Priming of human monocyte superoxide production and arachidonic acid metabolism by adherence to collagen- and basement membrane-coated surfaces

Subendothelial matrix components influence endothelial cell apoptosis in vitro

The programmed form of cell death (apoptosis) is essential for normal development of multicellular organisms. Dysregulation of apoptosis has been linked with embryonal death and is involved in the pathophysiology of various diseases. Specifically, endothelial apoptosis plays pivotal roles in atherosclerosis whereas prevention of endothelial apoptosis is a prerequisite for neovascularization in tumors and metastasis. Endothelial biology is intertwined with the composition of subendothelial basement membrane proteins. Apoptosis was induced by addition of tumor necrosis factor-α to cycloheximide-sensitized endothelial cells. Cells were either grown on polystyrene culture plates or on plates precoated with healthy basement membrane proteins (collagen IV, fibronectin, or laminin) or collagen I. Our results reveal that proteins of healthy basement membrane alleviate cytokine-induced apoptosis whereas precoating with collagen type I had no significant effect on apoptosis by addition of tumor necrosis factor-α to cycloheximide-sensitized endothelial cells compared with cells cultured on uncoated plates. Yet, treatment with transforming growth factor-β1 significantly reduced the rate of apoptosis endothelial cells grown on collagen I. Detailed analysis reveals differences in intracellular signaling pathways for each of the basement membrane proteins studied. We provide additional insights into the importance of basement membrane proteins and the respective cytokine milieu on endothelial biology. Exploring outside-in signaling by basement membrane proteins may constitute an interesting target to restore vascular function and prevent complications in the atherosclerotic cascade.

The arterial microenvironment: the where and why of atherosclerosis

The formation of atherosclerotic plaques in the large and medium sized arteries is classically driven by systemic factors, such as elevated cholesterol and blood pressure. However, work over the past several decades has established that atherosclerotic plaque development involves a complex coordination of both systemic and local cues that ultimately determine where plaques form and how plaques progress. Although current therapeutics for atherosclerotic cardiovascular disease primarily target the systemic risk factors, a large array of studies suggest that the local microenvironment, including arterial mechanics, matrix remodelling and lipid deposition, plays a vital role in regulating the local susceptibility to plaque development through the regulation of vascular cell function. Additionally, these microenvironmental stimuli are capable of tuning other aspects of the microenvironment through collective adaptation. In this review, we will discuss the components of the arterial microenvironment, how these components cross-talk to shape the local microenvironment, and the effect of microenvironmental stimuli on vascular cell function during atherosclerotic plaque formation.

Vascular extracellular matrix in atherosclerosis

The extracellular matrix (ECM) is an essential component of the human body that is responsible for the proper function of various organs. Changes in the ECM have been implicated in the pathogenesis of several cardiovascular conditions including atherosclerosis, restenosis, and heart failure. Matrix components, such as collagens and noncollagenous proteins, influence the function and activity of vascular cells, particularly vascular smooth muscle cells and macrophages. Matrix proteins have been shown to be implicated in the development of atherosclerotic complications, such as plaque rupture, aneurysm formation, and calcification. ECM proteins control ECM remodeling through feedback signaling to matrix metalloproteinases (MMPs), which are the key players of ECM remodeling in both normal and pathological conditions. The production of MMPs is closely related to the development of an inflammatory response and is subjected to significant changes at different stages of atherosclerosis. Indeed, blood levels of circulating MMPs may be useful for the assessment of the inflammatory activity in atherosclerosis and the prediction of cardiovascular risk. The availability of a wide variety of low-molecular MMP inhibitors that can be conjugated with various labels provides a good perspective for specific targeting of MMPs and implementation of imaging techniques to visualize MMP activity in atherosclerotic plaques and, most interestingly, to monitor responses to antiatheroslerosis therapies. Finally, because of the crucial role of ECM in cardiovascular repair, the regenerative potential of ECM could be successfully used in constructing engineered scaffolds and vessels that mimic properties of the natural ECM and consist of the native ECM components or composite biomaterials. These scaffolds possess a great promise in vascular tissue engineering.

Bone marrow mesenchymal stem cells can differentiate and assume corneal keratocyte phenotype

It remains elusive as to what bone marrow (BM) cell types infiltrate into injured and/or diseased tissues and subsequently differentiate to assume the phenotype of residential cells, for example, neurons, cardiac myocytes, keratocytes, etc., to repair damaged tissue. Here, we examined the possibility of whether BM cell invasion via circulation into uninjured and injured corneas could assume a keratocyte phenotype, using chimeric mice generated by transplantation of enhanced green fluorescent protein (EGFP)⁺ BM cells into keratocan null (Kera^−/−) and lumican null (Lum^−/−) mice. EGFP⁺ BM cells assumed dendritic cell morphology, but failed to synthesize corneal-specific keratan sulfate proteoglycans, that is KS-lumican and KS-keratocan. In contrast, some EGFP⁺ BM cells introduced by intrastromal transplantation assumed keratocyte phenotypes. Furthermore, BM cells were isolated from Kera-Cre/ZEG mice, a double transgenic mouse line in which cells expressing keratocan become EGFP⁺ due to the synthesis of Cre driven by keratocan promoter. Three days after corneal and conjunctival transplantations of such BM cells into Kera^−/− mice, green keratocan positive cells were found in the cornea, but not in conjunctiva. It is worthy to note that transplanted BM cells were rejected in 4 weeks. MSC isolated from BM were used to examine if BM mesenchymal stem cells (BM-MSC) could assume keratocyte phenotype. When BM-MSC were intrastromal-transplanted into Kera^−/− mice, they survived in the cornea without any immune and inflammatory responses and expressed keratocan in Kera^−/− mice. These observations suggest that corneal intrastromal transplantation of BM-MSC may be an effective treatment regimen for corneal diseases involving dysfunction of keratocytes.

Thromboxane receptor signaling is required for fibronectin-induced matrix metalloproteinase 9 production by human and murine macrophages and is attenuated by the Arhgef1 molecule

During an inflammatory response, resident and newly recruited tissue macrophages adhere to extracellular matrix and cell-bound integrin ligands. This interaction induces the expression of pro-inflammatory mediators that include matrix metalloproteinases (MMPs). Arhgef1 is an intracellular signaling molecule expressed by myeloid cells that normally attenuates murine macrophage MMP production in vivo and in vitro after cell culture on the extracellular matrix protein, fibronectin. In this study, we have extended the characterization of this fibronectin-induced Arhgef1-regulated signaling pathway in both human and murine myeloid cells. Our results show that MMP9 production by fibronectin-stimulated monocytes and macrophages depends on autocrine thromboxane receptor signaling and that under normal conditions, this signaling pathway is attenuated by Arhgef1. Finally, we show that the expression of ARHGEF1 by human peripheral blood monocytes varies between individuals and inversely correlates with fibronectin-mediated MMP9 production.

Strong glucose dependence of electron current in human monocytes

Reactive oxygen species (ROS) production by human monocytes differs profoundly from that by neutrophils and eosinophils in its dependence on external media glucose. Activated granulocytes produce vast amounts of ROS, even in the absence of glucose. Human peripheral blood monocytes (PBM), in contrast, are suspected not to be able to produce any ROS if glucose is absent from the media. Here we compare ROS production by monocytes and neutrophils, measured electrophysiologically on a single-cell level. Perforated-patch-clamp measurements revealed that electron current appeared after stimulation of PBM with phorbol myristate acetate. Electron current reflects the translocation of electrons through the NADPH oxidase, the main source of ROS production. The electron current was nearly abolished by omitting glucose from the media. Furthermore, in preactivated glucose-deprived cells, electron current appeared immediately with the addition of glucose to the bath. To characterize glucose dependence of PBM further, NADPH oxidase activity was assessed as hydrogen peroxide (H(2)O(2)) production and was recorded fluorometrically. H(2)O(2) production exhibited similar glucose dependence as did electron current. We show fundamental differences in the glucose dependence of ROS in human monocytes compared with human neutrophils.

Collagens in the progression and complications of atherosclerosis

Collagens constitute a major portion of the extracellular matrix in the atherosclerotic plaque, where they contribute to the strength and integrity of the fibrous cap, and also modulate cellular responses via specific receptors and signaling pathways. This review focuses on the diverse roles that collagens play in atherosclerosis; regulating the infiltration and differentiation of smooth muscle cells and macrophages; controlling matrix remodeling through feedback signaling to proteinases; and influencing the development of atherosclerotic complications such as plaque rupture, aneurysm formation and calcification. Expanding our understanding of the pathways involved in cell-matrix interactions will provide new therapeutic targets and strategies for the diagnosis and treatment of atherosclerosis.

Moraxella catarrhalis–Infected Alveolar Epithelium Induced Monocyte Recruitment and Oxidative Burst

The recruitment of monocytes appears to be a crucial factor for inflammatory lung disease. Alveolar epithelial cells contribute to monocyte influx into the lung, but their impact on monocyte inflammatory capacity is not entirely clear. We thus analyzed the modulation of monocyte oxidative burst by A549 and isolated human alveolar epithelial cells. Epithelial infection with Moraxella catarrhalis induced monocyte adhesion, transepithelial migration, and superoxide generation, whereas stimulation with lipopolysaccharide, tumor necrosis factor-alpha, interleukin-1beta, or interferon-gamma induced adhesion or transmigration, but failed to initiate monocyte burst. The effect of microbial challenge was mimicked by phorbol myristate acetate and inhibited by the protein kinase C inhibitor bisindoylmaleimide. Furthermore, evidence for a role of platelet-activating factor-signaling in monocytes is presented. Monocyte burst was neither induced by supernatant nor affected by fixation of A549 cells, excluding the contribution of epithelium-derived soluble factors but emphasizing the mandatory role of intercellular contact. The employment of blocking antibodies, however, denied a role for the adhesion molecules intercellular adhesion molecule-1 and vascular cell adhesion molecule-1, or CD11b/CD18 and CD49d/CD29. In essence, infection of alveolar epithelial cells with M. catarrhalis might amplify the inflammatory capacity of invading monocytes eliciting their superoxide production. The epithelial response to this microbial challenge thus clearly differed from that to proinflammatory cytokines.

Nicotine and fibronectin expression in lung fibroblasts: implications for tobacco‐related lung tissue remodeling

Tobacco-related lung diseases are associated with alterations in tissue remodeling and are characterized by increased matrix deposition. Among the matrix molecules found to be highly expressed in tobacco-related lung diseases is fibronectin, a cell adhesive glycoprotein implicated in tissue injury and repair. We hypothesize that nicotine, a component of tobacco, stimulates the expression of fibronectin in lung fibroblasts via the activation of intracellular signals that lead to increased fibronectin gene transcription. In support of this, we found that nicotine stimulated the expression of fibronectin in lung fibroblasts and that its stimulatory effect was associated with activation of protein kinase C and mitogen-activated protein kinases, increased levels of intracellular cAMP, and phosphorylation and DNA binding of the transcription factor CREB. Increased transcription of the gene was dependent on cAMP-response elements (CREs) present on the 5' end of its gene promoter. The stimulatory effect of nicotine on fibronectin expression was abolished by alpha-bungarotoxin, an inhibitor of alpha7 nicotinic acetylcholine receptors (alpha7 AChRs). Of note, nicotine increased the expression of alpha7 nAChRs on fibroblasts. Our data suggest that nicotine induces lung fibroblasts to produce fibronectin by stimulating alpha7 nAChR-dependent signals that regulate the transcription of the fibronectin gene.

Regulation of monocyte gene expression by the extracellular matrix and its functional implications

By binding to extracellular matrix (ECM) proteins, integrins integrate signals from outside the cell and transmit them inwards, thereby providing cells with information about location and allowing them to respond to stimuli in a manner appropriate to their environment. This is particularly important for monocytes and macrophages, given their wide distribution throughout the body and the vital role they play in immune and inflammatory responses. Integrin-mediated interaction of monocytes with ECM is a potent regulator of gene expression and is strongly synergized by the presence of growth factors. This synergy between growth factors and integrins is also apparent in the overlap seen in their signaling pathways. Integrin-mediated interaction with ECM results in increased expression of numerous inflammatory and immune response genes, revealing an important role for ECM-integrin interaction in affecting monocyte function and thus impacting on the development of pathologies. This is of particular relevance in the context of immune and inflammatory responses, where integrin-mediated adhesive interactions with the ECM-rich peripheral tissues are central to the localization of both resident and infiltrating monocytes at inflammatory sites. Here, we will review the functional effects of integrin-ECM interactions on monocytes, with particular attention to the regulation of gene expression by ECM and its functional implications.

Decreased superoxide production, degranulation, tumor necrosis factor alpha secretion, and CD11b/CD18 receptor expression by adherent monocytes from preterm infants

Preterm infants have an increased incidence of infection, which is principally due to deficiencies in neonatal host defense mechanisms. Monocyte adherence is important in localizing cells at sites of infection and is associated with enhanced antimicrobial functions. We isolated cord blood monocytes from preterm and full-term infants to study their adhesion and immune functions, including superoxide (O2-) generation, degranulation, and cytokine secretion and their adhesion receptors. O2- production and degranulation were significantly diminished, by 28 and 37%, respectively, in adherent monocytes from preterm infants compared to full-term infants (P < 0. 05); however, these differences were not seen in freshly isolated cells. We also observed a significant decrease of 35% in tumor necrosis factor alpha secretion by lipopolysaccharide-stimulated adherent monocytes from preterm infants compared to full-term infants (P < 0.05); however, this difference was not observed in interleukin-1beta or interleukin-6 production by the monocytes. The cell surface expression of the CD11b/CD18 adhesion receptor subunits was significantly decreased (by 60 and 52%, respectively) in monocytes from preterm infants compared to full-term infants (P < 0. 01). The cascade of the immune response to infection involves monocyte upregulation and adherence via CD11b/CD18 receptors followed by cell activation and the release of cytokines and bactericidal products. We speculate that monocyte adherence factors may be important in the modulation of immune responses in preterm infants.

The beta1-integrin cytosolic domain optimizes phospholipase A2-mediated arachidonic acid release required for NIH-3T3 cell spreading

Inhibition of PLA2 activity and rescue by addition of exogenous AA was used to demonstrate that AA production is essential for integrin-mediated NIH-3T3 murine cell spreading. Both AA release and cell spreading after attachment to a FN substrate were inhibited by the PLA2 inhibitor mepacrine. AA release was essential for signaling spreading since the inhibition of spreading induced by mepacrine was overcome by exogenous AA. Cells ectopically expressing full-length chicken beta1-integrins both released AA and spread fully on a substrate of anti-chicken beta1-integrin monoclonal antibody, and inhibition of PLA2 by mepacrine suppressed both spreading and AA release. Exogenous AA also reversed this mepacrine-induced inhibition of spreading. The role of the beta1-integrin cytosolic domain in AA release was examined by comparing responses of cells expressing full-length chicken beta1-integrins versus cells expressing a deletion mutant chicken beta1-integrin with a truncated cytosolic domain. Cells expressing a truncated chicken beta1-integrin released significantly less AA and failed to spread on the anti-chicken beta1-integrin antibody substrate. Furthermore, clustering full-length receptors with soluble antibody stimulated greater AA release than clustering of receptors having truncated cytosolic domains. These data suggest the beta1-integrin cytosolic domain is required for optimal PLA2 activation to produce AA necessary for cell spreading.

Cyclic strain effects on human monocyte interactions with endothelial cells and extracellular matrix proteins

Human vascular endothelial cells (ECs) are exposed to various levels of hemodynamic forces, cyclic strain, and shear stress in vivo. Here, we examined the in vitro effects of the various levels (0-6%, 7-16%, and 17-25%) of strain at 60, 30, and 15 cycles per minute (cpm) on human monocyte adherence to endothelial cells and extracellular matrix protein preabsorbed surfaces. Monocyte adhesion to endothelial cells under cyclic strain significantly increased. At both 30 and 60 cpm, ECs under strains of 7-16% and 17-25% showed >52% and >117% higher monocyte adhesion than endothelial cells under static condition when monocytes were added for 0.5 h. This increase in monocyte adhesion to ECs under cyclic strain remained significantly higher even after 24 h of incubation. Human monocyte adhesion to extracellular matrix protein preabsorbed surfaces differed depending on the specific extracellular matrix protein. Monocytes adhered to collagen type I and fibronectin preabsorbed surfaces >50% under 0-6% strain, >23% under 7-16% strain, and >52% under 17-25% strain at 15 and 30 cpm compared to the collagen type V preabsorbed surface. However, when extracellular protein preabsorbed surfaces under cyclic strain were compared to the control static condition, monocyte adhesion did not significantly change on most of other surfaces. These results suggest that cyclic strain may play a role in the regulation of monocyte-endothelial cells/extracellular matrix interactions in vivo.

Tissue inhibitors of metalloproteinases and metalloproteinases in atherosclerosis

The ability of the metalloproteinases to degrade extracellular matrix proteins is essential for the matrix remodelling that occurs during infiltration of inflammatory cells, intimal thickening, angiogenesis and plaque rupture which are a result of atherosclerosis. Increased metalloproteinase activity therefore requires stimulation of metalloproteinase expression by cytokines and growth factors, activation of metalloproteinases, and downregulation of tissue inhibitors of metalloproteinases. In addition, metalloproteinases may influence atherosclerosis by processing of proteins involved in inflammation and cell growth and death and the tissue inhibitors of metalloproteinases may also play a less inhibitory role by influencing cell growth and apoptosis.

Extracellular matrix modulates macrophage functions characteristic to atheroma: collagen type I enhances acquisition of resident macrophage traits by human peripheral blood monocytes in vitro

Activated resident macrophages sustain atheroma, and a high macrophage content is associated with plaque vulnerability. Factors leading to differentiation and activation of these blood-derived cells remain largely uncharacterized. We investigated the contribution of interaction with collagen type I, the predominant component of atherosclerotic matrix, to differentiation and modulation of characteristic macrophage functions, including intracellular lipid accumulation and production of the typical matrix-degrading enzyme matrix metalloproteinase (MMP)-9. When used as an adhesion substrate for human peripheral blood monocytes in vitro, collagen type I increased monocyte differentiation, assessed by analysis of CD71 expression and cell spreading. Culturing on collagen type I doubled the number of differentiated monocytes at 24 hours (44.9+/-1.4% versus 18.4+/-1.7% on uncoated dishes, P<.001, n=3 independent experiments) and was a stronger stimulus for differentiation than phorbol myristate acetate, a known inducer of monocyte differentiation. The effect of substrate on intracellular accumulation of modified lipoproteins was assessed by quantitative confocal microscopy of monocytes incubated with fluorescent acetylated LDL. The collagen type I substrate also doubled the number of macrophages containing intracellular lipid and significantly increased the individual intracellular loading. Monocytes cultured on collagen type I also released more MMP-9 than did cells plated directly on plastic. The role of monocyte spreading was further assessed by treatment with colchicine, an inhibitor of cytoskeletal function, or with genistein, a nonspecific inhibitor of tyrosine kinases, shown to participate in cell adhesion. Cell spreading was inhibited in 72.3+/-6.7% of colchicine-treated and in 62.4+/-6.4% of genistein-treated monocytes (n=3, P<.01 in both cases). The same conditions also decreased secretion of MMP-9, and genistein reduced the number of acetylated LDL-containing cells (from 286+/-7 to 184+/-8 cells/mm2 with genistein, n=3, P<.001). Data showed a strong correlation (r>.98) between monocyte spreading on collagen type I and intracellular lipid accumulation. Our results indicate that interaction with vascular matrix may play an important role in differentiation of peripheral blood monocytes into resident lipid-laden macrophages, which act as central stimulators throughout the natural history of atheroma.

Extracellular matrix and lung inflammation

Lung injury triggers an acute inflammatory response characterized by increased expression and deposition of extracellular matrix (ECM) components such as fibronectin and collagen. Although the function of newly deposited matrices in injured lungs is unknown, their ability to affect the migration, proliferation, differentiation, and activation state of cells in vitro suggests an important role in the initiation and maintenance of the inflammatory response in vivo. Interactions between immune and nonimmune cells with the lung ECM are mediated via cell surface receptors of the integrin family which link the ECM with intracellular molecules involved in signal transduction. Activation of integrin-mediated intracellular signals may promote inflammation by facilitating leukocyte recruitment and cytokine expression.

Monocyte adherence to the subendothelial basement membrane increases interleukin-8 gene expression and antigen release

The emigration of peripheral blood monocytes into the interstitium allows for contact with a variety of surfaces which may provide signals important for monocyte function in both normal and inflammatory states. In the present study, we examined the effect of adherence to an endothelial cell-derived basement membrane and to collagen I, the major collagen of the interstitium, on monocyte release and gene expression of the potent chemotactic cytokine Interleukin-8 (IL-8). We further evaluated neutrophil chemotactic activity of the conditioned media containing antigenic IL-8 from monocytes adherent to these same surfaces. Elutriation-purified monocytes were adhered for 1 hour to plastic tissue culture wells either uncoated (PL) or coated with bovine serum albumin (BSA), collagen type I (C-I), or endothelial cell-derived basement membrane (BM). Following removal of nonadherent cells, monocytes were further incubated in a serum-free media for 18 hours in the presence or absence of lipopolysaccharide (IPS). Following 18 hrs of incubation there were significantly less monocytes remaining adherent to BM when compared to other surfaces tested. In the absence of LPS, adherent monocytes released significant amounts of IL-8 that was not surface specific. In the presence of LPS, monocytes adherent to BM released significantly more IL-8, when corrected for adherent cell number, than monocytes adherent to PL, BSA, or C-I. Conditioned media from adherent monocytes expressed IL-8 dependent neutrophil chemotactic activity that was not influenced by the surfaces tested. Northern blot analysis indicated greater induction for IL-8 mRNA by monocytes adhered to BM after 18 hrs in the presence of LPS. These results suggest that monocyte adherence to the subendothelial basement membrane provides a priming signal for the induction and secretion of the chemotactic cytokine IL-8 in response to inflammatory stimuli.

Laminin SIKVAV peptide induction of monocyte/macrophage prostaglandin E2 and matrix metalloproteinases

The laminin-derived synthetic peptide containing the SIKVAV (Ser-Ile-Lys-Val-Ala-Val) amino acid sequence has been previously shown to regulate tumor invasion, metastasis, and angiogenesis. Here, we demonstrate that this peptide also modulates human monocyte responses. Moreover, the monocytic responses elicited by this peptide are influenced by the culture conditions. When elutriated monocytes were cultured on SIKVAV substrate or in suspension with this peptide, the synthesis of prostaglandin E2, interstitial collagenase, and gelatinase B was induced and was further enhanced in the presence of concanavalin A (ConA). However, when monocytes were adhered before adding soluble SIKVAV, the peptide alone failed to induce the production of prostaglandin E2 or matrix metalloproteinases. If adherent monocytes were exposed to SIKVAV in the presence of ConA, this peptide enhanced the ConA induced production of these mediators. In contrast to SIKVAV, the intact laminin molecule failed to influence these monocyte responses. This is the first demonstration that a laminin derived peptide is capable of inducing or enhancing monocyte inflammatory responses that may influence a number of biological activities such as wound healing or excessive connective tissue destruction associated with chronic inflammation.