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J Am Coll Cardiol, 1999; 33:436-443 © 1999 by the American College of Cardiology Foundation |
a Department of Internal Medicine and Pathology, Fukuoka University School of Medicine, Fukuoka 814-0180, Japan
* Department of Pediatrics, Ryukyus University School of Medicine, Okinawa 904-2300, Japan
Manuscript received April 15, 1998; revised manuscript received August 25, 1998, accepted October 2, 1998.
Reprint requests and correspondence: Keijiro Saku, Department of Internal Medicine, Fukuoka University School of Medicine, 7-45-1 Nanakuma Jonan-ku, Fukuoka 814-80, Japan
hh035399{at}msat.fukuoka-u.ac.jp
| Abstract |
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To examine the association between the fractional esterification rate of cholesterol (C) in low density lipoprotein- and very low density lipoprotein-depleted plasma (FERHDL) and coronary artery disease (CAD) and the influence of serum HDL-C levels.
Background
The function of HDL in reverse cholesterol transport is involved in the antiatherogenic action of HDL, and FERHDL is a newly established quantitative measure of HDL function in vivo.
Methods
Cases (n = 185, F/M: 43/142) and controls (n = 74, F/M:27/47) were defined as subjects with/without angiographically proven CAD, respectively.
Results
The cases had significantly (p < 0.05) higher FERHDL values (13.2 ± 0.3 %/h vs. 12.1 ± 0.5 %/h) and lower HDL-C levels (39.0 ± 1.0 mg/dL vs. 46.8 ± 1.4 mg/dL) than the controls. The associations of FERHDL and HDL-C with CAD were linear and significant (p < 0.05). Multiple logistic regression analysis indicated that the association of FERHDL with CAD varied with the HDL-C level: significant for the low HDL-C tertile (chi-square = 6.20, p < 0.05) but not significant for the middle and high HDL-C tertiles (chi-square = 0.08 and 0.03, n.s.). The risk of CAD, relative to that in patients with low FERHDL and high HDL-C, was higher in patients with low FERHDL and low HDL-C (odds ratio [95% confidence interval]: 2.37 [1.124.97], p < 0.05) and was highest in patients with high FERHDL and low HDL-C (3.85 [1.848.06], p < 0.01).
Conclusions
The functional assay of HDL (FERHDL) is an independent risk factor for CAD. The combination of FERHDL and HDL-C could be a potent indicator for CAD, and may reflect a potential mechanism of atherosclerosis.
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HDL particles differ in size, structure and function (8). Both HDL2 and HDL3 levels are reduced in patients with CAD (9,10). However, although some studies have reported altered relative proportions of the HDL2 and HDL3 subclasses, the functional aspects of HDL did not attract much attention until Dobiasova and Frohlich (8) established a functional assessment of HDL: the fractional esterification rate in low density lipoprotein (LDL)- and very low density (VLDL)-depleted plasma (FERHDL). They indicated that FERHDL is a functional test of HDL particle interaction (8), and suggested that angiographically proven CAD subjects had higher FERHDL values than controls (11).
A major hypothesis for explaining the antiatherogenic properties of HDL involves the role of HDL in reverse cholesterol transport (RCT) (12). Reverse cholesterol transport is a multistep process that results in the net movement of cholesterol from peripheral tissues back to the liver via the plasma compartment (3). The efflux of cholesterol from the plasma membrane of peripheral cells to HDL is the first step in the RCT pathway (13). Promotion of this step may be antiatherogenic because it reduces the possibility of the overaccumulation of cellular cholesterol, and this hypothesis has been supported by studies using genetic animal models of RCT (46). Two kinds of nascent HDL particles are believed to be secreted from the liver and (in humans) intestine: small spherical HDL3-like particles (14) and small lipid-poor complexes (15) that migrate with pre-ß mobility on agarose gel electrophoresis (16). These two subfractions of HDL remove cellular free cholesterol by distinct mechanisms (1719): diffusion-based or receptor-dependent (15).
Cholesterol ester that has accumulated in HDL may then transfer from HDL to apolipoprotein (apo)-B containing lipoproteins (LDL and VLDL) in exchange for triglycerides (TG) as a result of the activity of cholesterol ester transfer protein (CETP) with subsequent uptake of TG-rich lipoprotein remnants by the liver. In humans, this second step of RCT is illustrated by the dramatic accumulation of HDL in subjects with CETP deficiency (20,21). TG-rich HDL2 particles are subjected to hydrolysis by hepatic lipase and perhaps lipoprotein lipase (14) and are converted back to a small HDL3-like particle. Apolipoprotein (Apo) A-I can also be released from HDL2 to produce pre-ß1 HDL (22). Thus, the HDL3
HDL2
HDL3 cycle and pre-ß1 HDL
HDL3
HDL2
pre-ß1 HDL cycle are completed. Promotion of this second step of RCT is probably proatherogenic because CETP transfers cholesterol ester from "good" or "safe" lipoprotein HDL to atherogenic apo-B containing LDL and VLDL, thereby promoting cholesterol ester deposition (22). Therefore, we propose that both the quantity of HDL, as measured by HDL-C and the function of HDL in RCT, as quantitatively measured by FERHDL, play important roles in antiatherogenic properties of HDL.
In this case-control study, we tested the association of FERHDL with CAD and its interaction with HDL-C, after controlling for age, gender, conventional risk factors and other lipid parameters.
| Methods |
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2 after myocardial infarction), vascular disease (aortitis treated by prednisoline), hepatic dysfunction (virus and nonvirus, transaminases more than three times the normal value) or uncontrollable diabetes mellitus were excluded from the study. Patients with systolic or diastolic blood pressure >160 mm Hg or 95 mm Hg or who were under anti-hypertensive treatment were considered to have hypertension (HT). Patients under treatment for diabetes mellitus (DM) and/or with symptoms of DM and a fasting glucose concentration
126 mg/dL were considered to have DM. Otherwise, the results of a 75 gm glucose tolerance test were used to give a diagnosis of DM. About 98% of the women were in menopause but none were receiving hormone replacement therapy. None of the patients were being treated with lipid-lowering agents at the time of sampling. Coronary angiography. Coronary arteries were cannulated by the Judkins technique (23) with 5F catheters, and recorded on Kodak 35 mm cinefilm at a rate of 25 frames/s. Coronary arteries were divided into 15 segments, according to the classification of the American Heart Association Grading Committee. In this study, reliable and reproducible measurements were obtained. Coronary artery segments were carefully selected by two expert cardiologists on the basis of smooth luminal borders and the absence of stenotic changes. The presence of stenosis was determined using a computer-assisted coronary angiography analysis system (Mipron 1; Kontron Co., Tokyo, Japan) after the direct intracoronary injection of isosorbide dinitrate (ISDN) (2.5 mg/5 mL solution), as described previously (24). Arterial stenosis, that produced more than 50% luminal narrowing, was considered significant.
Determination of serum lipids, lipoproteins, and apolipoproteins. Blood was drawn in the morning after an overnight fast. Serum total cholesterol (TC) and triglyceride (TG) concentrations were determined enzymatically. HDL-C was determined by the heparin Ca2+ precipitation method (25). HDL subfraction (HDL2 and HDL3) were separated by standard sequential preparative ultracentrifugation techniques (26). Apo A-I, apo A-II, apo B, apo C-II, apo C-III and apo E were determined by the turbidity immunoassay method (27). Serum lipoprotein (a) (Lp[a]) levels were measured by an enzyme-linked immunosorbent assay using Tint Eliza Lp(a) (Biopool Co., Stockholm, Sweden) (28). For all measurements in our laboratory, the coefficients of interassay and intraassay variation were less than 5.0%, and blinded quality-control specimens were included in each assay.
Assay for FERHDL in plasma. VLDL-LDL-depleted plasma was prepared by precipitating apolipoprotein B-containing lipoproteins with phosphotungstate-MgCl2 (11). FERHDL was determined according to the method of Ohta et al. (29) with minor modifications. [3H] Free cholesterol (FC) was incorporated onto polystyrene tissue-culture wells (Corning, New York, New York) as follows: absolute ethanol (100 µl) containing 1 µCi of [3H] FC was placed in wells and dried off by flushing with nitrogen. Next, 100 µl of the VLDL- and LDL-depleted plasma samples, in 400 µl of PBS was added to each well and [3H] FC was equilibrated with the FC in each sample by incubation at 4° C for 16 h. [3H] FC-labeled VLDL- and LDL-depleted plasma samples were incubated in a shaking water bath for 3 h at 37° C. The enzyme reaction was stopped by immersing the sample tubes in an ice bath. The lipids in incubation samples were extracted with methanol/chloroform (2:1, v/v). The extract was dried by flushing it with nitrogen and was then dissolved in 60 µl of isopropanol. Aliquots (20 µl) of lipid extracts were spotted in duplicate on a thin-layer chromatography plate (Merck, West Point, Pennsylvania) and developed in n-hexane/diethyl ether/acetic acid/methanol (85:20:1:1, v/v). Spots corresponding to FC and cholesteryl ester (CE) were cut from the plate and the radioactivities were determined. The increase in [3H] CE was linear within 3 h of incubation. The fractional esterification rate was expressed as the difference between the percentage of radioactive cholesterol esterified before and after incubation at 37° C. The samples were measured in triplicate and the coefficient of variation of the assay was 0.75%. The coefficient of variation for the interassay variability was 6.2%.
Statistical analysis. Statistical analysis was performed using the SAS Software Package (Version 6, Statistical Analysis System, SAS Institute Inc., Cary, North Carolina). Categorical variables (such as gender) were compared between cases and controls by a chi-square analysis. Differences between cases and controls or among patients with 1-, 2-, and 3-vessel diseases were examined by an analysis of variance (ANOVA). Comparisons of 1-vessel and 2- or 3-vessel disease patients were performed with the multiple comparison test of Dunnett (30). Age and gender were adjusted for by an analysis of covariance (ANCOVA) (30). The logistic model was used to evaluate linear associations between CAD and lipid variables (continuous). In addition, odds ratios were simultaneously adjusted for age, gender and potentially confounding variables by a multiple logistic regression analysis (30). For all of the odds ratios, we calculated 95% confidence intervals (CI). For logistic regression coefficients, we show either 95% CI or the standard error. A multiple regression analysis was used to test the correlation between HDL-C or FERHDL and lipid variables while controlling for age, gender and other lipid parameters (30). Because some variables were not normally distributed, we used rank scores in the regression analysis to simplify the calculation (31). All p values are two-tailed. The significance level was considered to be 5% unless otherwise indicated.
| Results |
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) and without also adjusting for smoking, hypertension, diabetes and body mass index. For each variable shown in Table 3, the association with CAD was significantly linear: The relative risk of CAD (odds ratio) decreased with increasing serum levels of HDL-C, HDL2-C, HDL3-C, apo A-I, apo A-II, apo E, and HDL-FC and with decreasing serum Lp(a) levels and FERHDL values. After adjusting for conventional risk factors (Table 3, right columns), the association of HDL-C, apo A-I, apo E, Lp(a), and FERHDL with CAD remained significant, suggesting that these variables are independently associated with CAD.
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2 = 6.20, p = 0.01) but not in the middle and high tertiles (Wald
2 = 0.08 and 0.03, n.s.). Figure 1 shows a plot of the prevalence of CAD, FERHDL (in tertiles), and HDL-C (in tertiles). These results show that the association of FERHDL with CAD was modified by HDL levels. It is also possible that some other factor(s) mediate the relationship of both HDL-C and FERHDL with coronary atherosclerosis.
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| Discussion |
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HDL is remodeled during the process of RCT, with changes in structure and size (40), which are the most important factors in determining the rate of LCAT-catalyzed cholesterol esterification (8) and CETP-mediated transport of cholesterol ester (40). This is reflected in our findings that FERHDL is inversely related to HDL-C and that the association of FERHDL with CAD is modified by HDL levels. The net flux of cellular cholesterol to HDL particles is generated by a gradient of the cholesterol content between cells and the lipoprotein surface and provided by an LCAT reaction (41). Because the size of circulating HDL particles as reflected by FERHDL (8) depends on the balance between the opposing processes of cholesterol acceptance (which increases particle size) and lipolytic digestion (which reduces it), we can speculate that an increased FERHDL value reflects a detrimental condition that facilitates accumulation of intracellular cholesterol via LDL. This is confirmed by our finding that increased FERHDL is associated with an increased prevalence of CAD. However, a sufficient number of HDL particles may overcome this condition by retaining cholesterol esters in the HDL fraction. This is reflected in our finding that when HDL-C levels were high, increased FERHDL did not significantly increase the risk of CAD (Table 4, Fig. 1). Because low HDL-C levels also tended to be linked to a reduction in the expression of lipoprotein lipase and a rise in hepatic lipase, both of which are enzymatic changes that would lead to a decrease in triglyceride-rich particle lipolysis (12), increased FERHDL may cause an increased accumulation of cholesteryl ester-rich remnants of VLDL and chylomicra under conditions of low HDL-C and then confer an increased risk of CAD. This is reflected in our findings that patients with both low HDL-C and high FERHDL had the highest risk of CAD (Table 6, Fig. 1) and that the combination of both FERHDL and HDL is more strongly related to CAD than is either measure alone (Table 6).
Our results show that low HDL is a better indicator of CAD than high FERHDL, as is indicated by a better model fit for HDL than FERHDL (2 log likelihood: 21.31 vs. 16.35, Table 6) and as is apparent in Figure 1. This may be attributed to other functions of HDL particles, e.g., the antioxidant properties of HDL (which inhibits the oxidation of LDL particles) (42) and the ability of HDL particles to inhibit the expression of adhesion molecules on the surface of endothelial cells (43).
Limitations.
In this angiographic case-control study, we demonstrated that FERHDL is independently associated with CAD and this association was modified by HDL-C levels. However, whether or not FERHDL plays a causal role is unclear and cannot be determined from a case-control study. In this case-control study, cases were not matched with controls with regard to the number of patients or gender (Table 1). Although when the cost of sampling for cases and controls is equal and the relation between disease and exposure is not known beforehand, matching 1 case with 1 control can minimize the variance of the estimated odds ratio (44). We did not obtain a suitable number of controls due to limited funds. Differences in the gender ratio between cases and controls may have caused biased estimates of the odds ratio, since HDL-C levels were different (p < 0.05) between males and females among the controls (43.2 ± 2.0 mg/dL vs. 51.1 ± 2.6 mg/dL) and in all of the patients together (38.4 ± 0.90 mg/dL vs. 44.5 ± 1.5 mg/dL) after controlling for age. Although we tried to avoid this possible bias by adjusting for gender in the logistic regression analysis and found no gender interaction (group* gender: F value = 1.86, p = 0.17; HDL-C* gender:
2 = 0.01, p = 0.99), the variance of the estimated odds ratio may not be as low as that in a matched study.
We selected angiographically defined normal subjects as controls. However, a selection bias is known to exist: angiographically defined normal subjects generally have more risk factors for coronary disease than patients with clinical symptoms but who have not been selected for angiography, because a person with both a chest pain and a known risk factor, such as smoking, may be more likely to be referred for angiography than a person with just a clinical symptom (45). The lack of significance for the fairly substantial difference in the prevalence of diabetes between cases and controls and the lack of an association between FERHDL and the severity of coronary disease as judged by the number of involved vessels (Table 2) in the present study may be due to this bias. Previous angiographic studies such as the Coronary Artery Surgery Study have also failed to show associations between classic lipid risk factors and the severity of disease, presumably due to the biases involved in selecting patients for angiography. The controls were defined as having less than 25% luminal narrowing by conventional coronary angiography. However, since even mild coronary atherosclerotic lesions in the 25% range can result in significant acute events when based on unstable plaque (46), our controls are not absolute controls in the sense of having no significant coronary atherosclerosis or its eventual sequela. Thus, limitations regarding the controls may have limited the power of this study.
Conclusions. FERHDL, a quantitative measure of the HDL function, when combined with serum HDL-C levels, is a new epidemiological marker for the risk of CAD that is superior to HDL-C levels alone. Since FERHDL values are fairly constant, this test may be of great value in clinical screening. The clinical significance of this finding needs to be demonstrated in a prospective trial.
| Footnotes |
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