Expression of ICAM-1 and VCAM-1 and monocyte adherence in arteries exposed to altered shear stress

Cyclic-strain-induced endothelial cell expression of adhesion molecules and their roles in monocyte-endothelial interaction

Vascular endothelial cells (ECs) are constantly subjected to hemodynamic forces that may regulate monocyte-endothelial interaction in vivo. To examine the effects of cyclic strain on endothelial expression of monocyte adhesion molecules, E-selectin, intercellular adhesion molecule-1 (ICAM-1), and vascular cell adhesion molecule-1 (VCAM-1) ECs were exposed to physiologically relevant levels of cyclic strain. When ECs were under 25% maximal strain at 30 cycles/min for 24 h, the expression of E-selectin significantly (p<0.05) increased, by 83%, compared to control ECs under static conditions. Similarly, monocyte adhesion to ECs under strain (maximum of 15 or 25% at 30 and 60 cycles/min for 24 h) also significantly (p<0.05) increased, by >82%. This cyclic-strain-induced monocyte adhesion was substantially inhibited (83.5%) by anti-E-selectin antibody. ICAM-1 expression also significantly increased, by 62%, when ECs were under 25% maximal strain at 30 cycles/min for 3 h whereas VCAM-1 expression by ECs under strain (for 0.5, 3, and 24 h) did not change compared to static ECs. When ECs were treated with anti-ICAM-1 antibody and monocytes with anti-VLA-4 antibody, an increase in monocyte adhesion to ECs under cyclic strain was reduced significantly. These results demonstrate that cyclic strain can induce EC expression of monocyte adhesion molecules E-selectin, ICAM-1, and VCAM-1 in a time-dependent manner and thus can mediate monocyte adhesion.

Increased expression of matrix metalloproteinase-2 in the thickened intima of aged rats

-To characterize remodeling of elastic arteries with aging and to investigate its potential mechanisms, matrix metalloproteinase-2 (MMP-2), intracellular adhesive molecule-1 (ICAM-1), transforming growth factor-beta (TGF-beta), and fibronectin protein levels were measured in the aortas of young adult (6 months) and aged (30 months) Fischer 344XBN rats. At 30 versus 6 months, the thickness of the intima was 5-fold greater and contained marked increases in TGF-beta and ICAM-1, and fibronectin expression was enhanced throughout the aortic wall. Total MMP-2 protein (Western blot) of 30-month-old rats was increased 8-fold over that of 6-month-old rats (0.166+/-0.032 versus 0.020+/-0.006; P<0.01), and staining and activity were regionally localized to the intima, often near breaks in the internal elastic membrane and lamellae. Early passage, explanted smooth muscle cells (SMC) from aged aorta secreted more MMP-2 than those from young aorta; while basal MMP-2 production did not differ with age, after stimulation with cytokines (interleukin-1, tumor necrosis factor-alpha, or TGF-beta, 10 ng/mL each for 24 hours), MMP-2 production in SMC from 30-month-old rats increased to levels greater than those in 6-month-old rats. Thus, enhanced expression of TGF-beta, MMP-2, and ICAM-1 in the thickened vascular intima of aged rats may in part be produced by exaggerated SMC responses to cytokines and may have potential roles in intimal remodeling with aging.

The role of adhesion molecules in atherosclerosis

The progression of atherosclerosis is currently believed to involve the interaction of monocytes with the vascular endothelium. Within the last decade, the cell-surface proteins thought to control these interactions have been investigated. This review seeks to describe the nature of these interactions through what are known as adhesion molecules and their role in atherogenesis. It begins with the stages of atherogenesis from the movement of the monocyte to the endothelium, followed by the migration of smooth muscle cells from the media to the intima, and subsequently to the later stages of fibrofatty plaque formation and potential complications due to thrombosis and/or plaque fissure and embolism. The different structural classifications of the adhesion molecules, such as integrins, cadherins, selectins, and members of the immunoglobulin gene superfamily, are outlined, and interaction of binding domains are highlighted. The vascular endothelium and the basic role of adhesion molecules in dysfunction are considered. Discussion of the role of adhesion molecules in atherogenesis focuses on interactions of the endothelium, monocytes, and leukocytes, as well as the influences of cytokines, oxidized low-density lipoproteins, and genetic determinants. Finally, epidemiological risk factors associated with atherosclerosis such as hypertension and dyslipidemia are considered in light of their effects on adhesion molecule expression.

Expression of cell adhesion molecules and the appearance of adherent leukocytes on the left atrial endothelium with atrial fibrillation: rabbit experimental model

To assess the expression of cell adhesion molecules and the appearance of leukocytes adhering to the left atrial endothelium with atrial fibrillation (AF), 10 Japanese white rabbits were anesthetized and 3 pacing leads were placed in the right atrium. For the AF model, the right atrium was stimulated by electrical pacing (the stimulation frequency of each lead being adjusted to different intervals) for 8h while the control model was subjected to a sham operation without atrial stimulation. The left atrial appendage was excised from the heart and examined immunohistochemically. P-selectin staining of the endothelium in both models was linear and regional, and intracellular adhesion molecule-1 (ICAM-1) in the AF model was confined to leukocytes and endothelial cells with adherent leukocytes. The expression of P-selectin (p<0.05) and the appearance of positively ICAM-1 stained adherent leukocytes (p<0.05) were significantly greater in the AF model than in the control model. In conclusion, AF could regulate the expression of at least 2 critical adhesion molecules, P-selectin and ICAM-1, and the appearance of adherent leukocytes; suggesting that these molecules may play an important role in left atrial thrombus formation with AF. Although anticoagulant therapy has generally been carried out with warfarin in AF patients, neutralizing antibodies to cell adhesion molecules should be tried to prevent thromboembolic complications.

Mouse model of venous bypass graft arteriosclerosis

Saphenous vein grafts are widely used for treatment of severe atherosclerosis via aortocoronary bypass surgery, a procedure often complicated by later occlusion of the graft vessel. Because the molecular mechanisms of this process remain largely unknown, quantitative models of venous bypass graft arteriosclerosis in transgenic mice could be useful to study this process at the genetic level. We describe herein a new model of vein grafts in the mouse that allows us to take advantage of transgenic, knockout, or mutant animals. Autologous or isogeneic vessels of the external jugular or vena cava veins were end-to-end grafted into carotid arteries of C57BL/6J mice. Vessel wall thickening was observed as early as 1 week after surgery and progressed to 4-, 10-, 15-, and 18-fold original thickness in grafted veins at age 2, 4, 8, and 16 weeks, respectively. The lumen of grafted veins was significantly narrowed because of neointima hyperplasia. Histological and immunohistochemical analyses revealed three lesion processes: marked loss of smooth muscle cells in vein segments 1 and 2 weeks after grafting, massive infiltration of mononuclear cells (CD11b/18+) in the vessel wall between 2 and 4 weeks, and a significant proliferation of vascular smooth muscle cells (alpha-actin+) to constitute neointimal lesions between 4 and 16 weeks. Similar vein graft lesions were obtained when external jugular veins or vena cava were isografted into carotid arteries of C57BL/6J mice. Moreover, no significant intima hyperplasia in vein-to-vein isografts was found, although there was leukocyte infiltration in the vessel wall. Thus, this model, which reproduces many of the features of human vein graft arteriosclerosis, should prove useful for our understanding of the mechanism of vein graft disease and to evaluate the effects of drugs and gene therapy on vascular diseases.

Atherosclerosis and inflammation mononuclear cell recruitment and adhesion molecules with reference to the implication of ICAM-1/LFA-1 pathway in atherogenesis

Recent investigations have reanimated the view that there exists a possible link between atherosclerosis and inflammation. Adhesion of monocytes as well as T lymphocytes to the arterial endothelial surface, followed by their migration into the subendothelial space is a hallmark for experimental animals fed an atherogenic diet. Human studies show identical features in the arterial wall to the animal models of atherosclerosis. The recruitment of leukocytes into areas of inflammation is mediated by interacting sets of cell adhesion molecules. In atherosclerosis, focal expression of key adhesion molecules particularly triggered by plasma atherogenic lipoproteins has been detected, and these molecules may mediate the recruitment of mononuclear cells to the plaque. Among these adhesion molecules, ICAM-1, a protein of the Ig superfamily, and one of the ligands for LFA-1 have been suggested to play an important role in atherogenesis. In diet-induced hypercholesterolemic rats, we found that ICAM-1 expression is up-regulated mainly in lesion-prone areas of the aorta during the early stages of atherogenesis. Increased ICAM-1 expression was associated with a marked monocyte and T lymphocyte intimal recruitment. Further immunohistochemical studies have demonstrated that LFA-1 is expressed by more than 85% of macrophages in the lesions, and their presence therefore may point toward the involvement of the LFA-1/ICAM-1 receptor ligand pathway in the recruitment of mononuclear cells in the lesions. In order to verify this hypothesis, systemic administration of blocking antibodies was attempted; injection of anti-ICAM-1/LFA-1 monoclonal antibodies significantly reduced macrophage adherence and their emigration into the intima. Our current study suggests that ICAM-1 may act as an "athero-ELAM" for mononuclear cell intimal recruitment during atherogenesis.

Association between secondary flow in models of the aorto-celiac junction and subendothelial macrophages in the normal rabbit

In order to examine the association between arterial fluid dynamics and the distribution of subendothelial macrophages in the normal rabbit aorta, steady and pulsatile particle flow visualization was performed in a geometrically realistic model of the rabbit aorto-celiac junction region. Over a range of aorto-celiac steady flow ratios, particle pathlines along the upstream lateral aortic walls curved to enter the celiac orifice, while two asymmetric regions of reversing spiral secondary flow originated along the downstream lateral portions of the orifice flow divider. These regions increased in size as either the Reynolds number or flow into the celiac artery increased. In pulsatile flow studies, particles along the lateral aortic walls near the celiac orifice began to spiral into the branch during peak systole. During systolic deceleration, the size of this spiral flow region increased as particles reversed direction to enter the celiac orifice. This contrasted with flow patterns directly upstream and downstream of the orifice, which remained unidirectional throughout this period even along the distal lip of the orifice. The highest frequency of subendothelial white blood cells in the normal rabbit aorta was associated with regions where secondary flow patterns occurred, and where the orientation of endothelial cell nuclei deviated from the major direction of aortic flow. Secondary flow patterns may aid the accumulation of monocytes and macrophages about the lateral regions of the celiac artery flow divider by transporting monocytes to the walls, allowing them time to attach to the endothelial cells, or by stimulating the endothelial cells to express leukocyte adhesion molecules. These same regions are associated with increased endothelial permeability to low density lipoprotein and, under hypercholesterolemic conditions, lesion origination.

Effects of recirculating flow on U-937 cell adhesion to human umbilical vein endothelial cells

We used a sudden-expansion flow chamber to examine U-937 cell adhesion to unactivated and tumor necrosis factor (TNF)-alpha-activated human umbilical vein endothelial cells (HUVEC) in recirculating flow. For both unactivated and TNF-alpha-activated HUVEC, U-937 cells exhibited transient arrests within approximately 150 microm of flow reattachment. Few arrests occurred directly at the reattachment site. U-937 cell rolling was not observed. At all other locations within the recirculation zone, U-937 cells did not exhibit transient arrests or rolling. TNF-alpha activation increased the frequency of U-937 cell arrests near reattachment but did not change the median arrest duration. Numerically simulated cell trajectories failed to predict attachment near the reattachment point. Deviations between experiment and theory may result from the nonspherical shape and deformability of U-937 cells. These results demonstrate that U-937 cell transient arrests occur preferentially in the vicinity of the reattachment point in recirculating flow. Possible mechanisms for adhesion include low shear stress, curved streamlines, fluid velocity components normal to the endothelium, and formation of larger contact areas.

Expression of vascular adhesion molecules in saphenous vein coronary bypass grafts

BACKGROUND

Adhesion of blood elements to the endothelium is an important step in the development of vein graft disease. This study examines the expression of vascular adhesion molecules on explanted saphenous vein bypass grafts.

METHODS

Immunocytochemical staining was performed using explanted saphenous vein grafts from 28 patients. Antibodies against the endothelial markers CD31, von Willebrand factor, intercellular adhesion molecule-1, vascular adhesion molecule-1, and E-selectin were used.

RESULTS

Staining for CD31 and von Willebrand factor demonstrated the presence of endothelial cells in the lumen and the vasa vasorum. Expression of intercellular adhesion molecule-1 was variable between grafts, whereas vascular adhesion molecule-1 and E-selectin were almost always absent on the luminal endothelium. In contrast, the endothelium of the vasa vasorum stained positively for intercellular adhesion molecule-1 and vascular adhesion molecule-1, and was also seen on nonendothelial cells within the vessel wall. Expression of these adhesion molecules did not vary with the severity of vein graft disease.

CONCLUSIONS

This study highlights the blood vessels in the adventitia as possible sites for the adhesion and migration of cells into the vessel wall.

Vein grafts: haemodynamic forces on the endothelium--a review

OBJECTIVE

To review the mechanisms believed to be important in the development of vein graft stenosis, with particular attention placed on the adaptation of saphenous vein endothelium to a new haemodynamic environment.

DESIGN AND METHODS

Discussion based on review of published research.

RESULTS

The aetiology of vein graft stenosis remains to be established and appears to be multi-factorial. The increasing evidence for an important role of haemodynamic forces is discussed, particularly via the interaction of these force with the endothelium.

CONCLUSION

Further understanding of the interaction between haemodynamic forces, blood constituents and the newly implanted vein graft is required. Use of in vitro models is contributing increasing knowledge to this area, but ultimately better non-invasive methods of assessing haemodynamic forces in vivo are required.

Negative transcriptional regulation of the VCAM-1 gene by fluid shear stress in murine endothelial cells

To explore the mechanism of shear stress-induced downregulation of vascular cell adhesion molecule 1 (VCAM-1) expression in murine endothelial cells (ECs), we examined the effect of shear stress on VCAM-1 gene transcription and assessed the cis-acting elements involved in this phenomenon. VCAM-1 mRNA expression was downregulated at the transcriptional level as defined by nuclear run-on assay and transient transfection of VCAM-1 promoter-luciferase gene constructs. The luciferase assay on the VCAM-1 deletion mutants revealed that the cis-acting element is contained between -694 and -329 bp upstream from the transcription initiation site. Gel shift assay using overlapping oligonucleotide probes of this region showed that oligonucleotides containing a double AP-1 consensus sequence (TGACTCA) formed distinct complexes with nuclear proteins extracted from shear-stressed cells. Mutation of either one or both of two AP-1 consensus sequences completely abolished the ability of the promoter to respond to shear stress. These results suggest that fluid shear stress downregulates the transcription of the VCAM-1 gene via an upstream cis-element, a double AP-1 consensus sequence, in murine lymph node venule ECs.

Remodeling with neointima formation in the mouse carotid artery after cessation of blood flow

The ability of gene targeting in the mouse species presents a powerful tool to determine the role of specific molecules in vascular biology. Using a denuding-injury procedure, we recently reported that intimal lesions can be induced in the carotid artery of outbred mice. The technical challenge associated with achieving complete denudation and the relatively small size of the developing lesions prompted us to design the present model of neointima formation and vascular remodeling in the carotid artery of the inbred FVB mouse strain. Complete ligation of the vessel near the carotid bifurcation induced rapid proliferation of medial smooth muscle cells, leading to extensive neointima formation in the presence of an endothelial lining. Thrombus formation was not observed except in the most distal part of the vessel adjacent to the ligature. At 4 weeks after ligation, luminal area was reduced by approximately 80% through a combination of decreased vessel diameter and neointima formation. Ultrastructural analysis provided evidence for cell death in the developing neointima as well as the remodeling media. The present model might be useful in identifying those genes important for neointima formation and vascular remodeling.

Platelet-derived growth factor ligand and receptor expression in response to altered blood flow in vivo

Blood flow and the tractive force shear stress are important determinants of artery caliber, and reduced shear predisposes arteries to intimal thickening and atherosclerosis. The molecular basis for shear-induced changes in artery wall structure is poorly defined. A number of factors associated with normal and pathological artery wall remodeling are induced by shear stress in endothelial cell cultures. These include platelet-derived growth factor (PDGF), a potent mitogen, chemoattractant, and vasoconstrictor. To determine whether similar changes occur in vivo, we examined the effects of reduced blood flow on endothelial cell PDGF expression and proliferation in the rat carotid artery. Branches of the right internal and external carotid arteries were ligated, reducing common carotid artery blood flow from 8.0+/-0.6 to 0.5+/-0.1 mL/min while increasing flow in the left carotid from 7.1+/-0.6 to 10.8+/-0.7 mL/min. Shear stress following the procedure was 1.4+/-0.2 and 33.4+/-1.1 dyne/cm2 in carotids with reduced blood flow (RF) and increased blood flow (IF), respectively. Arteries were harvested 6, 24, 48, or 72 hours after ligation, perfusion-fixed, and opened longitudinally. Endothelial cell proliferation (bromodeoxyuridine [BrdU] labeling) was assessed en face at 24, 48, and 72 hours; expression of mRNA for PDGF-A and -B chains and PDGF alpha- and beta-receptors (in situ hybridization) was determined at 6, 48, and 72 hours after unilateral flow reduction. RF induced endothelial cell proliferation, which peaked at 48 hours (RF BrdU labeling: 24 hours, 0.4+/-0.2%; 48 hours, 7.2+/-2.0%; and 72 hours, 4.1+/-0.6%; n=5). PDGF-B expression increased in RF compared with IF endothelium within 48 hours and persisted at 72 hours (percent labeling [RF/IFx100]: 6 hours, 76+/-20%; 48 hours, 395+/-179%; and 72 hours, 208+/-44%; n=3). PDGF-A expression was similarly increased in RF endothelium. In contrast, expression of PDGF alpha- and beta-receptors was undetectable in RF and IF endothelium at all times. We conclude that endothelial cell PDGF ligand expression is induced by reduced shear stress in vivo and may play an important role in flow-mediated remodeling and atherogenesis.

Differential activation of NF-kappa B in human aortic endothelial cells conditioned to specific flow environments

Endothelial cell-monocyte interaction plays an important role in atherogenesis. The expressions of some endothelial cell adhesion molecules involved in endothelial cell-monocyte interactions are regulated by transcription factor NF-kappa B. Because low shear stress has been known to influence endothelial monocyte adhesion, the differential activation of NF-kappa B under different flow regimens across time (0.5-24 h) was investigated. Nuclear proteins from flow-conditioned human aortic endothelial cells (HAEC) were analyzed by electrophoretic mobility shift assay using [gamma-32P]dATP-labeled NF-kappa B-specific oligonucleotide. Our results demonstrated that NF-kappa B activation was significantly elevated in HAEC exposed to prolonged (> 2 h) steady low shear (2 dyn/cm2) and pulsatile low shear (2 +/- 2 dyn/cm2) compared with HAEC exposed to high shear (16 dyn/cm2). In contrast, at 30 min, high shear-exposed HAEC exhibited an early, transient increase in NF-kappa B activity, relative to low shear-exposed cells, which reversed on continued exposure to high shear. Maximum activity in both low shear- and pulsatile low shear-conditioned HAEC was observed at 16 h compared with HAEC exposed to prolonged high shear. These results indicate that exposure of HAEC to prolonged low shear conditions is associated with significantly increased and prolonged NF-kappa B activity. This observation might provide a mechanism to explain the increased monocyte adhesion in atherosclerosisprone arterial sites exposed to chronic low-shear flow patterns.

Fluid shear stress-mediated signal transduction: how do endothelial cells transduce mechanical force into biological responses?

We propose a model for signaling events induced by fluid shear stress that incorporates many of the features discussed in this paper (FIG. 4). First, heterotrimeric G-proteins, as well as a small G-proteins, are activated by flow. Indeed, a G protein appears to be required for ERK1/2 activation by flow because ERK1/2 activation is completely inhibited by GDP-beta S. Then, flow activates phospholipase C and generates IP3 and diacylglycerol (DG). IP3 releases Ca2+ from internal Ca2+ stores via IP3 receptor and DG activates PKC. Nollert and colleagues have shown that flow activates PLC and increases IP3. It is possible that several different PKC isozymes are activated by flow including both Ca(2+)-dependent and Ca(2+)-independent isozymes. These different isozymes may have specific downstream substrates. For example, PKC-epsilon may be involved in activation of ERK1/2, while the PKC isozyme responsible for activation of JNK remains unknown. It is also possible that these PKC isozymes may be important in gene transcription events. For example, PKC-zeta has been suggested to be involved in NF-kappa B-mediated gene transcription. Longer term changes in endothelial cell morphology and structure are likely to involve separate kinases. Important candidates for these changes include members of the c-Src and FAK families. c-Src is now considered to be a component of the focal adhesion complex and regulate focal adhesion formation and/or cytoskeletal rearrangement. Recently, stretch, another mechanostress, has been shown to activate c-Src in fetal rat lung cells. It has been clarified that ERK1/2 and JNK are regulated by the small G-proteins, Ras and Rac/Cdc42H, respectively, and their effectors in parallel with each other. Rac and Rho are also thought to be involved in membrane ruffling and/or cytoskeletal rearrangement. Fluid shear stress causes stress fiber formation and focal adhesion rearrangement. Recent study by Malek and Izumo suggested the importance of microtubules in shear stress-induced morphological change and actin stress fiber formation. It is clear that the focal adhesion complex plays an important role in shear stress-induced signal and it is interesting to speculate that shear stress-induced signaling has cross-talk with signaling induced by integrins. As a general model we propose that the integration between the rapid events stimulated by shear stress and the longer term events is mediated by tyrosine kinases that serve to regulate these multiple signal transduction pathways.

Endothelial cell injury in cardiovascular surgery: atherosclerosis

Most of the indications for cardiovascular operation and many of its complications are in large part due to advanced atherosclerosis. The pathogenesis of atherosclerosis involves inflammatory infiltration of the vessel wall, cellular proliferation, fibrous plaque formation, and ultimately plaque rupture and occlusive thrombosis. Many of these events are linked, at least initially, to chronic injury of the vascular endothelium. Endothelial cell injury from hypertension, diabetes mellitus, hyperlipidemia, fluctuating shear stress, smoking, or transplant rejection disrupts normal endothelial cell function. This results in the loss of the constitutive protective mechanisms and an increase in inflammatory, procoagulant, vasoactive, and fibroproliferative responses to injury. These changes promote vasospasm, intimal proliferation, and thrombus formation, all of which play a significant role in the initiation, progression, and clinical manifestations of atherosclerosis. Understanding the role of the chronically injured endothelium and its interactions with circulating immune cells and the underlying smooth muscle cells may lead to novel therapeutic interventions for the prevention and treatment of atherosclerosis.

Shear stress induction of the tissue factor gene

Using flow channel, we report that the application of a laminar shear stress induced a transient increase of tissue factor (TF) procoagulant activity in human umbilical vein endothelial cells (HUVEC), which was accompanied by a rapid and transient induction of the TF mRNA in the HUVEC. Functional analysis of the 2.2 kb TF 5' promoter indicated that a GC-rich region containing three copies each of the EGR-1 and Sp1 sites was required for induction. Mutation of the Sp1 sites, but not the EGR-1 sites, attenuated the response of TF promoter to shear stress. Thus, Sp1 is a newly defined shear stress responsive element. Electrophoretic mobility shift assays showed there was no increase in binding of nuclear extracts from sheared cells to an Sp1 consensus site. In contrast, immunoblotting of these nuclear extracts with antibody against transcription factor Sp1 demonstrated that shear stress increased the phosphorylation of Sp1. We also showed that shear stress, like the phosphatase inhibitor okadaic acid, increased the transcriptional activity of Sp1. These findings suggest that the shear stress induction of TF gene expression is mediated through an increased Sp1 transcriptional activity with a concomitant hyperphosphorylation of Sp1.

Effects of curvature and stenosis-like narrowing on wall shear stress in a coronary artery model with phasic flow

To gain insight into the details of intracoronary flow we have used computational fluid dynamic techniques to determine the velocity and wall shear stress distributions in both steady- and phasic-flow models of a curved coronary artery with several degrees of stenosis. The steady-flow Reynolds number was 500 and the peak phasic flow Reynolds number was 700. Without stenosis and at 25% (area) stenosis wall shear stress and velocities are higher at the outer wall than the inner wall but retain the same direction as the superimposed flow. At higher stenoses laminar flow separation occurs and the inner wall is exposed to shear stresses that vary widely, both temporally and spatially.

Spatial relationships in early signaling events of flow-mediated endothelial mechanotransduction

Blood flow interactions with the vascular endothelium represent a specialized example of mechanical regulation of cell function that has important physiological and pathological cardiovascular consequences. The endothelial monolayer in vivo acts as a signal transduction interface for forces associated with flowing blood (hemodynamic forces) in the acute regulation of artery tone and chronic structural remodeling of arteries, including the pathology of atherosclerosis. Mechanisms related to spatial relationships at the cell surfaces and throughout the cell that influence flow-mediated endothelial mechanotransduction are discussed. In particular, flow-mediated ion channel activation and cytoskeletal dynamics are considered in relation to topographic analyses of the luminal and abluminal surfaces of living endothelial cells.

Hemodynamic modulation of monocytic cell adherence to vascular endothelium

Hemodynamic shear stress is hypothesized to contribute to the localization of atherosclerotic plaques to certain arterial sites. Monocyte recruitment to these sites is an early event in artherogenesis. To determine the possible mechanisms by which shear stress modulates monocyte adhesion in vivo, studies of human monocytic cell adherence to endothelium were conducted under different shear conditions in a parallel-plate flow chamber. The number of monocytes capable of developing firm adhesive contacts with endothelium decreased as shear stress-induced drag forces increased over the range of 0.5 to 30 dynes/cm2. The number of adherent monocytic cells at a given shear stress was highly dependent on the activation state of the endothelium. To test the direct effect of shear stress on endothelial cell adhesivity, endothelial cells were presheared for 2 to 6 hr at 2, 10, or 30 dynes/cm2, and monocytic cell adherence was quantified at 1 dyne/cm2. Adherence increased 330% or 370% when endothelial cells were presheared for 2 hr at 2 or 10 dynes/cm2, respectively, as compared to unsheared endothelium. In contrast, when endothelial cells were presheared at 30 dynes/cm2, monocytic cell adherence at 1 dyne/cm2 was not significantly different from unsheared controls. Increased monocytic cell adherence to presheared endothelium was via a vascular cell adhesion molecule 1 (VCAM-1)/alpha(4) beta(1) mechanism, and enzyme-linked immunosorbent assay studies showed that preshearing at 2 dynes/cm2 for 2 hr increased endothelial VCAM-1 expression by 38%. These data demonstrate that low levels of shear stress induce endothelial VCAM-1 expression and increase monocytic cell adherence via a VCAM-1/alpha 4 beta 1 mechanism. Thus, shear stress can modulate monocyte adherence to vascular endothelium through drag forces that affect the establishment and maintenance of adhesive bonds and by directly modulating the expression of endothelial VCAM-1. This dual effect of shear stress produces the most favorable conditions for adhesion at low-shear regions, where drag forces are low and induction of VCAM-1 is likely. The preferential adherence of monocytes to these regions may contribute to the localization of atherosclerotic plaques to low-shear regions of the arterial circulation in vivo.

A mechanism for heterogeneous endothelial responses to flow in vivo and in vitro

Exposure of endothelium to a nominally uniform flow field in vivo and in vitro frequently results in a heterogeneous distribution of individual cell responses. Extremes in response levels are often noted in neighboring cells. Such variations are important for the spatial interpretation of vascular responses to flow and for an understanding of mechanotransduction mechanisms at the level of single cells. We propose that variations of local forces defined by the cell surface geometry contribute to these differences. Atomic force microscopy measurements of cell surface topography in living endothelium both in vitro and in situ combined with computational fluid dynamics demonstrated large cell-to-cell variations in the distribution of flow-generated shear stresses at the endothelial luminal surface. The distribution of forces throughout the surface of individual cells of the monolayer was also found to vary considerably and to be defined by the surface geometry. We conclude that the endothelial three-dimensional surface geometry defines the detailed distribution of shear stresses and gradients at the single cell level, and that there are large variations in force magnitude and distribution between neighboring cells. The measurements support a topographic basis for differential endothelial responses to flow observed in vivo and in vitro. Included in these studies are the first preliminary measurements of the living endothelial cell surface in an intact artery.

Effects of wall shear stress and fluid recirculation on the localization of circulating monocytes in a three-dimensional flow model

There is a correlation between the location of early atherosclerotic lesions and the hemodynamic characteristics at those sites. Circulating monocytes are key cells in the pathogenesis of atherosclerotic plaques and localize at sites of atherogenesis. The hypothesis that the distribution of monocyte adhesion to the vascular wall is determined in part by hemodynamic factors was addressed by studying monocyte adhesion in an in vitro flow model in the absence of any biological activity in the model wall. Suspensions of U937 cells were perfused (Re = 200) through an axisymmetric silicone flow model with a stenosis followed by a reverse step. The model provided spatially varying wall shear stress, flow separation and reattachment, and a three-dimensional flow pattern. The cell rolling velocity and adhesion rates were determined by analysis of videomicrographs. Wall shear stress was obtained by numerical solution of the equations of fluid motion. Cell adhesion patterns were also studied in the presence of chemotactic peptide gradients. The cell rolling velocity varied linearly with wall shear stress. The adhesion rate tended to decrease with increasing local wall shear stress, but was also affected by the radial component of velocity and the dynamics of the recirculation region and flow reattachment. Adhesion was increased in the vicinity of chemotactic peptide sources downstream of the expansion site. Results with human monocytes were qualitatively similar to the U937 experiments. Differences in the adhesion rates of U937 cells occurring solely as a function of the fluid dynamic properties of the flow field were clearly demonstrated in the absence of any biological activity in the model wall.

Shear stress-mediated changes in the expression of leukocyte adhesion receptors on human umbilical vein endothelial cells in vitro

Extensive monocyte recruitment is an early phenomenon associated with the development of atherosclerotic lesions, suggesting an active role for the involvement of adhesion receptors expressed by endothelial cells. In this study we describe the contribution of hemodynamic shear forces in regulating the expression of a few of the monocyte adhesion receptors, including intercellular adhesion molecule (ICAM-1), vascular cell adhesion molecule (VCAM-1), and E-selectin on endothelial cells. A parallel plate flow chamber and recirculating flow loop device was used to expose human umbilical vein endothelial cells (HUVECs) to different levels of shear (2-25 dyn/cm2). Subsequently the cells were analyzed either for shear induced changes in the mRNA levels of adhesion receptors by Northern blot analyses or for changes in the surface expression of ICAM-1 using flow cytometry. Results from the fluorescence analysis showed a transient increase in the surface expression of ICAM-1, 12 hr after exposure to 25 dyn/cm2 shear, returning to basal levels within 24 hr. This was quite different from the time dependent response of ICAM-1 to lipopolysaccharide (LPS), where ICAM-1 expression was maximally induced 18-24 hr post-stimulus. ICAM-1 mRNA level appeared slightly elevated after exposure to shear for 1 hr, compared to basal values, but dropped below basal levels within 6 hr. This biphasic response was seen irrespective of the magnitude of applied shear stress. VCAM-1 mRNA expression, in contrast, decreased below the baseline expression within an hour after onset of flow, and appeared to be considerably down-regulated within 6 hr. After exposure to shear for 24 hr, no increase in mRNA levels could be detected for either molecule, at any shear magnitude. E-selectin mRNA was less responsive to shear stress, especially at the lower magnitudes of shear. After an hour of exposure to flow E-selectin mRNA level appeared slightly reduced compared with control levels, but it remained at this level even after 6 hr of flow. These results indicate that the expression of adhesion receptors is sensitive to local shear stresses in a manner that is molecule specific in the short term even though prolonged exposure to flow results in similar down-regulation for both ICAM-1 and VCAM-1.