Levels of Soluble Cell Adhesion Molecules in Patients With Dyslipidemia
Background Increased expression of cell adhesion molecules (CAMs) on the vascular endothelium has been postulated to play an important role in atherogenesis. Both in vitro and in vivo studies have suggested that dyslipidemia may increase expression of CAMs.
Methods and Results To determine whether dyslipidemia is associated with increased expression of CAMs, we examined the levels of soluble intercellular adhesion molecule 1 (sICAM-1), soluble vascular cell adhesion molecule 1 (sVCAM-1), and soluble E-selectin (sE-selectin) in individuals with either hypercholesterolemia or hypertriglyceridemia and in control subjects matched for age and sex. Patients with hypertriglyceridemia had significantly higher levels of sVCAM-1 (739±69 ng/mL) compared with patients with hypercholesterolemia (552±63 ng/mL) and control subjects (480±56 ng/mL). Levels of sICAM-1 were significantly increased in both the hypercholesterolemic and hypertriglyceridemic groups (298±29 and 342±31 ng/mL, respectively) compared with the control group (198±14 ng/mL). Levels of sE-selectin were significantly increased in hypercholesterolemic patients (74±9 ng/mL) compared with control subjects (48±5 ng/mL). Ten hypercholesterolemic patients were treated aggressively with atorvastatin alone or a combination of colestipol and either atorvastatin or simvastatin for a mean of 42 weeks and had an average LDL cholesterol reduction of 51%. Comparison of soluble CAMs before and after treatment showed a significant reduction only in sE-selectin (77±11 versus 56±6 ng/mL, P≤.03) but not for sVCAM-1 or sICAM-1.
Conclusions Although severe hyperlipidemia is associated with increased levels of soluble CAMs, aggressive lipid-lowering treatment had only limited effects on the levels. Increased levels of soluble CAMs in patients with hyperlipidemia may be a marker for atherosclerosis.
Adhesion and transendothelial migration of circulating leukocytes are critical early events in the pathogenesis of atherosclerosis.1 VCAM-1, ICAM-1, and E-selectin are CAMs that are expressed on endothelial cells and mediate the adhesion of leukocytes to vascular endothelium. In vitro studies have shown that the rolling of monocytes on endothelial cells is mediated at least in part by the interaction of E-selectin and VCAM-1.2 VCAM-1 interacts with the integrins α4β1 and α4β7 present on monocytes and lymphocytes, whereas ICAM-1 interacts with the β2-integrins CD11a, CD11b, and CD11c.3 The expression of CAMs is stimulated in vitro by cytokines such as interleukin-1, tumor necrosis factor, and interferon-γ,4 and pathological studies of human atherosclerosis have shown increased expression of VCAM-1 and ICAM-1 on endothelial cells, smooth muscle cells, and macrophages in human atherosclerotic plaques and in the endothelium of adventitial vessels adjacent to plaques.5 6 7 E-selectin expression is also increased in atherosclerosis but is confined to the vascular endothelium. Animal studies of hyperlipidemia and diabetes mellitus have demonstrated increased expression of VCAM-1 and E-selectin associated with atherosclerosis.8 Lysophosphatidylcholine, a component of modified LDL, has been shown to upregulate VCAM-1 expression,9 and recent reports suggest that fatty acids may also modulate expression of VCAM-1.10 11 Unfortunately, determining whether dyslipidemia leads to increased expression of endothelial CAMs has been difficult because of the inability to assess the level of adhesion molecule expression of the vascular endothelium in vivo.
Soluble forms of these adhesion molecules (sVCAM-1, sICAM-1, and sE-selectin) can be detected in the serum and are increased in conditions with an inflammatory component, such as pulmonary fibrosis, vasculitis, melanoma, and heart transplantation.12 13 The mechanism by which levels of soluble CAMs are increased is unknown, but the soluble levels are increased in conditions in which expression on the cell membrane has also been shown to be increased, such as after heart or liver transplantation.13 14 The purpose of this study was twofold. First, we wanted to determine whether patients with severe dyslipidemia due to defects in either LDL or TG metabolism had increased levels of soluble CAMs, and second, we wanted to determine whether aggressive lowering of LDL-C by drug therapy in a subset of patients would lead to a reduction in the levels of soluble CAMs.
Three groups of subjects were recruited into a protocol approved by the institutional review board for human subjects. HC subjects (n=14) were identified on the basis of LDL-C level >200 mg/dL and fasting TG level <200 mg/dL on an American Heart Association step I diet without drug therapy for at least 30 days. HTG patients (n=13) were identified on the basis of fasting TG level >475 mg/dL and LDL-C level <180 mg/dL without lipid-lowering therapy. Healthy control subjects (n=13) without major cardiovascular risk factors and with LDL-C level <160 mg/dL and fasting TG level <150 mg/dL were selected to approximate the age and sex distribution of the other groups. Exclusion criteria for all subjects included hypothyroidism, renal disease, malignancy, treatment with immunosuppressive drugs, connective tissue disease, cardiovascular event within 6 months, and any acute illness. In patients with hyperlipidemia, a medical history was taken and a physical examination was performed to determine if they had clinical evidence or symptoms of advanced coronary artery disease, peripheral vascular disease, or cerebrovascular disease. In addition, a risk factor score was derived for each patient by assigning one point for each of the following: diabetes mellitus, hypertension, smoking, clinically manifest atherosclerosis, and hyperlipidemia. Diabetes was defined as a fasting blood glucose level >115 mg/dL or treatment with a hypoglycemic agent. Hypertension was defined as systolic blood pressure >140 mm Hg, diastolic blood pressure >90 mm Hg, or treatment with an antihypertensive agent.
Treatment of Hypercholesterolemic Subjects
Of the 14 HC subjects, 10 received aggressive treatment for their hypercholesterolemia for a period of 37 to 46 weeks (mean, 41.6±1.1 weeks). Four patients received atorvastatin, an experimental 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor, as monotherapy at a dosage of 80 mg/d. Three patients were treated for 16 weeks with colestipol 20 g/d, then with a combination of colestipol 20 g/d and atorvastatin 40 mg/d for an additional 21 to 30 weeks (mean, 25.7±2.6 weeks). One of these 3 patients discontinued the colestipol after 17 weeks because of gastrointestinal complaints and remained on atorvastatin 40 mg/d. The final 3 patients received colestipol 20 g/d for 16 weeks followed by a combination of colestipol 20 g/d and simvastatin 40 mg/d for an additional 21 to 30 weeks (mean, 26±2.6 weeks).
Blood samples were obtained by standard venipuncture after a 12-hour fast from all subjects at baseline. Plasma TC was measured with either a Hitachi 747 or Cobas-Fara II analyzer according to CDC reference procedures.15 The same technique was used to measure HDL-C in control and HC subjects after precipitation of apo B–containing lipoproteins from samples by the use of dextran sulfate and magnesium.16 In the HTG subjects, HDL particles were separated by ultracentrifugation at a density of 1.125 g/mL, and cholesterol concentration then was determined by the same methods as described for TC. Plasma TG was measured after preparation with lipase, glycerol phosphate oxidase, and peroxidase.17 LDL-C was calculated in the HC patients and control subjects by the Friedewald equation.18 In the HTG patients, LDL-C was measured directly after fractionation of the lipoproteins by differential ultracentrifugation at density 1.006 g/mL. In the 10 HC subjects who received drug therapy, a second 20-mL blood sample was collected after treatment, and values for TC, HDL-C, TG, and LDL-C were obtained by the same techniques described above. Levels of sICAM-1, sVCAM-1, and sE-selectin were determined by the use of monoclonal antibody-based ELISA assays (R and D Systems) on frozen serum collected at baseline from all subjects and after treatment from the patients who received medication. All samples and controls were performed in duplicate, and concentrations of samples were determined by analyzing standards with known concentrations of recombinant adhesion molecules coincident with samples and plotting a curve of signal versus concentration.
ANOVA with the Bonferroni-Dunn comparison19 was performed to determine the differences among the three subject groups in age, baseline lipid levels, and baseline levels of sICAM-1, sVCAM-1, and sE-selectin. Nonparametric tests (ie, ANOVA on the ranks rather than on the raw data20 ) were used for TG, HDL-C, and sICAM-1 because of the large difference in variability among the groups in these parameters. Simple regression analysis was used to examine the relation among sICAM-1, sVCAM-1, and sE-selectin levels. ANOVA on ranked data (Kruskal-Wallis test) was used to determine if the levels of sICAM-1, sVCAM-1, and sE-selectin differed among the risk factor categories of the subjects. For all ANOVAs, pairs of groups were compared only if the overall comparison was significant (P<.05). Soluble CAM levels in subjects with atherosclerosis were compared with those in hyperlipidemic (HC and HTG) subjects without clinical atherosclerosis using the unpaired Student’s t test. The paired Student’s t test was used to compare lipid levels, sICAM-1, sVCAM-1, and sE-selectin before and after treatment in the subjects who received drug treatment for elevated LDL-C. Results are presented as mean±SE.
Baseline characteristics of the three groups are shown in the Table⇓. There were no significant differences in age or sex among the three groups. The HC group compared with the control group had a significantly higher TC (345±16 versus 201±9 mg/dL) and LDL-C (269±16 versus 114±9 mg/dL) and significantly lower HDL-C (44±2 versus 67±6 mg/dL). The HTG group compared with the control group had significantly higher TC (311±16 versus 201±9 mg/dL) and TG (911±111 versus 97±7 mg/dL) and significantly lower HDL-C (20±2 versus 67±6 mg/dL); LDL-C level (116±13 versus 114±9 mg/dL) was not significantly different from that in the control group.
The level of sVCAM-1 was significantly increased in the HTG group (739±69 ng/mL) compared with the HC (552±63 ng/mL) and control (480±56 ng/mL) groups (Fig 1⇓, top). Levels of sICAM-1 were increased in both the HC and HTG groups (289±29 and 342±31 ng/mL, respectively) compared with the control group (198±14 ng/mL). Level of sE-selectin was increased in the HC group (74±9 ng/mL) compared with the control group (48±5 ng/mL). More patients in the HTG group had other risk factors or clinical evidence of atherosclerosis; therefore, a second analysis that considered the presence of these factors was performed to compare levels of soluble CAMs in the control subjects, subjects with hyperlipidemia alone (either elevated LDL-C or TG, n=19), and subjects with hyperlipidemia plus any other risk factor (n=8). Patients with hyperlipidemia and at least one other risk factor had significantly higher levels of both sVCAM-1 and sICAM-1 than the other groups, as shown in the bottom of Fig 1⇓. The 5 patients with hyperlipidemia and atherosclerosis had significantly increased sVCAM-1 compared with the other 22 patients with hyperlipidemia (977±143 versus 566±37 ng/mL, P≤.0004). In the overall study population, there was a significant correlation between levels of sICAM-1 and sVCAM-1 (r=.56, P≤.001) and between levels of sICAM-1 and sE-selectin (r=.61, P≤.0001) but not between sVCAM-1 and sE-selectin (r=.19, P=.25). Because of the small sample size of this study, we did not perform separate analyses to examine the role of additional risk factors in patients with hypercholesterolemia versus hypertriglyceridemia, nor did we examine the relative impact of specific risk factors such as diabetes versus hypertension.
Lipid-lowering drug treatment effects in the HC patients evaluated by comparing pretreatment and posttreatment lipid levels showed reductions of TC (327±15 versus 200±17 mg/dL [39%]) and LDL-C (252±14 versus 124±17 mg/dL [51%]) without significant changes in TG (139±19 versus 113±13 mg/dL) or HDL-C (47±3 versus 53±4 mg/dL). Comparison of soluble CAMs before and after treatment showed a significant reduction in sE-selectin (77±11 versus 56±6 ng/mL, P≤.03) but no significant change in sVCAM-1 (626±76 versus 672±51 ng/mL) or sICAM-1 (314±36 versus 342±36 ng/mL) (Fig 2⇓).
In this report, we have shown for the first time that severe dyslipidemia is associated with elevated levels of soluble CAMs. Patients with marked elevations of LDL-C had increased levels of sICAM-1 and sE-selectin, whereas patients with severely elevated TG had increased levels of sVCAM-1 and sICAM-1. Patients with severe hypertriglyceridemia also had markedly reduced levels of HDL-C, thus making it unclear which aspect of dyslipidemia was related to the change in soluble CAM levels. One of the questions raised by the association between dyslipidemia and soluble CAM levels is whether dyslipidemia causes endothelial dysfunction that results in increased expression of CAMs and increased release of CAMs into the plasma or whether the increased levels of soluble CAMs are a consequence of atherosclerosis induced by the dyslipidemia. Compared with other subjects, patients with hyperlipidemia and at least one other risk factor, who are more likely to have a greater extent of atherosclerosis, also had increased levels of sVCAM-1 and sICAM-1. In addition, patients who had clinical evidence of atherosclerosis (angina, claudication, or history of carotid endarterectomy) had the highest levels of sVCAM-1 and sICAM-1. Other studies have suggested that atherosclerosis may be associated with increased levels of CAMs. A recent study by Blann and McCollum21 found higher levels of sICAM-1 in patients with peripheral vascular disease (P=.0003) and in patients with ischemic heart disease (P=.0059) than in age-matched control subjects but no significant difference in sVCAM-1. We predicted that if the increased levels of soluble CAMs were secondary to endothelial dysfunction resulting from hyperlipidemia, reduction of LDL-C might lead to reduction in the levels of soluble CAMs. Successful lipid-lowering therapy, which reduced LDL-C a mean of 51%, led to a significant reduction in sE-selectin but no significant reduction in sVCAM-1 or sICAM-1. We hypothesize that this may be because E-selectin differs in mechanisms of gene regulation as reflected by a pattern of expression restricted to endothelial cells, whereas VCAM-1 and ICAM-1 are expressed on endothelial cells and on other cells that are present in atherosclerotic lesions, such as leukocytes and smooth muscle cells. These genes exhibit differences in regulation at the level of transcription in response to oxidation-sensitive pathways as previously reported for in vitro endothelial expression of VCAM-1, ICAM-1, and E-selectin.22 23 We predict that a significant percentage and perhaps the majority of soluble ICAM-1 and VCAM-1 may not arise from arterial endothelial cells. Although one might argue that the length of therapy was insufficient to alter endothelial function, 6 months of less effective LDL-C reduction with lovastatin was sufficient to show improvement in endothelially mediated vasodilatation in response to acetylcholine.24 These data suggest that the increased levels of sICAM-1 and sVCAM-1 in patients with dyslipidemia may be related to underlying atherosclerosis. If the levels of soluble CAMs were related to the extent of vascular disease (atherosclerotic burden) or the activity of atherosclerosis (the rate of progression), then treatment for 6 months would be of insufficient duration to show a significant impact. The Scandinavian Simvastatin Survival Study did not show a significant reduction in clinical events, ie, activity of atherosclerosis, until after 1.5 to 2 years of treatment.25 Angiographic and ultrasound studies show very modest changes in the extent of atherosclerosis even after 2 to 4 years of aggressive lipid-lowering therapy.26 Future studies should examine the effects of longer periods of lipid-lowering therapy on the level of soluble CAMs.
Although treatment of hyperlipidemia in patients with documented atherosclerosis is believed to be cost-effective, considerable controversy exists about the cost-effectiveness of drug treatment for lipids in primary prevention, particularly in women. Noninvasive tests that would help to identify individuals with atherosclerosis who would be at high risk for cardiovascular events would improve the cost-effectiveness of lipid-lowering therapy. Studies are in progress using sera from individual in the Atherosclerosis Risk in Communities study27 to determine whether levels of soluble CAMs can be used as biochemical markers in conjunction with traditional risk factors to identify asymptomatic individuals at high risk for developing cardiovascular events because of atherosclerosis.
Selected Abbreviations and Acronyms
|CAM||=||cell adhesion molecule|
|sICAM-1||=||soluble intercellular adhesion molecule-1|
|sVCAM-1||=||soluble vascular cell adhesion molecule-1|
This work was supported in part by NHLBI grant HL-42550 (C.M.B.); American Heart Association, National Sanofi Winthrop Awardee (C.M.B.); Zeneca Pharmaceuticals (C.M.B.); Parke-Davis (W.I.); and Jiohi Medical School (Y.A.). We are grateful to Kerrie Jara for editorial assistance and Rima Maghes for manuscript preparation.
The guest editor for this article was Wayne Alexander, MD, PhD, Emory University Medical School, Atlanta, Ga.
- Received August 14, 1995.
- Revision received February 5, 1996.
- Accepted February 5, 1996.
- Copyright © 1996 by American Heart Association
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