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Lipids and Lipoproteins |
Departments of
1
Biochemistry I and
2
Internal Medicine III, Nagoya City University Medical School, Kawasumi 1, Mizuho-cho, Mizuho-ku, Nagoya 467-8601, Japan.
a Author for correspondence. Fax 81-52-841-3480; e-mail syokoyam{at}med.nagoya-cu.ac.jp.
| Abstract |
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50% in their coronary arteries was not significantly
different from that of the 26 patients with luminal stenosis <50%. No
other lipoprotein-related measurement except HDL-cholesterol
differentiated the two groups. Among 40 hypercholesterolemic patients,
no lipoprotein-related measurement other than LDL-cholesterol was found
to positive correlate with the CETP. Before and after the treatment of
23 patients with simvastatin 5 mg a day for 4 weeks, plasma CETP
markedly decreased in those whose pretreatment CETP was
3 mg/L; no
change was observed for those with lower pretreatment CETP. In the
former group, negative correlation between CETP and HDL-cholesterol was
demonstrated only in the posttreatment plasma. | Introduction |
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In such a context, CETP would help the removal of cholesterol from a atherosclerotic vascular lesion. However, it also lowers HDL and may raise LDL in plasma by transferring CE from HDL, which is inconsistent with the expected beneficial function of CETP in counteratherogenesis. This issue is indeed still controversial in animal experiments and clinical observations. The reports on atherosclerosis in transgenic mice expressing CETP are in conflict (7)(8). A heterozygous genetic defect of CETP is reportedly atherogenic when compared on the basis of the same HDL concentration (9). A few more recent reports indicated that the CETP deficiency is not protective against atherosclerosis despite the marked hyperalphalipoproteinemia that accompanies it (10)(11). Thus, more clinical data are needed to understand the relationship of CETP to atherosclerosis. A reliable method to measure plasma CETP concentrations is therefore primarily important.
We have raised monoclonal antibodies (mAbs) against CETP isolated from rabbit plasma, and two of them cross-reacted with human CETP (12)(13). In this study, we report a new ELISA method that uses these two mAbs, 311D and 148F. We also include the results of CETP measurement using this method in plasma samples from patients with coronary heart disease and hypercholesterolemia.
| Materials and Methods |
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CETP was isolated from human plasma by Dr. Taira Ohnishi, Kagawa Medical University, Miki-cho, Japan, according to the method previously described (14). The plasma of the homozygous patient with CETP deficiency (intron 14 splicing defect) (15) was kindly provided by Dr. T. Miida, Niigata University, School of Medicine, Niigata, Japan.
MAb 311D (0.5 µg in 50 µL of 50 mmol/L sodium bicarbonate, pH 9.6) was put into microwells of the assay plates (Falcon PRO-BINDTM Assay Plates, 96 flat-bottom wells, Becton Dickinson) and left at 4 °C overnight. The unbound antibody was removed, the wells were rinsed twice with phosphate-buffered saline (PBS; 20 mmol/L sodium phosphate buffer, pH 7.4, containing 154 mmol/L NaCl) to which 0.5 mL/L Tween 80 was added (PBS-T), and the blocking solution of 10 g/L bovine serum albumin and 14 g/L NaN3 in PBS was added. The blocking mixture was left for 1 h at 37 °C and removed, and the wells were rinsed three times with PBS-T. The samples, appropriately diluted in 20 mmol/L phosphate buffer containing 1 mmol/L NaCl and 0.5 mL/L Tween 80, were then added at 50 µL/well and allowed to stand at 37 °C for 4 h. The antigen solution was removed, and the plates were rinsed five times with PBS-T. The detection antibody, mAb 148F (0.5 µg conjugated with horseradish peroxidase) (16), was added to 50 µL of the same sodium phosphate buffer as the samples. After incubation for 1.5 h at 37 °C, the unbound antibody was removed, and the wells were rinsed five times with PBS-T. The freshly prepared color reagent solution (0.4 g/L o-phenylenediamine and 0.6 g/L hydrogen peroxide solution in 58 mmol/L sodium phosphate, 21 mmol/L citrate buffer, pH 5.6) was added, and the plate was incubated for 20 min at 37 °C. The reaction was stopped by adding 50 µL of 2 mol/L sulfuric acid. The absorbance at 490650 nm was measured for each well in a SPECTRE MAX 340(TM) (Molecular Devices).
For the coronary trial, 63 Japanese men between 31 and 59 years of age
(51 ± 6 years, mean ± SD) consecutively underwent elective
coronary angiography for diagnosis of anginal chest pain at the Nagoya
City University Hospital. The coronary angiogram of each patient was
reviewed by a panel of three cardiologists blinded to the identity of
subjects, their clinical histories, and laboratory data. The subjects
were classified into two groups: those who had luminal stenosis
50%
in one or more coronary arteries (group 1) and those who had stenosis
<50% (group 2). Blood samples were taken before angiographic
examination. For the hyperlipidemia trial, 40 Japanese patients, 16 men
and 24 women between the ages of 26 and 81 years (55 ± 14.4
years, mean ± SD), were treated for 8 weeks with dietary calorie
intake restricted to 30 cal/kg standard weight (22 x [height
(m)]) with a composition of 26.6% protein, 20.3%
fat, and 53.1% carbohydrate by weight. Additional treatment with 5 mg
a day of simvastatin for 4 weeks was given to the 23 patients (4 men
and 19 women) who were willing to take the drug among those resistant
to dietary treatment and who had LDL-cholesterol >1.2 g/L. Blood
samples were obtained at the end of the dietary treatment and at the
end of the drug period. All the patients who participated in the trials
provided written informed consent.
Fasting venous blood was collected into EDTA-containing glass tubes. Plasma total cholesterol and TG concentrations were determined by enzymatic methods. The plasma HDL-cholesterol concentration was measured after precipitation of apolipoprotein B-containing lipoproteins with dextran sulfate and magnesium chloride. Plasma LDL-cholesterol was calculated according to the equation of Friedewald et al. (17). CETP activity was measured by the method described by Albers et al. (18), monitoring the transfer of radiolabeled CE (C-cholesteryl oleate, New England Nuclear) from the CE-donor HDL to the unlabeled acceptor LDL.
| Results |
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Table 2
shows a summary of the subjects with coronary heart disease.
Group 1 represents those who had luminal stenosis
50% in one or more
coronary arteries (37 men). Group 2 includes those who had luminal
stenosis <50% (26 men). The concentration of HDL-cholesterol was
significantly higher in the Group 2 than the Group 1
(P <0.05). Otherwise, there was no significant
difference in total cholesterol, LDL-cholesterol, TG, and CETP activity
between the two groups. CETP concentration showed neither a difference
between the two groups nor a correlation with HDL-cholesterol in either
group. In addition, the CETP concentration of either group was not
different from that separately measured for the reference subjects
mentioned earlier. The distribution profiles of the CETP of groups 1
and 2 are shown in Fig. 2
. There may be a slight shift of the peak to the higher
concentration in group 2, which has not reached statistical
significance.
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Forty hypercholesterolemic patients were instructed to follow a
restricted calorie intake for an 8-week period. Among those in whom
diet therapy failed to decrease LDL-cholesterol to 1.2 g/L or less, 23
patients took simvastatin 5 mg a day. Table 3
summarizes the plasma lipids, lipoprotein, and CETP
concentration, as well as the CETP activity, of these patients. The
average CETP concentration of the original 40 patients was higher than
that of the normolipidemic control group, although not significantly
(2.13 ± 1.01 vs 1.92 ± 0.53 mg/L). This may have been due
to the subgroup of patients who had a high concentration of CETP (Fig. 3
A); this view was supported by significant positive correlation
between the CETP and LDL-cholesterol concentrations (r
= 0.555, P <0.001; Fig. 3B
). However, there was no
significant correlation between CETP and HDL-cholesterol among these
patients (Fig. 3C
). As mentioned above, 23 of the 40 patients also
underwent the simvastatin treatment. Lipid and lipoprotein values and
CETP were monitored after the 4-week treatment (Table 3
). Before the
drug treatment, the CETP concentration of this subgroup was higher than
that of those who did not receive the drug therapy (2.57 ± 0.77
vs 1.53 ± 1.02 mg/L, P <0.001), perhaps reflecting
their plasma LDL-cholesterol concentration. By the end of the treatment
with simvastatin, plasma total cholesterol, LDL-cholesterol, and CETP
activity decreased, whereas HDL-cholesterol increased significantly
(Table 3
).
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The average CETP concentration was also decreased by the treatment
(Table 3
). The distribution profile of the CETP concentration indicated
the two distinct subgroups of CETP concentration in the pretreatment
stage (Fig. 4
and Table 4
). As shown in Fig. 4A
1, patients of group A whose pretreatment
concentration was <3 mg/L showed essentially the same distribution
profile as those of the normolipidemic control group (2.10 ± 0.44
vs 1.92 ± 0.53 mg/L), whereas the eight patients (all female) in
group B (Fig. 4B
1) were distinct from group A, having a pretreatment
concentration
3 mg/L. The CETP concentration was not substantially
changed by the treatment in group A (2.01 ± 0.52 mg/L) despite
the change of LDL and HDL (Fig. 4A
2). In contrast, the CETP
concentration of group B was markedly decreased by the simvastatin
treatment, from 3.46 ± 0.29 mg/L (range, 3.063.77 mg/L) to
2.46 ± 0.64 mg/L (range, 1.213.32 mg/L; P =
0.0074 by t-test and 0.012 by Wilcoxon test; Fig. 4B
2). No
apparent difference was demonstrated between groups A and B in their
lipid and lipoprotein profile before and after the treatment (Table 4
).
In group B, significant correlations between CETP and HDL-cholesterol
appeared in the posttreatment stage (r = -0.81). No
significant correlation was observed between CETP and HDL-cholesterol
for the pretreatment condition in either group or between posttreatment
CETP and HDL-cholesterol in group A.
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| Discussion |
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To investigate this problem in clinical studies, various methods have been reported for immunomeasurement of the CETP mass in plasma (19)(20)(21)(22)(23)(24)(25)(26)(27). Not many methods reported have used polyclonal antibodies, perhaps because the CETP isolated from human plasma has not been completely proven homogeneous. Immunoassay of human CETP was first established as a competitive solid-phase radioimmunoassay by the use of a mAb against human plasma CETP (20), and it was used to demonstrate the lack of CETP in the plasma of those with its genetic defect (15). Another report introduced second oligoclonal antibodies raised against the fragment peptides of CETP for a sandwich-type immunoassay (26). Thus, availability of antibodies suitable for the immunoassay seemingly is of primary importance, and the mAbs stably expressed by the established cell lines should be the most reliable source of the antibodies. An enzyme-linked colorimetric system would be an additional advantage, avoiding the use of radioisotopes to make the assay system conventional. Therefore, we have established a new ELISA for CETP in human plasma by using two different mAbs, 311D and 148F, both of which had been raised against rabbit plasma CETP and characterized for binding to human CETP and inhibition against the lipid transfer by human CETP (13).
Using this technique, we have measured the plasma CETP concentration of patients with coronary heart disease. The distribution profile of plasma CETP did not reach significant statistical difference between the patient groups with coronary stenosis of different degrees of severity, whereas HDL-cholesterol clearly differentiated these groups. Plasma CETP was also measured in hypercholesterolemic patients before and after simvastatin treatment. In agreement with previous observations (28)(29), CETP showed positive correlation with plasma LDL concentrations and no correlation with the HDL concentration. The hypercholesterolemic patients apparently included a subgroup, with high CETP concentrations, who were distinct from those having a CETP distribution within reference values. The simvastatin treatment markedly reduced the CETP concentration of the high CETP group but did not affect the concentration of the group whose pretreatment CTEP concentration was within reference values. These results are consistent with the reports that simvastatin reduced the average plasma CETP activity in hyperlipoproteinemia but showed no clear correlation between the changes in CETP activity and plasma lipoproteins (30) and that cholestyramine treatment of hypercholesterolemic patients produced a reduction of their plasma CETP concentrations (31).
The reason for the positive correlation between plasma CETP activity and LDL-cholesterol (28)(29) is unknown, whether LDL increases as a result of transfer of CE from HDL or CETP is increased by the increase of LDL. Inconsistency has also been observed in some pathological states: decreased CETP activity in non-insulin-dependent diabetes mellitus patients (32) but increased activity in insulin-dependent diabetes mellitus patients (33), despite the high LDL in both groups; reduced CETP activity in chronic renal diseases (34); and the decrease in CETP mass in patients treated with hemodialysis (35), in spite of the increase of their LDL.
Changes in CETP concentration throughout those with the normal CETP genotype and heterozygotes of CETP deficiency are unlikely to contribute to direct regulation of HDL-cholesterol, although complete CETP deficiency results in a very high HDL-cholesterol concentration (36)(37). However, the plasma CETP concentration correlates with HDL-cholesterol in hypertriglyceridemic humans (38) and monkeys (39), indicating a notable contribution of the exchange of CE with TG to the regulation of HDL-cholesterol by CETP. Nevertheless, the slight increase of CETP by probucol has been viewed as a potential cause of the HDL reduction by this drug (40)(41).
Many other metabolic factors may influence the plasma CETP concentration independently. Chronic alcoholic intake reduces CETP activity, which may account for the increase of HDL (42)(43). The effect of cigarette smoking is controversial (44)(45), as is the role of estrogen (20)(28)(46)(47). Reports are also controversial about correlation of CETP activity with the body mass index (48)(49).
The most important clinical questions is whether CETP plays a major role in atherogenesis. The finding with transgenic mice that expressed monkey CETP supported the hypothesis that CETP is atherogenic (7), whereas the finding with human CETP-transgenic mice did not (8). On the other hand, inhibition of CETP prevented atherogenesis in cholesterol-fed rabbits (27). However, the study on the heterozygotes of the genetic CETP deficiency showed that low CETP has negative impact on the prevention of coronary heart disease as long as the HDL-cholesterol concentration matches that of the control (0.40.6 g/L) (9). More recent studies by Hirano and co-workers (10)(11) also concluded that CETP deficiency was not necessarily antiatherogenic. No definitive data are available for the role of CETP in atherogenesis among those with the unaffected CETP gene.
Thus, more clinical data are needed for understanding the role of CETP in atherogenesis. The ELISA described in this report would provide a new conventional tool for measuring CETP concentrations in various biological samples. In our preliminary observation, plasma CETP concentration is neither a regulating factor of HDL concentration nor an indicative risk for coronary heart disease. Also implicated is that there is a subgroup of hypercholesterolemic patients who have distinctly high plasma CETP concentrations. Treatment of hypercholesterolemia of such patients produces a notable reduction of CETP, whereas it has little influence on the CETP of the group with CETP concentrations within reference values. Thus, the CETP concentration should be analyzed more carefully in clinical trials, taking into account the heterogeneous background of the plasma CETP concentration.
| Acknowledgments |
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| Footnotes |
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| References |
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