21(Cip1/Waf1) and p27(Kip1). Cleavage of xIAP by caspases during endothelial cell apoptosis disables both of these biological functions of xIAP. Thus, caspase-mediated cleavage of xIAP interrupts a positive regulatory cytoprotective loop between NF-kappaB and xIAP and increases the vulnerability of the cell to apoptosis by releasing it from an xIAP-mediated quiescent state.

Detection of dissection and remodeling of atherosclerotic lesions in rabbits after balloon angioplasty by magnetic-resonance imaging

OBJECTIVE

To assess the usefulness of magnetic-resonance imaging (MRI) for monitoring acute changes after angioplasty of preexisting lesions in rabbits with basal lesions similar to those observed in humans.

METHODS

A combination of Fogarty balloon injury (1 week after initiation of diet) and a mildly hypercholesterolemic diet (0.2% cholesterol and 5% peanut oil) was used to promote the rapid formation of atherosclerotic lesions in 16 New Zealand white rabbits. After 5 months of the diet, angioplasty was performed on these lesions with a Grüntzig catheter in both iliac arteries and the abdominal aorta. MRI was used to monitor the initial formation of lesions after 3 and 5 months of the diet, and 2 days, 2 weeks, and 1 and 2 months after angioplasty.

RESULTS

The combination of early Fogarty injury and mildly hypercholesterolemic diet induced fibroproliferative lesions similar to type Vb atherosclerotic lesions seen in humans. Angioplasty induced deep dissections at the shoulders of lesions in the majority of animals. These dissections often extended into the media. The cellular, proliferative response after angioplasty was localized and limited to sites of dissection. A significant increase in area of arterial wall was observed after angioplasty at sites of dissection without any loss of lumen. In contrast, proximal and distal to the sites of injury, there was no change in wall area but a transient reduction in lumen area.

CONCLUSIONS

Comparison of MRI results with histology confirmed that changes in the wall and lumen, including small linear dissections in the lesions and arterial remodeling, are detectable by MRI.

Endothelial cells of hematopoietic origin make a significant contribution to adult blood vessel formation

Granulation tissue formation is an example of new tissue development in an adult. Its rich vascular network has been thought to derive via angiogenic sprouting and extension of preexisting vessels from the surrounding tissue. The possibility that circulating cells of hematopoietic origin can differentiate into vascular endothelial cells (ECs) in areas of vascular remodeling has recently gained credibility. However, no quantitative data have placed the magnitude of this contribution into a physiological perspective. We have used hematopoietic chimeras to determine that 0.2% to 1.4% of ECs in vessels in control tissues derived from hematopoietic progenitors during the 4 months after irradiation and hematopoietic recovery. By contrast, 8.3% to 11.2% of ECs in vessels that developed in sponge-induced granulation tissue during 1 month derived from circulating hematopoietic progenitors. This recruitment of circulating progenitors to newly forming vessels would be difficult to observe in standard histological studies, but it is large enough to be encouraging for attempts to manipulate this contribution for therapeutic gain.

The extracellular matrix can regulate vascular cell migration, proliferation, and survival: relationships to vascular disease

The extracellular matrix (ECM) of the normal artery wall is a collection of fibrous proteins and associated glycoproteins embedded in a hydrated ground substance of glycosaminoglycans and proteoglycans. These distinct molecules are organized into a highly ordered network that are closely associated with the vascular cells that produce them. In addition to providing the architectural framework for the artery wall that imparts mechanical support and viscoelasticity, the ECM can regulate the behaviour of vascular cells, including their ability to migrate, proliferate and survive injury. The composition of the ECM is different within intimal lesions of atherosclerosis, which are composed of monocytes and lymphocytes from the circulation and smooth muscle cells (SMC) that migrate from the media to the intima (Ross 1993, 1999), and these differences may contribute to the altered phenotype of vascular cells within lesions. This review will briefly outline the ECM changes observed in atherosclerosis and restenosis and the potential relationship of these changes to altered vascular cell functions.

The extracellular matrix dynamically regulates smooth muscle cell responsiveness to PDGF

Focal accumulation of smooth muscle (SMC) within the arterial intima contributes to the formation of lesions of atherosclerosis. Platelet-derived growth factor (PDGF) is a potent stimulant of SMC migration and proliferation in culture that may play a role in the accumulation of SMC in atherogenesis. SMCs normally reside in the media of the artery wall surrounded by extracellular matrix (ECM), including type I collagen. In atherogenesis, the ECM is degraded, new ECM components, such as fibronectin, are synthesized and assembled, and these alterations in ECM components are associated with changes in SMC phenotype. To model the changes in ECM in normal and diseased arteries, we have analyzed SMCs cultured on different forms of type I collagen. Our studies demonstrate that integrin-mediated signals from various forms of type I collagen lead to specific and rapid modulation of the integrin signaling complex, including cytoskeletal connections, and of the responsiveness of SMC to PDGF stimulation.

Apoptosis overrides survival signals through a caspase-mediated dominant-negative NF-kappa B loop

The transcription factor NF-kappa B is an important regulator of gene expression during immune and inflammatory responses, and can also protect against apoptosis. Here we show that endothelial cells undergo apoptosis when deprived of growth factors. Surviving viable cells exhibit increased activity of NF-kappa B, whereas apoptotic cells show caspase-mediated cleavage of the NF-kappa B p65/ReIA subunit. This cleavage leads to loss of carboxy-terminal transactivation domains and a transcriptionally inactive p65 molecule. The truncated p65 acts as a dominant-negative inhibitor of NF-kappa B, promoting apoptosis, whereas an uncleavable, caspase-resistant p65 protects the cells from apoptosis. The generation of a dominant-negative fragment of p65 during apoptosis may be an efficient pro-apoptotic feedback mechanism between caspase activation and NF-kappa B inactivation.

Cleavage of beta-catenin and plakoglobin and shedding of VE-cadherin during endothelial apoptosis: evidence for a role for caspases and metalloproteinases

Growth factor deprivation of endothelial cells induces apoptosis, which is characterized by membrane blebbing, cell rounding, and subsequent loss of cell-matrix and cell-cell contacts. In this study, we show that initiation of endothelial apoptosis correlates with cleavage and disassembly of intracellular and extracellular components of adherens junctions. beta-Catenin and plakoglobin, which form intracellular links between vascular endothelial cadherin (VE-cadherin) and actin-binding alpha-catenin in adherens junctions, are cleaved in apoptotic cells. In vitro incubations of cell lysates and immunoprecipitates with recombinant caspases indicate that CPP32 and Mch2 are involved, possibly by initiating proteolytic processing. Cleaved beta-catenin from lysates of apoptotic cells does not bind to endogenous alpha-catenin, whereas plakoglobin retains its binding capacity. The extracellular portion of the adherens junctions is also altered during apoptosis because VE-cadherin, which mediates endothelial cell-cell interactions, dramatically decreases on the surface of cells. An extracellular fragment of VE-cadherin can be detected in the conditioned medium, and this "shedding" of VE-cadherin can be blocked by an inhibitor of metalloproteinases. Thus, cleavage of beta-catenin and plakoglobin and shedding of VE-cadherin may act in concert to disrupt structural and signaling properties of adherens junctions and may actively interrupt extracellular signals required for endothelial cell survival.

Upregulation of VCAM-1 and ICAM-1 at atherosclerosis-prone sites on the endothelium in the ApoE-deficient mouse

Focal recruitment of monocytes and lymphocytes is one of the earliest detectable cellular responses in the formation of lesions of atherosclerosis. This localized accumulation of leukocytes is a multistep process in which the endothelium remains intact and may regulate leukocyte recruitment by expressing specific adhesion molecules. To examine the relationship of adhesion molecule expression to initiation factors and the sites of lesion formation, we analyzed the expression of vascular cell adhesion molecule-1 (VCAM-1), intercellular adhesion molecule-1 (ICAM-1), and platelet-endothelial cell adhesion molecule-1 (PECAM-1) en face on the aortic endothelium of control mice and homozygous apolipoprotein E-deficient (ApoE -/-) mice that develop complex lesions of atherosclerosis similar to those in humans. In control mice, VCAM-1 staining was weak and limited to sites of altered blood flow. In contrast, in the ApoE -/- mice, VCAM-1 appeared to be localized over the surface of groups of endothelial cells in lesion-prone sites. Expression of VCAM-1 preceded lesion formation, and increased expression above control levels appeared to be correlated with the extent of exposure to plasma cholesterol. Although ICAM-1 was the most prominent adhesion molecule in lesion-prone sites, its expression appeared to be independent of plasma cholesterol levels and was upregulated in both ApoE -/- and control mice. At lesion-prone sites associated with altered blood flow, ICAM-1 was located over the surface of each endothelial cell and on microvilli, whereas VCAM-1 was confined to the cell periphery in non-lesion-prone sites. PECAM-1 was localized at the cell periphery throughout the aorta, and its expression did not appear to be regulated. Thus, the levels, localization, and characteristics of expression of VCAM-1, ICAM-1, and PECAM-1 appear to be differentially regulated. Upregulation of VCAM-1 and ICAM-1 is associated with sites of lesion formation.

Cleavage of p21Cip1/Waf1 and p27Kip1 mediates apoptosis in endothelial cells through activation of Cdk2: role of a caspase cascade

Apoptosis of human endothelial cells after growth factor deprivation is associated with rapid and dramatic up-regulation of cyclin A-associated cyclin-dependent kinase 2(cdk2) activity. In apoptotic cells, the C termini of the cdk inhibitors p21Cip1/Waf1 and p27Kip1 are truncated by specific cleavage. The enzyme involved in this cleavage is CPP32 and/or a CPP32-like caspase. After cleavage, p21Cip1/Waf1 loses its nuclear localization sequence and exits the nucleus. Cleavage of p21Cip1/Waf1 and p27Kip1 results in a substantial reduction in their association with nuclear cyclin-cdk2 complexes, leading to a dramatic induction of cdk2 activity. Dominant-negative cdk2, as well as a mutant of p21Cip1/Waf1 resistant to caspase cleavage, partially suppress apoptosis. These data suggest that cdk2 activation, through caspase-mediated cleavage of cdk inhibitors, may be instrumental in the execution of apoptosis following caspase activation.

Caspase-mediated cleavage of focal adhesion kinase pp125FAK and disassembly of focal adhesions in human endothelial cell apoptosis

Normal endothelial and epithelial cells undergo apoptosis when cell adhesion and spreading are prevented, implying a requirement for antiapoptotic signals from the extracellular matrix for cell survival. We investigated some of the molecular changes occurring in focal adhesions during growth factor deprivation-induced apoptosis in confluent monolayers of human umbilical vein endothelial cells. Among the first morphologic changes after initiation of the apoptotic process are membrane blebbing, loss of focal adhesion sites, and retraction from the matrix followed by detachment. We observe a specific proteolytic cleavage of focal adhesion kinase (pp125FAK), an important component of the focal adhesion complex, and identify pp125FAK as a novel substrate for caspase-3 and caspase-3-like apoptotic caspases. The initial cleavage precedes detachment, and coincides with loss of pp125FAK and paxillin from focal adhesion sites and their redistribution into the characteristic membrane blebs of apoptotically dying cells. Cleavage of pp125FAK differentially affects its association with signaling and cytoskeletal components of the focal adhesion complex; binding of paxillin, but not pp130(Cas) (Cas, Crk-associated substrate) and vinculin, to the COOH terminally truncated pp125FAK is abolished. Therefore, caspase-mediated cleavage of pp125FAK may be participating in the disassembly of the focal adhesion complex and actively interrupting survival signals from the extracellular matrix, thus propagating the cell death program.

Proteoglycan distribution in lesions of atherosclerosis depends on lesion severity, structural characteristics, and the proximity of platelet-derived growth factor and transforming growth factor-beta

The accumulation of proteoglycans (PGs) in atherosclerosis contributes to disease progression and stenosis and may partly depend on local regulation by growth factors such as platelet-derived growth factor (PDGF) and transforming growth factor (TGF)-beta. In this study, the distribution of the major extracellular PGs is compared with that of PDGF and TGF-beta isoforms in developing lesions of atherosclerosis from hypercholesterolemic nonhuman primates. Strong immunostaining for decorin, biglycan, versican, and hyaluronan is observed in both intermediate and advanced lesions. Perlecan staining is weak in intermediate lesions but strong in advanced lesions in areas bordering the plaque core. Immunostaining for PDGF-B and TGF-beta1 is particularly prominent in macrophages in intermediate and advanced lesions. In contrast, TGF-beta2 and TGF-beta3 and PDGF-A are present in both macrophages and smooth muscle cells. Overall, PG deposits parallel areas of intense growth factor immunostaining, with trends in relative localization that suggest interrelationships among certain PGs and growth factors. Notably, decorin and TGF-beta1 are distributed similarly, predominantly in the macrophage-rich core, whereas biglycan is prominent in the smooth muscle cell matrix adjoining TGF-beta1-positive macrophages. Versican and hyaluronan are enriched in the extracellular matrix adjacent to both PDGF- and TGF-beta1-positive cells. These data demonstrate that PG accumulation varies with lesion severity, structural characteristics, and the proximity of PDGF and TGF-beta.

The mitogen-activated protein kinase pathway can mediate growth inhibition and proliferation in smooth muscle cells. Dependence on the availability of downstream targets

Activation of the classical mitogen-activated protein kinase (MAPK) pathway leads to proliferation of many cell types. Accordingly, an inhibitor of MAPK kinase, PD 098059, inhibits PDGF-induced proliferation of human arterial smooth muscle cells (SMCs) that do not secrete growth-inhibitory PGs such as PGE2. In striking contrast, in SMCs that express the inducible form of cyclooxygenase (COX-2), activation of MAPK serves as a negative regulator of proliferation. In these cells, PDGF-induced MAPK activation leads to cytosolic phospholipase A2 activation, PGE2 release, and subsequent activation of the cAMP-dependent protein kinase (PKA), which acts as a strong inhibitor of SMC proliferation. Inhibition of either MAPK kinase signaling or of COX-2 in these cells releases them from the influence of the growth-inhibitory PGs and results in the subsequent cell cycle traverse and proliferation. Thus, the MAPK pathway mediates either proliferation or growth inhibition in human arterial SMCs depending on the availability of specific downstream enzyme targets.

Platelet-derived growth factor: a key regulator of connective tissue cells in embryogenesis and pathogenesis

Studies of PDGF-A, PDGF-B and PDGF receptor-beta knock-out mice have revealed critical functions for the PDGF-PDGF receptor signaling systems in the ontogeny of connective tissue cells: the mesangial cells of kidney glomeruli and the alveolar smooth muscle cells (SMC) of the lung. The phenotypes of the PDGF mutant mice have also shed light on the identity and functions of these cell types, as well as revealed analogies suggesting that common morphogenetic principles have evolved for use in different organs, involving related growth factors and cell types. Although the lethality of PDGF knock-out mice has not allowed an investigation of the role of PDGF in SMC of the vessel wall, regulation of PDGF and its receptors in adult vessels following injury is consistent with a role for PDGF in the fibroproliferative response in the intima that occurs as part of the pathogenesis of atherosclerosis. PDGF modulation of connective tissue synthesis may thus be critical to connective tissue phenotype and proliferation in both embryogenesis and pathogenesis.

Expression of a disintegrin-like protein in cultured human vascular cells and in vivo

Metalloproteinase-like, disintegrin-like, and cysteine-rich proteins (MDCs) are potential novel regulators of cell-cell and cell-matrix interactions, as well as of matrix degradation. We have asked whether MDCs are expressed in cultured diploid vascular cells, and have identified MDC 15 in human aortic smooth muscle (SMC) and umbilical vein endothelium (HUVEC). MDC 15 mRNA is expressed at higher levels in HUVECs than in SMCs. In cultured SMCs, MDC 15 mRNA levels are not regulated by PDGF or IGF-I or by adherence to different extracellular matrices. Nor is regulation of MDC 15 mRNA levels observed in HUVEC monolayers at different cell densities, after multi-scratch wounding, or after treatment with TNF-alpha, LPS, or thrombin. However, differences in proteolytic processing of MDC 15 are observed in different HUVEC strains. In contrast to cultured arterial cells, MDC 15 protein is not expressed in vivo in normal vessels, but is up-regulated in lesions of atherosclerosis. These findings suggest that MDC 15 may be a potential regulator of vascular cell function and may be involved in the development of lesions of atherosclerosis.

Fibrillar collagen inhibits arterial smooth muscle proliferation through regulation of Cdk2 inhibitors

Arterial smooth muscle cells (SMCs) are arrested in the G1 phase of the cell cycle on polymerized type I collagen fibrils, while monomer collagen supports SMC proliferation. Cyclin E-associated kinase and cyclin-dependent kinase 2 (cdk2) phosphorylation are inhibited on polymerized collagen, and levels of the cdk2 inhibitors p27Kip1 and p21Cip1/Waf1 are increased compared with SMCs on monomer collagen. p27Kip1 associates with the cyclin E-cdk2-p21Cip1/Waf1 complex in SMCs on polymerized collagen. Monovalent blocking antibodies to alpha2 integrins, integrins that mediate adhesion to both forms of collagen, mimic these effects on monomer collagen. Furthermore, polymerized collagen rapidly suppresses p70 S6 kinase, a possible regulator of p27Kip1. Thus, fibrillar collagen specifically regulates early integrin signaling that may lead to up-regulation of cdk2 inhibitors and inhibition of SMC proliferation.

Multiple growth factors are associated with lesions of atherosclerosis: specificity or redundancy?

Within the last five years, a number of specific growth factors have been localized in developing lesions of atherosclerosis. This localization of growth factors that is not observed in normal vessels, together with the pleotrophic activities of growth factors, have suggested a role for growth factors in atherosclerotic lesion progression. However, based on in vitro studies, many of the growth factors identified in lesions have overlapping target cells and are derived from the same cellular sources. What is the relative role of the specific growth factors identified? How is the their activity altered by the local conditions in the vessel wall? How do different risk factors for atherosclerosis alter the balance between growth factors and their natural regulators? Evidence for the involvement of specific growth factors in the progression of lesions of atherosclerosis is discussed, as well as the multiple levels at which the activities of these growth factors may be regulated by the vessel wall.

Platelet-derived growth factor stimulates protein kinase A through a mitogen-activated protein kinase-dependent pathway in human arterial smooth muscle cells

The abilities of platelet-derived growth factor (PDGF) and insulin-like growth factor (IGF-I) to regulate cAMP metabolism and mitogen-activated protein kinase (MAP kinase) activity were compared in human arterial smooth muscle cells (hSMC). PDGF-BB stimulated cAMP accumulation up to 150-fold in a concentration-dependent manner (EC50 approximately 0.7 nM). The peak of cAMP formation and cAMP-dependent protein kinase (PKA) activity occurred approximately 5 min after the addition of PDGF and rapidly declined thereafter. Incubating cells with PDGF and 3-isobutyl-1-methylxanthine (IBMX, a phosphodiesterase inhibitor) enhanced the accumulation of cAMP and PKA activity by an additional 2.5-3-fold, whereas IBMX alone was essentially without effect. The PDGF-stimulated increase in cAMP was prevented by addition of the cyclooxygenase inhibitor indomethacin, consistent with release of prostaglandins stimulating cAMP. PDGF, but not IGF-I, stimulated MAPK activity, cytosolic phospholipase A2 (cPLA2) phosphorylation, and cAMP synthesis which indicated a key role for MAP kinase in the activation of cPLA2. Further, PDGF stimulated the rapid release of arachidonic acid and synthesis of prostaglandin E2 (PGE2) which could be inhibited by a cPLA2 inhibitor (AACOCF3). Calcium mobilization was required for PDGF-induced arachidonic acid release and PGE2 synthesis but not for MAPK activation, whereas PKC was required for PGE2-mediated activation of PKA. In summary, these results demonstrated that PDGF increases cAMP formation and PKA activity through a MAP kinase-mediated activation of cPLA2, arachidonic acid release, and PGE2 synthesis in human arterial smooth muscle cells.

Magnetic resonance imaging to study lesions of atherosclerosis in the hyperlipidemic rabbit aorta

We have developed a high resolution magnetic resonance (MR) imaging technique to serially assess lesions of atherosclerosis in a rabbit model. A volume phased array coil was designed and used to image the abdominal aortas of six atherosclerotic rabbits and two age-, sex-, and weight-matched controls. Lesions of atherosclerosis were induced by a combination of repeat balloon injury and a hyperlipidemic diet. All animals were imaged on at least two occasions 9-16 months after initiation of atherosclerosis. In addition, animals were imaged immediately after sacrifice. Anatomic dissection and histology were performed to verify the MR findings. The volume phased array coil improves the image signal-to-noise ratio over existing extremity coils and resulted in higher resolution images of the abdominal aorta. Proton density-weighted images acquired with 2D/3D fast spin-echo are the most useful sequence to outline the vessel wall and to differentiate wall from lumen and background. Progressive wall thickening and lumen stenosis were observed in the serial images of the diseased rabbits. Wall thickness and lumen area derived noninvasively from the in vivo MR images correlate with postmortem MR images and sections of aorta examined by dissection microscopy and histology. Spin-echo and fast spin-echo imaging with a phased array body coil can be used to accurately assess plaque dimensions, and potentially can be used to image intraplaque features and to monitor lesion progression or regression. It should also be possible to adapt these techniques to assess human disease, especially for peripheral vascular problems.

Inhibition of hypercholesterolemia-induced atherosclerosis in the nonhuman primate by probucol. II. Cellular composition and proliferation

In nonhuman primates (Macaca nemestrina) treated with the antioxidant probucol during diet-induced hypercholesterolemia, intimal lesion area in the thoracic aorta was decreased, with increased resistance of plasma LDL to oxidation. The cellular and molecular changes associated with the decrease in lesion size in the probucol-treated hypercholesterolemic animals are quantitatively evaluated in this study. Lesions from the probucol-treated animals appear less mature and have altered lipid distribution. Abundant lipid-laden smooth muscle cells are found in the intima and media of the probucol-treated animals, with fewer medial lipid-laden macrophages, compared with lesions at similar sites in the control hypercholesterolemic animals. In both the control and probucol-treated animals, macrophages are the predominant cells in most lesions, but the ratio of macrophages to smooth muscle cells is decreased in the lower thoracic and upper abdominal aortic sites in the probucol-treated animals. Lesions at all aortic sites in the probucol-treated animals have a 35% to 80% reduction in the percentage of cells in cell cycle traverse, as indicated by immunostaining for proliferating cell nuclear antigen (% PCNA-positive). In both groups, macrophages and smooth muscle cells are PCNA-positive, but the majority (> 60%) are macrophages. No difference in % PCNA-positive cells is seen in the iliac arteries, where the most advanced lesions were present at the time probucol administration was initiated. Limited Northern analysis of growth-regulatory molecules possibly involved in the cellular changes associated with lesions shows a 30% to 50% decrease in mRNA levels of platelet-derived growth factor (PDGF) B-chain, PDGF beta-receptor, colony-stimulating factor type 1, and monocyte chemotactic protein 1. Thus, a potential role for an antioxidant such as probucol in the treatment of atherosclerosis may be to alter the early inflammatory fibroproliferative processes of the disease. Whether these effects are directly related to the antioxidant properties or some other activity of probucol is not yet known.

Platelet-derived growth factor. Distinct signal transduction pathways associated with migration versus proliferation

Figure 2 summarizes our current interpretation of data concerning signals from the activated PDGF receptor involved in directed migration and proliferation of human arterial SMC. Binding of PDGF (PDGF-BB or PDGF-AA) causes PDGF-receptor dimerization, tyrosine autophosphorylation, and subsequent binding of several molecules containing SH2 domains to the activated receptor. Binding and activation of PLC gamma by the PDGF receptor leads to PIP2 hydrolysis, resulting in generation of diacylglycerol (DAG) and IP3. Subsequently, intracellular levels of calcium are elevated as a result of IP3-mediated calcium release from intracellular compartments. The decreased levels of PIP2 and increased levels of calcium both favor actin-filament disassembly by inducing capping of actin-filament barbed ends and actin-monomer sequestration. A localized, and transient, actin-filament disassembly enables the cell to extend filopodia towards PDGF, thereby enabling chemotaxis to take place. At a later time and/or in a different compartment, actin-filament assembly is promoted by PDGF by a mechanism that is not completely understood, but that may involve small GTP-binding proteins, such as Rho, and formation of DAG. Migration on collagen requires functional alpha 2 beta 1 integrins, which may either constitute a permissive state required for a cell to migrate, or which may be actively involved in intracellular signals leading to migration. PDGF-induced DNA synthesis and proliferation involves activation of Ras, MAP kinase kinase, and MAP kinase. Cross-talk between PKA signaling and tyrosine-kinase receptor signaling results in PKA inhibition of the MAP kinase cascade, probably at the level of Raf. Activation of PI 3-kinase, or a PI 3-kinase-like enzyme, is also likely to contribute to the mitogenic effects of PDGF in these cells (Bornfeldt, unpublished observation). What determines if a SMC will migrate and/or proliferate in response to PDGF? Results are starting to emerge that show regulation of expression of molecules involved in intracellular signaling with different phenotypic states of SMC. For example, expression of PLC gamma is very low in intact vascular wall (where SMC show a "contractile phenotype"), and induced when SMC are converted to a "synthetic phenotype" in culture. Proliferation and expression of MAP kinase, but not calcium signaling, appear to be regulated by the extracellular matrix, and the profile of integrin expression is different in SMC in culture compared to SMC in the vascular wall. Thus, the relation between expression of signaling molecules involved in migration and signaling molecules involved in proliferation, as well as cross-talk between different signal-transduction pathways, may determine the net effect of PDGF.

Sphingosine-1-phosphate inhibits PDGF-induced chemotaxis of human arterial smooth muscle cells: spatial and temporal modulation of PDGF chemotactic signal transduction

Activation of the PDGF receptor on human arterial smooth muscle cells (SMC) induces migration and proliferation via separable signal transduction pathways. Sphingosine-1-phosphate (Sph-1-P) can be formed following PDGF receptor activation and therefore may be implicated in PDGF-receptor signal transduction. Here we show that Sph-1-P does not significantly affect PDGF-induced DNA synthesis, proliferation, or activation of mitogenic signal transduction pathways, such as the mitogen-activated protein (MAP) kinase cascade and PI 3-kinase, in human arterial SMC. On the other hand, Sph-1-P strongly mimics PDGF receptor-induced chemotactic signal transduction favoring actin filament disassembly. Although Sph-1-P mimics PDGF, exogenously added Sph-1-P induces more prolonged and quantitatively greater PIP2 hydrolysis compared to PDGF-BB, a markedly stronger calcium mobilization and a subsequent increase in cyclic AMP levels and activation of cAMP-dependent protein kinase. This excessive and prolonged signaling favors actin filament disassembly by Sph-1-P, and results in inhibition of actin nucleation, actin filament assembly and formation of focal adhesion sites. Sph-1-P-induced interference with the dynamics of PDGF-stimulated actin filament disassembly and assembly results in a marked inhibition of cell spreading, of extension of the leading lamellae toward PDGF, and of chemotaxis toward PDGF. The results suggest that spatial and temporal changes in phosphatidylinositol turnover, calcium mobilization and actin filament disassembly may be critical to PDGF-induced chemotaxis and suggest a possible role for endogenous Sph-1-P in the regulation of PDGF receptor chemotactic signal transduction.

TNF-alpha stimulation of fibroblast proliferation. Dependence on platelet-derived growth factor (PDGF) secretion and alteration of PDGF receptor expression

TNF-alpha stimulates DNA synthesis and proliferation of cultured human fibroblasts. Maximal DNA synthesis in response to TNF-alpha occurs approximately 28 h after addition of TNF-alpha to quiescent fibroblasts, a delay of about 12 to 14 h as compared with DNA synthesis elicited by platelet-derived growth factor (PDGF). TNF-alpha induces PDGF A chain gene expression with a maximum at 4 h. DNA synthesis is abrogated in response to TNF-alpha by a goat anti-PDGF IgG but not by nonimmune goat IgG, suggesting induction of an autocrine PDGF-AA loop by TNF-alpha. The response to PDGF-AA requires the presence of PDGF receptor alpha-receptors. TNF-alpha does not significantly affect PDGF alpha-receptor mRNA or protein expression, nor does it alter the proliferative response to externally added PDGF-AA. In contrast, TNF-alpha reduces the levels of PDGF beta-receptor mRNA, protein expression, and cell proliferation in response to PDGF-BB. Thus, DNA synthesis in response to TNF-alpha depends upon autocrinely induced PDGF-AA. At the same time, TNF-alpha may alter the response to PDGF-BB from exogenous sources.

TNF-alpha stimulation of fibroblast proliferation. Dependence on platelet-derived growth factor (PDGF) secretion and alteration of PDGF receptor expression

TNF-alpha stimulates DNA synthesis and proliferation of cultured human fibroblasts. Maximal DNA synthesis in response to TNF-alpha occurs approximately 28 h after addition of TNF-alpha to quiescent fibroblasts, a delay of about 12 to 14 h as compared with DNA synthesis elicited by platelet-derived growth factor (PDGF). TNF-alpha induces PDGF A chain gene expression with a maximum at 4 h. DNA synthesis is abrogated in response to TNF-alpha by a goat anti-PDGF IgG but not by nonimmune goat IgG, suggesting induction of an autocrine PDGF-AA loop by TNF-alpha. The response to PDGF-AA requires the presence of PDGF receptor alpha-receptors. TNF-alpha does not significantly affect PDGF alpha-receptor mRNA or protein expression, nor does it alter the proliferative response to externally added PDGF-AA. In contrast, TNF-alpha reduces the levels of PDGF beta-receptor mRNA, protein expression, and cell proliferation in response to PDGF-BB. Thus, DNA synthesis in response to TNF-alpha depends upon autocrinely induced PDGF-AA. At the same time, TNF-alpha may alter the response to PDGF-BB from exogenous sources.

Biology of atherosclerotic plaque formation: possible role of growth factors in lesion development and the potential impact of soy

The advanced lesions of atherosclerosis occlude the affected artery by increasing the thickness of the intima. The focal thickening of the intima is due to a large increase in smooth muscle cells, formation of new connective tissue matrix by these smooth muscle cells and, in hyperlipidemic individuals, the accumulation of intracellular and extracellular lipid. Additionally, monocytes and T lymphocytes infiltrate the artery wall. Various forms of "injury" may lead to cellular infiltration and proliferation. Localized cellular infiltration of monocytes and T cells may be due to changes in adhesive properties of the endothelial surface, involving the expression of specific adhesion molecules. The directed cell migration and proliferation may represent the cells' response to polypeptide growth factors, acting singly or in concert. These peptide growth factors also modulate matrix synthesis and degradation, angiogenesis, cell-cell adhesion and cellular metabolism, including lipid uptake. In atherosclerosis, growth factors may be delivered by infiltrating cells or by activation of cells within the artery wall. Normally, growth factors and their cell-surface receptors are expressed at low or undetectable levels. Their up-regulation in early and developing atherosclerotic lesions suggests a pathogenic role for these molecules. Increased levels of isoflavonoids, in particular genistein, which are associated with consumption of soy-based diets, inhibit cell adhesion, alter growth factor activity and inhibit cell proliferation involved in lesion formation.

The adhesive and migratory effects of osteopontin are mediated via distinct cell surface integrins. Role of alpha v beta 3 in smooth muscle cell migration to osteopontin in vitro

Osteopontin is an arginine-glycine-aspartate containing acidic glycoprotein postulated to mediate adhesion, migration, and biomineralization in diverse tissues. The mechanisms explaining this multifunctionality are not well understood, although it is known that one osteopontin receptor is the alpha v beta 3 integrin. In this work, we studied human smooth muscle cells varying in alpha v beta 3 levels to identify additional osteopontin receptors. We report that, in addition to alpha v beta 3, both alpha v beta 5 and alpha v beta 1 are osteopontin receptors. Moreover, the presence or absence of alpha v beta 3 on the cell surface altered the adhesive and migratory responses of smooth muscle cells to osteopontin. Adhesion of alpha v beta 3-deficient cell populations to osteopontin was only half that of cells containing alpha v beta 3, and migration toward an osteopontin gradient in the Boyden chamber was dependent on cell surface alpha v beta 3. Although alpha v beta 3-deficient smooth muscle cells were unable to migrate to osteopontin, they did migrate significantly in response to vitronectin and fibronectin. These findings represent the first description of alpha v beta 5 and alpha v beta 1 as osteopontin receptors and suggest that, while adhesion to osteopontin is supported by integrins containing beta 1, beta 3, and beta 5, migration in response to osteopontin appears to depend on alpha v beta 3. Thus, interaction with distinct receptors is one mechanism by which osteopontin may initiate multiple functions.

Serial magnetic resonance imaging of experimental atherosclerosis detects lesion fine structure, progression and complications in vivo

A major problem in the study of lesions of atherosclerosis is the difficulty of imaging noninvasively the lesions and following their progression in vivo. To address this problem, we have developed advanced magnetic resonance techniques to noninvasively and serially image advanced lesions of atherosclerosis in the rabbit abdominal aorta. Both lumen and wall were imaged with high resolution. Progression of disease, resulting in increase in lesion mass, decrease in arterial lumen, or stenosis, and intralesion complications, can be detected. Images acquired in vivo correlate with the fine structure of the lesions of atherosclerosis, including the fibrous cap, necrotic core, and lesion fissures, as verified by gross examination, dissection microscopy, and histology. The ability to noninvasively identify the features of atherosclerotic plaques, has significant implications for determining risks and benefits associated with different therapeutic approaches.

Dynamic expression of alpha 1 beta 1 and alpha 2 beta 1 integrin receptors by human vascular smooth muscle cells. Alpha 2 beta 1 integrin is required for chemotaxis across type I collagen-coated membranes

Vascular smooth muscle cells (SMCs) in the media of normal arteries express alpha 1 beta 1 integrin with no detectable alpha 2 beta 1 as determined by immunocytochemistry. In contrast, immunoprecipitation of integrins expressed by human SMCs cultured from medial explants shows strong expression of alpha 2 beta 1 and no expression of alpha 1 beta 1. The apparent reciprocal expression of these two collagen and laminin receptors was confirmed by flow cytometric analysis of fluorescent labeled cells. Freshly isolated SMCs had detectable alpha 1, alpha 3, alpha 5, and alpha v subunits with low levels of detectable beta 3 and no detectable alpha 2. Cultured SMCs expressed alpha 2, alpha 3, alpha 5 and alpha v subunits with little or no alpha 1 or beta 3. Neither alpha 4 nor alpha 6 were detectable in freshly isolated or cultured cells. Expression of alpha 2 beta 1 receptors by cultured SMCs appears to be required for the migration of these cells on type I collagen. Migration of cultured SMCs across a type I collagen-coated membrane toward two different chemotactic stimuli, platelet-derived growth factor-BB (1 nmol/L) and insulin-like growth factor-(1 nmol/L), was Mg2+ dependent and inhibited by preincubation of cells with an affinity-purified polyclonal anti-alpha 2 beta 1 antibody or by monoclonal antibodies directed against the individual alpha 2 or beta 1 subunits. Attachment to type 1 collagen membranes was not affected by antibodies under conditions where migration was significantly impeded. The combined data show that SMC expression of alpha 1 beta 1 and alpha 2 beta 1 integrin receptors for collagen and laminin is dynamic and reciprocal and may be important with respect to SMC migration on type I collagen. These findings are potentially important in understanding the pathophysiology of vascular diseases, for example, atherosclerosis and restenosis following balloon angioplasty, where SMC migration is a contributing factor.

Inhibition of hypercholesterolemia-induced atherosclerosis in the nonhuman primate by probucol. I. Is the extent of atherosclerosis related to resistance of LDL to oxidation?

Lipoprotein oxidation is believed to play an important role in atherogenesis. To investigate whether inhibition of oxidation of low density lipoprotein (LDL) would alter atherogenesis in the nonhuman primate, we administered probucol, a potent antioxidant, to Macaca nemestrina fed a high-fat, high-cholesterol diet. Probucol was administered to half of the 16 monkeys 14 wk after starting the hypercholesterolemic diet, and was given daily until they were sacrificed after 11 mos. To evaluate the antioxidant effect of probucol, the resistance of isolated plasma LDL to in vitro oxidation was evaluated. Probucol significantly increased the resistance of LDL to oxidative modification, as shown by an increase in the lag time required for conjugated diene formation. Lesions in the probucol-treated animals appeared less mature, and increased accumulation of lipid was observed in smooth muscle cells. Comparison of all control and probucol-treated monkeys demonstrated that intimal lesion areas in the thoracic aortas of the probucol-treated monkeys were reduced by 43% (P < 0.0001), but no significant difference in lesion area was found in the abdominal aortas or in the iliac arteries. However, the lag phase of conjugated diene formation was not prolonged in 2 of the 8 probucol-treated animals. A plot of intimal lesion size versus lag phase of all 16 animals showed a trend that lesion size was inversely related to oxidation resistance for all anatomic sites. The strong inverse relationship between intimal lesion size and resistance of LDL to oxidation supports a role for lipoprotein oxidation in the development and progression of lesions of atherosclerosis. The possibility that some of the effect is due to other biological properties of probucol cannot be ruled out.

Olfactory ensheathing glia and platelet-derived growth factor B-chain reactivity in the transplanted rat olfactory bulb

Using a monoclonal antibody against the B-chain of platelet-derived growth factor as a marker, we have examined the behavior of olfactory ensheathing glia in the normal and transplanted rat olfactory bulb. In the normal postnatal olfactory bulb, these glia are found to ensheath the bundles of incoming primary olfactory nerve fibers as well as those in the olfactory nerve layer. Olfactory marker protein antibody was used to identify the olfactory nerve proper. Within the transplant, the same glia: (1) ensheath bundles of both primary olfactory and non-primary olfactory axons, (2) ensheath axonal bundles deep within the donor tissue, and (3) eventually permit radiation of individual axons from bundles to surrounding neuropil. We believe that ensheathing glia (being rich in growth-related factors and extracellular matrix molecules) may be useful in providing trophic support and guidance for the reconstruction of developmentally or traumatically damaged neuronal pathways not directly related to the olfactory system. The evidence presented here indicates that ensheathing glia are capable of existing in deep brain areas and ensheathing other than primary olfactory axons. The special molecular characteristics of these glia along with the morphological findings presented here provide a foundation for further studies of these unique glia and their potential utility in the restoration of damaged neural pathways.

Platelet-derived growth factor (PDGF) receptor alpha-subunit mutant and reconstituted cell lines demonstrate that transforming growth factor-beta can be mitogenic through PDGF A-chain-dependent and -independent pathways

Mitogenic stimulation of connective tissue cells by transforming growth factor-beta (TGF-beta) has two unusual properties; entry into S-phase is delayed compared with that induced by other mitogens, and the dose response is biphasic, with low concentrations stimulating and high concentrations inhibiting or having no effect. A hypothesis that provides an explanation for both of these properties is that TGF-beta stimulates proliferation indirectly by inducing synthesis of platelet-derived growth factor (PDGF) A-chain, which in turn stimulates proliferation via autocrine activation of the PDGF receptor alpha-subunit (PDGFR alpha). High concentrations of TGF-beta reduce PDGFR alpha expression and break the autocrine loop. We tested this hypothesis by determining whether TGF-beta and interleukin-1 alpha can induce DNA synthesis in connective tissue (3T3) cells derived from the Patch mouse line in which the PDGFR alpha gene is deleted. We found that these cells do respond mitogenically to TGF-beta and interleukin-1 alpha, indicating that PDGF A-chain induction is not the sole mechanism of mitogenic stimulation. Reestablishing PDGFR alpha expression via transfection with a human PDGFR alpha construct enhanced the response to TGF-beta. Neutralizing anti-PDGF antiserum reduced TGF-beta stimulation of PDGFR alpha-expressing 3T3 cells by about 35%. We conclude that induction of PDGF A-chain/PDGFR alpha autocrine stimulation does contribute to the ability of TGF-beta to stimulate connective tissue cells, but that there is, in addition, a PDGF-independent pathway.

Insulin-like growth factor-I and platelet-derived growth factor-BB induce directed migration of human arterial smooth muscle cells via signaling pathways that are distinct from those of proliferation

Directed migration or chemotaxis of arterial smooth muscle cells (SMC) contributes to intimal SMC accumulation, a key event in the development of atherosclerotic lesions and in restenosis after angioplasty. The present study compares and contrasts insulin-like growth factor I (IGF-I) and platelet-derived growth factor (PDGF-BB) as chemoattractants and mitogens for human arterial SMC. Compared with PDGF-BB, IGF-I is a weaker SMC mitogen. Thus, PDGF-BB, but not IGF-I, evokes a strong and rapid activation of mitogen-activated protein (MAP) kinase kinase and MAP kinase. However, IGF-I is a potent stimulator of directed migration of human arterial SMC, as measured in a Boyden chamber assay. The half-maximal concentration for migration is similar to the Kd for IGF-I receptor interaction. An IGF-I receptor-blocking antibody blocks the effects of IGF-I, IGF-II, and insulin, indicating that the effects are indeed mediated through the IGF-I receptor. The maximal effect of IGF-I on directed migration ranges between 50% and 100% of the effect of PDGF-BB, the strongest known chemoattractant for SMC. The ability of IGF-I and PDGF-BB to induce chemotaxis coincides with their ability to stimulate phosphatidylinositol turnover, diacylglycerol formation, and intracellular Ca2+ flux and suggests that these signaling pathways, but not activation of the MAP kinase cascade, are required for chemotaxis of human arterial SMC.

Lysophosphatidylcholine upregulates the level of heparin-binding epidermal growth factor-like growth factor mRNA in human monocytes

Lysophosphatidylcholine is increased in the plasma of hypercholesterolemic patients, is a component of oxidatively modified low-density lipoprotein, and, as such, may play an important role in atherosclerosis. Here we demonstrate that in human monocytes, lysophosphatidylcholine increases the level of mRNA encoding the heparin-binding epidermal growth factor-like growth factor (HB-EGF), a potent smooth muscle mitogen. Lysophosphatidylcholine treatment also enhances the release of heparin-binding mitogenic activity by these cells in culture. The anti-inflammatory glucocorticoid dexamethasone inhibits the upregulation of HB-EGF mRNA induced by either lysophosphatidylcholine or bacterial lipopolysaccharide in cultured monocytes. However, the responses induced by lysophosphatidylcholine and by lipopolysaccharide differ in their kinetics. In addition, the response to lysophosphatidylcholine is resistant to the action of cycloheximide, whereas the response to lipopolysaccharide is not, suggesting that the activation mechanisms induced by these two stimuli are different. Since a nuclear run-on assay showed no effect of lysophosphatidylcholine on the transcription of the HB-EGF gene, we speculate that lysophosphatidylcholine may increase the level of HB-EGF mRNA by altering the processing or degradation of primary or mature transcripts. Lysophosphatidylcholine enhancement of monocyte production of HB-EGF may represent an important result of the interactions among oxidized low-density lipoprotein and monocyte-derived macrophages and may play a role in initiation of smooth muscle proliferation in atherogenesis.

ApoE-deficient mice develop lesions of all phases of atherosclerosis throughout the arterial tree

Initial description of apolipoprotein (apo) E-deficient transgenic mice demonstrated the development of severe hypercholesterolemia due to probable delayed clearance of large atherogenic particles from the circulation. Examination of these mice demonstrated foam cell accumulation in the aortic root and pulmonary arteries by 10 weeks of age. In the present study, the animals were fed either chow or a high-fat, Western-type diet and examined at ages ranging from 6 to 40 weeks. Gross examination by dissection microscopy revealed a predilection for development of lesions in the aortic root, at the lesser curvature of the aortic arch, the principal branches of the aorta, and in the pulmonary and carotid arteries. Monocyte attachment to endothelial cells was observed by light and electron microscopic examination at 6 weeks, the earliest time point examined. Foam cell lesions developed as early as 8 weeks, and after 15 weeks advanced lesions (fibrous plaques) were observed. The latter consisted of a fibrous cap containing smooth muscle cells surrounded by connective tissue matrix that covered a necrotic core with numerous foamy macrophages. Mice fed the Western-type diet generally had more advanced lesions than those fed a chow diet. The apoE-deficient mouse contains the entire spectrum of lesions observed during atherogenesis and is the first mouse model to develop lesions similar to those in humans. This model should provide numerous opportunities to study the pathogenesis and therapy of atherosclerosis in a small, genetically defined animal.

Mitogenic signaling mechanisms of human cementum-derived growth factors

Cementum-derived growth factor (CGF) is a M(r) 23,000 protein, which is sequestered in the mineralized matrix of tooth cementum. We have investigated the mitogenic signaling reactions induced by CGF using quiescent human gingival fibroblasts as target cells. Cells activated with CGF were compared with those treated with CGF plus epidermal growth factor (EGF) and other growth factors. CGF caused a transient increase in cytoplasmic Ca2+ concentration, and this was accompanied by enhancement of membrane protein kinase C activity, myelin basic protein and S6 kinase activities, inositol phosphate levels, and activation of c-fos and jun-B gene expression. Membranes obtained from cells activated with CGF contained several protein bands, which cross-reacted with antiphosphotyrosine antibody; however, proteins corresponding to a putative phosphorylated CGF receptor were not detected. DNA synthesis induced by CGF was inhibited by 65% in cells treated with pertussis toxin but only 25-29% in cultures exposed to H7 or 12-O-tetradecanoylphorbol-13-acetate; these values were different from those obtained when EGF, PDGF, or fetal bovine serum were used as mitogens. CGF and TGF-beta, but not EGF, caused an increase of PDGF-A chain mRNA expression 4 h after mitogen addition. However, while CGF was mitogenic for gingival fibroblasts, TGF-beta was not. Kinetics of DNA stimulation and experiments with anti-PDGF antibodies indicated that PDGF-A expression does not contribute significantly to CGF-induced DNA synthesis. When the stimulation of various signaling pathways induced by CGF and other growth factors was compared, the pattern of stimulation by CGF was different from other growth factors. The characteristic signaling reactions of CGF are likely to be important components of the mechanisms that regulate the formation and regeneration of cementum and adjacent connective tissues.

Protein kinase A antagonizes platelet-derived growth factor-induced signaling by mitogen-activated protein kinase in human arterial smooth muscle cells

Stimulation of aortic smooth muscle cells with platelet-derived growth factor BB homodimer (PDGF-BB) leads to the rapid activation of mitogen-activated protein kinase (MAPK) and MAPK kinase (MAPKK). Compounds that increase cAMP and activate protein kinase A (PKA)--prostaglandin E2, isoproterenol, cholera toxin, and forskolin--were found to inhibit the PDGF-BB-induced activation of MAPKK and MAPK. Forskolin, but not the inactive analogue 1,9-dideoxyforskolin, inhibited PDGF-BB-stimulated MAPKK and MAPK activation in a dose-dependent manner. PKA antagonism of MAPK signaling was observed at all doses of PDGF-BB or PDGF-AA. PKA did not inhibit MAPKK and MAPK activity in vitro, and MAPKK and MAPK from extracts of forskolin-treated cells could be activated normally with purified Raf-1 and MAPKK, respectively, suggesting that PKA blocked signaling upstream of MAPKK. Neither PDGF-BB-stimulated tyrosine autophosphorylation of the PDGF receptor beta subunit nor inositol monophosphate accumulation was affected by increased PKA activity, suggesting that PKA inhibits events downstream of the PDGF receptor. This study provides an example of cross talk between two important signaling systems activated by physiological stimuli in smooth muscle cells--namely, the PKA pathway and the growth factor-activated MAPK cascade.

Glucocorticoid inhibits thrombin-induced expression of platelet-derived growth factor A-chain and heparin-binding epidermal growth factor-like growth factor in human aortic smooth muscle cells

Proliferation of smooth muscle cells (SMCs) in atherosclerosis may be modulated by several growth regulatory molecules. At least two mitogens for SMCs, platelet-derived growth factor (PDGF) A-chain and heparin-binding epidermal growth factor-like growth factor (HB-EGF), can be produced by SMCs themselves and may stimulate smooth muscle proliferation in an autocrine or paracrine fashion. We examined the effects of thrombin, which may be generated at the site of vascular injury during atherogenesis, and the potent anti-inflammatory glucocorticoid, dexamethasone (DEX), on the expression of the genes encoding these two growth factors. Since both PDGF A-chain and HB-EGF have affinity for heparin, we also examined the effect of thrombin and DEX on the release of heparin binding mitogenic activity from SMCs. Treatment of SMCs with thrombin resulted in increases both in the level of the PDGF-A and HB-EGF transcripts in the cells, as well as in released heparin-binding growth factor activity. DEX inhibits the thrombin-stimulated release of mitogenic activity in a dose-dependent manner. An enzyme-linked immunoadsorbent assay showed that DEX inhibits both constitutive and thrombin-stimulated release of PDGF-AA. DEX also decreases both constitutive and thrombin-stimulated mRNA levels for PDGF A-chain and HB-EGF and destabilizes the transcripts for both growth factors. A nuclear run-on assay revealed that DEX acts, in addition, to inhibit constitutive and thrombin-stimulated transcription of the PDGF A-chain and HB-EGF genes. Thus, these findings indicate that expression of PDGF A-chain and HB-EGF may be regulated by thrombin and glucocorticoid at the transcription level. Our results are consistent with the involvement of thrombin-induced growth factor expression in neointimal SMC proliferation and suggest the possibility that intimal proliferation may be attenuated by glucocorticoids.

Role of endogenous platelet-derived growth factor in arterial smooth muscle cell migration after balloon catheter injury

The process of intimal thickening after de-endothelializing injury to the rat carotid artery is dependent on the migration of smooth muscle cells from the media. Recent reports have suggested that platelet-derived growth factor may be an important mediator of migration after injury. We have addressed this issue by directly determining smooth muscle cell migration in injured arteries of animals depleted of platelets and after administration of an antibody that blocks platelet-derived growth factor. Because there is a reported association between plasminogen activator synthesis and smooth muscle cell migration, we assayed the activity levels of plasminogen activators after arterial injury and also assessed the effect of a plasmin inhibitor on migration. The data suggest that platelet-derived growth factor, released by platelets at sites of arterial injury, is an endogenous mediator of smooth muscle cell migration; that plasmin generation, catalyzed by tissue-type plasminogen activator, is necessary for migration; and that one way in which platelet-derived growth factor may act is by stimulation of the synthesis of tissue-type plasminogen activator by smooth muscle cells.