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(Hypertension. 1996;28:386-391.)
© 1996 American Heart Association, Inc.

Articles

Cyclic Strain Enhances Adhesion of Monocytes to Endothelial Cells by Increasing Intercellular Adhesion Molecule-1 Expression

Jing-Jy Cheng; Being-Sun Wung; Yeun-Jen Chao; Danny Ling Wang

the Cardiovascular Division, Institute of Biomedical Sciences, Academia Sinica (Y.-J.C., D.L.W.), and Graduate Institute of Life Sciences, National Defense Medical Center (J.-J.C., B.-S.W.), Taipei, Taiwan, ROC.

Correspondence to Danny Ling Wang, PhD, Cardiovascular Division, Institute of Biomedical Sciences, Academia Sinica, Taipei, Taiwan, ROC 11529. E-mail lingwang@ibms.sinica.edu.tw.

	Abstract

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Since endothelial cells are constantly subjected to pressure-induced strain, we examined how cyclic strain affects the expression of intercellular adhesion molecule-1 (ICAM-1). Endothelial cells grown on a flexible membrane base were deformed by different sinusoidal negative pressures (-10, -15, or -20 kPa) to produce an average strain of 9%, 11%, and 12%, respectively, for various times. The release of the soluble form of ICAM-1 from strained endothelial cells increased in a time- and strain-dependent manner. Using flow cytometric analysis, we showed the induction of ICAM-1 expression on the endothelial cell surface to depend on both time and the amount of strain. Northern blot analysis revealed a sustained, approximately 1.8-fold increase in ICAM-1 mRNA levels in 11% strained cells. Strain-induced expression of ICAM-1 correlated with a strain-dependent increase in adhesion of monocytic cells to strained cells. This increase in monocytic cell adhesion could be partially inhibited by pretreatment of strained cells with antibody to ICAM-1. These results indicate that mechanical strain can stimulate the expression of ICAM-1 by endothelial cells and thus contribute to the increased adhesion of monocytes to strained cells. Such strain-induced expression of ICAM-1 may contribute to the trapping of monocytes on local vascular walls where strain is high and to the initiation of atherogenesis, thus providing a possible link between hypertension and atherogenesis.

Key Words: intercellular adhesion molecule-1 • genes • monocytes • stress, mechanical

	Introduction

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Vascular ECs, which synthesize and release substances important for maintaining normal vascular homeostasis, are constantly exposed to mechanical forces, including blood flow–induced shear stress and pressure-induced strain. These mechanical forces play an important role in modulating various important physiological processes, including regulation of vascular tone and maintenance of normal hemostasis.¹ The effects of shear stress on endothelial biology and gene expression have been intensively studied.¹ Among those genes that can be modulated by shear stress are endothelin-1,² ICAM-1,^{3 4} platelet-derived growth factor,⁵ and MCP-1.⁶ Despite the intensive studies on shear stress effects, the effects of pressure-induced strain on ECs are less well understood. Recent data^{4 7 8} and our previous studies^{9 10} have demonstrated that fluid shear stress and mechanical strain can have significantly different effects in the modulation of gene expression. In contrast to the downregulation of endothelin-1 and transitory upregulation of MCP-1 and ICAM-1 by physiological fluid shear stress, cyclic strain can induce a sustained elevated expression of endothelin-1^{4 9 11} and MCP-1 mRNAs,¹⁰ with a subsequent increase in the secretion of both proteins into culture media.^{9 12} The modulation of both genes can have a significant effect on vascular integrity, particularly in atherogenesis and other cardiovascular disorders. Augmented MCP-1 release from strained ECs can result in an increased monocytic adhesion to ECs¹⁰ and has been suggested to play an essential role during atherogenesis.¹³

In addition to MCP-1 production induced by mechanical forces, the gene expression of ICAM-1, but not of VCAM-1 and E-selectin, can be modulated by fluid shear stress.^{3 4} The effects of cyclic strain in the expression of adhesion molecules on ECs, however, have not been reported. Our previous studies^{10 12} have demonstrated that cyclic strain can induce MCP-1 mRNA expression and stimulate MCP-1 release from ECs. Although strain-induced MCP-1 production can contribute to the increased adhesion of monocytes to strained ECs,¹⁰ other adhesion molecules that are required for monocytic adhesion may also be modulated by strain treatment and thus contribute to monocytic adhesion and transmigration. Our studies have aimed to clarify these possibilities. The present studies clearly demonstrate that ECs subjected to strain increase the expression and release of ICAM-1 by ECs. This leads to an increased monocytic adhesion that can be partially inhibited by treating ECs with antibody to ICAM-1. Thus, in addition to the stimulation of MCP-1 production, strain treatment can also increase the expression of ICAM-1 by ECs, which will facilitate the monocytic adhesion and transmigration through the endothelial layer. This strain-induced ICAM-1 expression may play an important role in the recruitment of leukocytes/monocytes into vessel wall during atherogenesis.

	Methods

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Materials
The ICAM-1 enzyme-linked immunosorbent assay kit, monoclonal antibody to ICAM-1, and ICAM-1 cDNA probes were obtained from R&D Systems, Inc. The human monocytic cell line THP-1 was obtained from the American Type Culture Collection and maintained in RPMI-1640 (GIBCO-BRL) supplemented with 10% fetal calf serum, 2 mmol/L L-glutamine, 10⁵ U/L penicillin, 150 µmol/L streptomycin, and 100 µmol/L ß-mercaptoethanol. All other chemicals of reagent grade were purchased from Sigma Chemical Co.

EC Culture
Human umbilical vein ECs were isolated from human umbilical cords as previously described.⁹ ECs were grown in Petri dishes for 3 days and then seeded onto the silicon membrane base of a cell plate (Flexcell International Co) until the monolayer became confluent. The culture medium was then changed to medium 199 containing 2% fetal calf serum, and the cells were incubated overnight before the experiment. Experiments were conducted with ECs pooled from several human umbilical cords. Different EC batches responded similarly under strain.

In Vitro Cyclic Strain on Cultured ECs
The strain unit (Flexcell FX-2000), which consisted of a vacuum unit linked to a valve controlled by computer, has been previously described in detail.^{14 15} Strain profile analysis¹⁵ indicates that radial strain responds nonlinearly to increasing vacuum pressure. Peak radial strain occurs at the radial distance of 9.5 mm from the center.¹⁵ The flexible membrane supporting the cultured cells was deformed by a sinusoidal negative pressure with a peak level of -10 kPa, which produced a strain on cells ranging from minimal strain at the center of the membrane to a peak value of 17% at the periphery (maximal strain, 17%; average strain, 9%), -15 kPa (maximal strain, 20%; average strain, 11%), or -20 kPa (maximal strain, 25%; average strain, 12%) at a frequency of 1 Hz (60 cycles per minute) for various durations. The average strain was calculated by averaging radial strain over the total plate surface area. For the purpose of simplification, we designated these three groups as 9%, 11%, and 12% average strain groups. Subsequent to the strain exposure, the conditioned supernatant was assessed for soluble adhesion molecule content, and the total RNAs were assayed for ICAM-1 mRNA concentration. For some experiments, THP-1 cells were added directly onto strained ECs for monocytic adhesion studies. After strain treatment, ECs remained intact on the flexible membrane, as shown by trypan blue staining and by the quantities and qualities of the total RNA collected.

Enzyme-Linked Immunoassays
The soluble form of ICAM-1 in collected culture medium was analyzed as described in the immunoassay kits (R&D Systems). Typical sensitivity of these assays was less than 1 ng/mL. Soluble ICAM-1 in culture medium containing 2% fetal calf serum was used as a blank control. The standard concentration curve by this method was linear from 2 to 45 ng/mL.

Immunofluorescence
After strain treatment, ECs were washed three times with medium 199. Cells were detached by treatment with buffer containing Versene (EDTA) for 5 minutes. Suspended cells were centrifuged and the pellets washed twice with phosphate-buffered saline (PBS) containing 0.5% bovine serum albumin. Cells were resuspended and incubated in 0.2 mL PBS containing monoclonal anti–human ICAM-1 (IgG_1a, 0.14 µmol/L). After incubation at 4°C for 30 minutes, cells were centrifuged at 13 000 rpm and washed twice with PBS. Cells were subsequently incubated with fluorescein isothiocyanate–labeled goat anti-mouse IgG for 30 minutes at 4°C. ECs were washed twice with PBS, fixed in 2% paraformaldehyde, and analyzed with a fluorescence-activated cell sorter (FAC-Scan, Becton Dickinson) using 10⁴ cells per sample. After correction for nonspecific binding, specific mean fluorescence intensity was obtained.

Cell Adherence Measurements
THP-1 cells suspended in RPMI-1640 containing 0.1% fetal bovine serum were labeled with 1 µCi [³H]thymidine overnight (specific activity, 23 Ci/mmol; Amersham). Cells were washed three times in fresh RPMI-1640 medium. THP-1 cells (3x10⁵) were then added to each well containing ECs and incubated for 1 hour. Nonadherent THP-1 cells were removed by washing with medium 199. ECs with adherent THP-1 cells were lysed with lysis buffer, and radioactivity was counted by a scintillation counter.

RNA Isolation and Northern Hybridization
Total RNA isolation was obtained with guanidine thiocyanate as described previously.^{9 10} In some experiments, RNA was collected from cells at the periphery of the well by removing cells at the center (area is within 0.4 cm radius from the center) with a button punch or cotton ball. Radial strain profile analysis indicates that this center area has low strain versus the high-strain region at the periphery.¹⁵ RNA thus collected was transferred onto a membrane by a vacuum blotting system (VacuGene XL, Pharmacia). After hybridization with the ³²P-labeled ICAM-1 cDNA, the membrane was washed with 1x SSC containing 1% sodium dodecyl sulfate at room temperature for 15 minutes and exposed to x-ray film (Kodak X-Omat-AR) at -70°C. Autoradiographic results were scanned and analyzed with a densitometer (Computing Densitometer 300S, Molecular Dynamics).

Statistical Analysis
Statistical analyses were performed with Student's t test for experiments consisting of two groups only and with ANOVA for experiments consisting of more than two groups. Data are expressed as mean±SE. Statistical significance was defined at a value of P<.05.

	Results

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Cyclic Strain Increases Release of Soluble ICAM-1 From ECs
Cultured ECs maintained in medium 199 with 2% serum constantly secreted ICAM-1 into the cultured medium in a time-dependent manner (Fig 1). ECs under 9% strain did not significantly increase their ICAM-1 release after 36 and 48 hours of treatment compared with unstrained control ECs. In contrast, 11% and 12% strain for 36 and 48 hours significantly increased ICAM-1 release (Fig 1). Furthermore, ICAM-1 released from cells treated with 12% strain was significantly higher than from cells under 11% strain. Basal release rate, as calculated from the rise in the ICAM-1 concentration in medium from unstrained control cells between 36 and 48 hours, was about 0.015 ng/h per 10⁵ cells. Cells under 11% strain increased their ICAM-1 release rate by 33%, to 0.02 ng/h per 10⁵ cells. The ICAM-1 release rate of cells treated with 12% strain increased further to 0.03 ng/h per 10⁵ cells, double the rate in unstrained control cells. Lipopolysaccharide treatment of ECs as a positive control significantly induced ICAM-1 release. Although the strain-induced ICAM-1 release was low compared with lipopolysaccharide treatment of ECs, mechanical strain could stimulate ICAM-1 release in a dose-dependent manner. These results indicate that mechanical strain dose-dependently induces the release of soluble ICAM-1.

View larger version (39K): [in this window] [in a new window]   Figure 1. Cyclic strain increases soluble ICAM-1 release in a time- and strain-dependent manner. ECs were subjected to strain of 9%, 11%, or 12% for 24, 36, and 48 hours, respectively. Cells treated with lipopolysaccharide (LPS) (1 ng/mL) for 24 hours were used as a positive control. Each column represents mean±SE from a total of 18 wells from at least three separate experiments. *P<.05 vs unstrained control cells; #P<.05 vs unstrained control group and cells under 9% and 11% strain.

Cyclic Strain–Induced ICAM-1 Expression on ECs Is due to the Increase of ICAM-1 mRNA
To demonstrate that ICAM-1 expression on the surface of ECs increased after strain treatment, we measured surface ICAM-1 expression on strained ECs by flow cytometry as indicated by fluorescence intensity. As shown in Fig 2, for cells subjected to 11% strain, the relative mean fluorescence intensity increased significantly from 8.4±0.8 in unstrained control groups to 9.1±0.6 after 12 hours and 10.4±1.2 after 24 hours. ECs subjected to 12% strain increased the surface ICAM-1 fluorescence intensity further to 12.1±0.7 and 21.9±3.0 after 12- and 24-hour treatment, respectively. This strain-induced ICAM-1 expression on the EC surface, however, was significantly less than that induced by lipopolysaccharide. To study whether strain-induced ICAM-1 release and expression on the cell surface were due to the increased gene induction, we analyzed RNA collected from control and strained groups for ICAM-1 mRNA content. As shown in Fig 3, ECs under 11% strain for 1 hour did not significantly increase ICAM-1 mRNA levels. However, ECs under continuous strain for 3 and 24 hours showed 1.5- and 1.8-fold increases in ICAM-1 mRNA levels, respectively, compared with the control groups. Thus, this elevated ICAM-1 mRNA level can be sustained if strain is maintained. Since this strain device provided higher strain at the periphery of each flexible well^{12 15} and to demonstrate further that this ICAM-1 induction depended on the amount of strain, we collected the cells at the periphery and center of each well separately and analyzed mRNA levels for each group. As shown in Fig 3, ECs at the periphery of the well (SP) expressed a higher level of ICAM-1 mRNA than cells at the center (SC). Thus, we conclude that the strain treatment of ECs induces ICAM-1 expression on the EC surface in a time- and strain-dependent manner and that this elevated expression is due to a sustained increase in ICAM-1 mRNA level in the strained ECs.

View larger version (29K): [in this window] [in a new window]   Figure 2. Cyclic strain increases surface ICAM-1 expression as represented by mean fluorescence intensity (MFI). ECs were subjected to strain of 11% or 12% for 12 and 24 hours, respectively. Cells treated with lipopolysaccharide (LPS) (1 ng/mL) for 24 hours were used as a positive control. Each column represents mean±SE from three to four separate experiments. *P<.05 vs unstrained control cells; #P<.05 vs ECs under 12% strain for 12 hours.

View larger version (35K): [in this window] [in a new window]   Figure 3. Cyclic strain increases ICAM-1 gene expression in ECs. Confluent ECs grown on flexible membrane bases were mechanically strained by 11% for 1 (S1), 3 (S3), and 24 (S24) hours. In some experiments, cells were removed from the center of the well, followed by immediate extraction of RNA from cells remaining at the periphery (SP). In certain wells, ECs at the periphery were removed, and cells at the center were collected for RNA extraction (SC). Total RNA was isolated for Northern blot analysis with 32P-labeled ICAM-1 cDNA as a probe. The 18S rRNA staining indicates that equal amounts of RNA were loaded. Autoradiographic results were analyzed by densitometry; data are presented as percent change in relative band density normalized to 18S RNA band and are shown as mean±SE from three to four experiments. *P<.05 vs unstrained control cells (C).

Increased ICAM-1 Expression Results in Increased THP-1 Cell Adhesion to Strained ECs
ECs subjected to strain were later incubated with THP-1 cells. Cells under 9% strain did not significantly increase THP-1 cell adhesion (data not shown) compared with unstrained cells. However, increased THP-1 cell adhesion to cells under 11% and 12% strain was demonstrated to be strain dependent (Fig 4a). We have previously demonstrated that MCP-1 can be induced by strain and its release into culture medium can stimulate monocyte adhesion to ECs.¹⁰ To demonstrate that in addition to MCP-1, strain-induced ICAM-1 expression on the EC surface could also enhance monocyte adhesion, we removed culture medium from strained cells, replaced it with fresh medium, and subsequently incubated ECs with THP-1 cells for 1 hour. The previously strained cells were still able to enhance adhesion of the THP-1 cells, although to a lesser degree (Fig 4b). ECs previously under 12% strain were shown to have a higher ability to induce THP-1 cell adhesion. When the medium from ECs treated with 12% strain was replaced with control medium, the number of adhesive monocytes decreased from 250% to 158% of the number in unstrained control groups (Fig 4c). Furthermore, when the medium from control ECs was replaced with medium from the cells treated with 12% strain, a 50% increase in monocyte adhesion resulted. To further evaluate whether the increased adhesion of monocytes to strained cells was due to the increase of ICAM-1 expression, we pretreated the strained cells containing the original medium with anti–ICAM-1 before the addition of THP-1 cells. As shown in Fig 4d, although anti–ICAM-1 pretreatment decreased THP-1 cell adhesion to control ECs, strained ECs were shown to have a much larger reduction in THP-1 adhesion compared with that of strained but untreated ECs (Fig 4a). Therefore, strain-induced ICAM-1 expression contributes to increased monocytic adhesion to strained ECs.

View larger version (15K): [in this window] [in a new window]   Figure 4. Bar graph shows increase of THP-1 cells adhering to strained ECs. ECs were subjected to strain of 11% or 12% for 24 hours before addition of 3H-labeled THP-1 cells. After incubation of THP-1 cells with strained ECs for 1 hour, all adhered cells were lysed, and total radioactivity was counted by liquid scintillation. THP-1 cells were applied directly onto strained cells (a). Culture medium was replaced with fresh medium before incubation with THP-1 cells (b). Culture medium from unstrained control ECs and strained ECs was exchanged before addition of THP-1 cells (c). Anti–ICAM-1 (20 µg/mL) was preincubated with control and strained ECs containing original medium before the introduction of THP-1 cells (d). ECs pretreated with lipopolysaccharide (LPS) (1 ng/mL) for 24 hours were used as a positive control. Each column represents mean±SE from a total of 12 wells from three to four separate experiments. *P<.05 vs unstrained controls; #P<.05 vs respective ECs under 11% strain; +P<.05 vs respective strained ECs in (a).

	Discussion

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It has been reported that flow-induced shear stress can selectively upregulate the gene expression of ICAM-1 but not of VCAM-1 and E-selectin.^{3 4} However, this flow-induced ICAM-1 expression was shown to be transient.⁴ In contrast to the shear stress stimuli, the effects of mechanical strain and the involved intracellular signals and gene regulation in ECs are less well understood. We previously reported that cyclic strain induced a sustained expression of endothelin-1^{9 11} and MCP-1 genes.¹⁰ In addition, cyclic strain induced MCP-1 secretion in a time- and strain-dependent manner,¹² and this increased MCP-1 secretion resulted in an increased monocytic adhesion that could be partially inhibited by pretreatment of ECs with anti–MCP-1.¹⁰ The collected culture medium containing higher MCP-1 concentration could augment the monocyte adhesion to ECs, and this increased adhesion could be partially blocked by preincubation of the collected culture medium with anti–MCP-1. However, the adhesion of monocytes and subsequent transmigration to the subendothelial space of the ECs require the expression of adhesion molecules, which may include ICAM-1, VCAM-1, and E-selectin, on the EC surface.¹⁶ The present report focuses exclusively on ICAM-1 expression on strained cells and its adhesive role during monocyte-EC interaction. Our studies demonstrate that mechanical strain can elevate ICAM-1 expression on ECs and increase the release of soluble ICAM-1 into culture medium. The expression and release of ICAM-1 depend on time and the amount of strain. These strained cells when replenished with fresh medium could still promote monocyte adhesion, and preincubation of the strained cells with anti–ICAM-1 reduced the adhesiveness of monocytes, both clearly indicating that strain-induced ICAM-1 expression on the EC surface plays an important adhesive role during monocyte-EC interaction. Furthermore, increased ICAM-1 release and expression on the EC surface correlated with increased monocytic adhesion, which also occurred in a strain-dependent manner. Thus, in addition to the MCP-1 induction, cyclic strain upregulates ICAM-1 expression, which acts in concert with MCP-1 production to facilitate monocytic adhesion to strained ECs.

ECs expressing various adhesion molecules including ICAM-1 have been studied intensively.¹⁶ With the expression on activated ECs, leukocyte adherence and transmigration are enhanced. During atherogenesis, monocyte-EC interaction in stimulated ECs may be mediated in part by both ICAM-1 and VCAM-1.¹⁷ Studies have also demonstrated that soluble ICAM-1 can be released from cytokine-stimulated ECs. Furthermore, elevated soluble ICAM-1 levels in serum have been noted in patients with melanoma^{18 19} or during inflammatory states.²⁰ Raised soluble ICAM-1 has also been found in patients with vascular and ischemic heart disease and has been suggested to be a useful index of EC activation in clinically manifested atherosclerosis.²¹ Despite its elevation in sera from various pathological states, the mechanism of the release of soluble ICAM-1 and its pathological importance still require clarification. It has been suggested that soluble ICAM-1 is the result of proteolytic cleavage of membrane-bound ICAM-1,²² since the molecular weight of the releasing molecule is consistent with the size by cleavage at a site close to the point of membrane insertion.²³ A recent study indicates that ICAM-1 binds to the cytoskeleton through linkage with -actinin.²⁴ Proteolytic activity of strained ECs may activate proteolytic enzymes that act on those membrane insertion points. Since no storage form of ICAM-1 has been found, the induced expression of ICAM-1 probably depends on the synthesis of new mRNA and protein. Increasing release of soluble ICAM-1 from strained ECs may be the consequence of elevated ICAM-1 production. In contrast to VCAM-1 and E-selectin, ICAM-1 is constitutively expressed on the surface of ECs.¹⁶ The increased expression of ICAM-1 followed by its release from ECs may be an autoprotective mechanism that prevents any further leukocyte adhesion. Our results indicate that ECs under 9% strain did not increase ICAM-1 release or expression. In contrast, we observed graded responses in ECs under 11% and 12% strain. This graded response to strain is consistent with previous reports which show that tissue plasminogen activator activity²⁵ and MCP-1 release¹² can only be detected in cells subjected to more than 7% and 10% cyclic strain, respectively. These results, together with the observation of expression of higher ICAM-1 mRNA levels in cells near the periphery of wells where strain is high, strongly indicate that ICAM-1 production depends on the amount of strain and in turn are consistent with the results of dose-dependent increases in the surface expression of the cell. This strain-induced graded ICAM-1 response also results in a dose-dependent increase in THP-1 cell adhesion.

The cyclic strain–induced ICAM-1 gene expression is relatively low compared with that induced by cytokines. This may be attributed to our stretch device, which does not provide uniform stretch.¹⁵ In our previous reports,^{10 12} we have demonstrated that cells grown at the periphery of wells where strain is greatest show higher MCP-1 gene expression than those cells at the center where strain is least. The present results represented a mixture of high- and low-strained cells, and this may contribute to the lower ICAM-1 induction observed in the present study. Unfortunately, no in vitro model can completely reproduce the in vivo milieu. In vivo, ECs are constantly subjected to hemodynamic forces, including flow-induced shear stress and pressure-produced strain. It has been reported that c-fos gene induction in ECs is higher under pulsatile flow than under steady flow treatment.²⁶ Since rhythmic distension of the vessel wall is a component of pulsatile flow, cyclic strain on vessel walls may thus play an important role in the modulation of gene expression. Furthermore, studies indicate that strains are significantly higher at branch sites than in the straight segments of vessel walls.²⁷ Thus, local ICAM-1 expression may be higher at certain geometric areas. Whether those branch sites subjected to higher strain induce a higher basal ICAM-1 expression in vivo remains to be investigated.

Earlier reports indicate that hypertension is usually associated with an increase in both the extent and severity of atherosclerosis.²⁸ Earlier studies²⁹ have demonstrated that hypertension is accompanied by an increased EC turnover and a concomitant enhancement of permeability to macromolecules, particularly in the branching regions of the aorta. Recent studies indicate that hypertension may enhance the responsiveness of the endothelium to factors that promote monocyte adhesion.³⁰ Increased soluble E-selectin in essential hypertension has been documented.³¹ Although the basic mechanism of hypertension-induced vascular disorder remains complicated and unclear, the results from our in vitro model may provide some insight into the initial mechanisms by which certain geometric locations subjected to higher strain in the vessel wall may be more vulnerable to vascular disorder. Whether the above-normal strain applied to vascular walls by high blood pressure contributes to these vascular abnormalities in hypertensive individuals remains to be elucidated.

A previous report has demonstrated that fluid shear stress can modulate ICAM-1 expression.^{4 32} Laminar flow induces a significant time-dependent increase in the surface expression of ICAM-1 on ECs. It has been suggested that a shear stress–responsive element (SSRE) in the 5' promoter region is involved in those shear stress–inducible genes, including ICAM-1.^{3 33} The transcription factor nuclear factor-B (NF-B) was later found to be activated by shear stress.³⁴ Further studies indicate that after cytokine treatment, the activator protein-1 (AP-1) and NF-B binding regions in 5' promoter regions are involved in ICAM-1 gene expression.³⁵ Strain-activating nuclear factors binding to the AP-1 region have also been reported.³⁶ Whether mechanical strain–induced ICAM-1 expression is mediated through these binding activities remains to be determined. It would be of interest to further elucidate the mechanism of intracellular signals and gene regulation involved in this mechanical strain–induced gene expression. Understanding the effects of mechanical strain on ECs and the intracellular responses will provide a fundamental knowledge of vascular biology in normal and pathological states, including hypertension-induced vascular disorders.

In summary, cyclic strain stimulates the expression of ICAM-1 and the release of soluble ICAM-1 from human ECs. The increase of ICAM-1 expression by cyclic strain results in an enhanced adhesion of THP-1 monocytic cells. These results clearly indicate that hemodynamic forces can play a significant role in the modulation of gene expression of ECs and thus affect the leukocyte-EC interaction.

	Selected Abbreviations and Acronyms

 

EC = endothelial cell ICAM-1 = intercellular adhesion molecule-1 MCP-1 = monocyte chemotactic protein-1 VCAM-1 = vascular adhesion molecule-1

	Acknowledgments

 
This work was supported in part by grant NSC85-2231-B-001-021 from the National Science Council, Taiwan, ROC. We thank Dr H.J. Hsieh, Department of Chemical Engineering, National Taiwan University, for his help on calculating the average strain.

Received November 17, 1995; first decision January 10, 1996; accepted April 19, 1996.

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