Enzyme-linked immunosorbent assay for cholesteryl ester transfer protein in human serum

HDL from CETP-deficient subjects shows enhanced ability to promote cholesterol efflux from macrophages in an apoE- and ABCG1-dependent pathway

Genetic deficiency or inhibition of cholesteryl ester transfer protein (CETP) leads to a marked increase in plasma levels of large HDL-2 particles. However, there is concern that such particles may be dysfunctional in terms of their ability to promote cholesterol efflux from macrophages. Recently, the ATP-binding cassette transporter ABCG1, a macrophage liver X receptor (LXR) target, has been shown to stimulate cholesterol efflux to HDL. We have assessed the ability of HDL from subjects with homozygous deficiency of CETP (CETP-D) to promote cholesterol efflux from macrophages and have evaluated the role of ABCG1 and other factors in this process. CETP-D HDL-2 caused a 2- to 3-fold stimulation of net cholesterol efflux compared with control HDL-2 in LXR-activated macrophages, due primarily to an increase in lecithin:cholesterol acyltransferase-mediated (LCAT-mediated) cholesteryl ester formation in media. Genetic knockdown or overexpression of ABCG1 showed that increased cholesterol efflux to CETP-D HDL was ABCG1 dependent. LCAT and apoE contents of CETP-D HDL-2 were markedly increased compared with control HDL-2, and increased cholesterol esterification activity resided within the apoE-HDL fraction. Thus, CETP-D HDL has enhanced ability to promote cholesterol efflux from foam cells in an ABCG1-dependent pathway due to an increased content of LCAT and apoE.

Mass concentration of plasma phospholipid transfer protein in normolipidemic, type IIa hyperlipidemic, type IIb hyperlipidemic, and non-insulin-dependent diabetic subjects as measured by a specific ELISA

Mean plasma phospholipid transfer protein (PLTP) concentrations were measured for the first time by using a competitive enzyme-linked immunosorbent assay. PLTP mass levels and phospholipid transfer activity values, which were significantly correlated among normolipidemic plasma samples (r=0.787, P<0.0001), did not differ between normolipidemic subjects (3.95+/-1.04 mg/L and 575+/-81 nmol. mL-1. h-1, respectively; n=30), type IIa hyperlipidemic patients (4. 06+/-0.84 mg/L and 571+/-43 nmol. mL-1. h-1, respectively; n=36), and type IIb hyperlipidemic patients (3.90+/-0.79 mg/L and 575+/-48 nmol. mL-1. h-1, respectively; n=33). No significant correlations with plasma lipid parameters were observed among the various study groups. In contrast, plasma concentrations of the related cholesteryl ester transfer protein (CETP) were higher in type IIa and type IIb patients than in normolipidemic controls, and significant, positive correlations with total and low density lipoprotein cholesterol levels were noted. Interestingly, plasma PLTP mass concentration and plasma phospholipid transfer activity were significantly higher in patients with non-insulin-dependent diabetes mellitus (n=50) than in normolipidemic controls (6.76+/-1. 93 versus 3.95+/-1.04 mg/L, P<0.0001; and 685+/-75 versus 575+/-81 nmol. mL-1. h-1, P<0.0001, respectively). In contrast, CETP levels did not differ significantly between the 2 groups. Among non-insulin-dependent diabetes mellitus patients, PLTP levels were positively correlated with fasting glycemia and glycohemoglobin levels (r=0.341, P=0.0220; and r=0.382, P=0.0097, respectively) but not with plasma lipid parameters. It is proposed that plasma PLTP mass levels are related to glucose metabolism rather than to lipid metabolism.

Human cholesteryl ester transfer protein measured by enzyme-linked immunosorbent assay with two monoclonal antibodies against rabbit cholesteryl ester transfer protein: plasma cholesteryl ester transfer protein and lipoproteins among Japanese hypercholesterolemic patients

Plasma cholesteryl ester transfer protein (CETP) concentrations were measured in Japanese subjects by an ELISA with two different monoclonal antibodies that were raised against rabbit CETP and cross-reacted against human CETP. Among 63 patients who consecutively underwent coronary angiography, the plasma CETP of 37 patients with luminal stenosis ≥50% in their coronary arteries was not significantly different from that of the 26 patients with luminal stenosis &lt;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.

Skipping of exon 14 and possible instability of both the mRNA and the resultant truncated protein underlie a common cholesteryl ester transfer protein deficiency in Japan

Among the Japanese population, a G-to-A mutation at the beginning of intron 14 of the human cholesteryl ester transfer protein (CETP) gene is a frequent cause of CETP deficiency characterized by markedly increased HDL cholesterol. The resulting abnormalities responsible for null CETP deficiency were studied in detail. The CETP mRNA transcripts amplified by polymerase chain reaction from the monocyte-derived macrophages of two homozygous patients were both found to be normal except for the whole deletion of exon 14. The deletion causes a shift of reading frame and introduces a premature termination codon downstream. Examination of the macrophage RNA from heterozygotes suggested the increased instability of the abnormal mRNA in the cytoplasm, because the amount of the aberrant transcript was nearly one third that of a normal transcript in the cytoplasm, while they were equal in the nucleus. Although this indicated the synthesis of a mutant CETP that lacks about 15% at its carboxy terminus, immunoblot analysis demonstrated that the abnormal CETP was virtually absent in both the media and cell lysates of transfected COS-1 cells, which massively expressed the mutant CETP mRNA. These results elucidate the primary abnormality due to the common CETP splicing mutation to be the exon skipping of mRNA, which decreases the level of mRNA and produces a truncated protein that should be rapidly degraded intracellularly.

Mouse monoclonal antipeptide antibodies specific for cholesteryl ester transfer protein (CETP)

A synthetic peptide whose amino acid sequence corresponds to residues 131-142 of human cholesteryl ester transfer protein (CETP) was used as an immunogen to generate a panel of monoclonal antibodies (MAbs) specific for the intact CETP molecule. Spleen cells from BALB/c mice immunized with the peptide conjugated with keyhole limpet hemocyanin (KLH) were fused with SP2/0 myeloma cells. Two MAbs that bound fixed peptide in an enzyme-linked immunoabsorbent assay (ELISA) were partially characterized regarding their specificity and biological activity. ATM192 of the IgG1 subclass and J16-14 of the IgG3 subclass were used in a Western blot assay as well as in the ELISA. We have also shown through the use of immunoprecipitation that ATM192 can remove CETP enzyme activity from human serum without destroying the enzyme's activity. We have also shown that the antibodies can bind CETP from rabbits. The specificity studies and the lack of inhibition of enzymatic activity suggest that the MAbs bind a structural area of the CETP molecule not a part of the active binding site of the enzyme. We conclude that these antibodies can be valuable as tools for studying CETP levels in human serum as well as in tissue homogenates from rabbits and humans.

CETP is a determinant of serum LDL-cholesterol but not HDL-cholesterol in healthy Japanese

Cholesteryl ester transfer protein (CETP) is one of the factors that regulate plasma levels of HDL-cholesterol. To identify the factors that may regulate CETP activity, and to determine to what extent CETP is correlated with physiologic concentrations of lipoprotein, we performed an epidemiologic study in 586 healthy volunteers (317 males and 269 females mean age 52.2 +/- 10.9 years). CETP activity in these subjects was 192.96 +/- 48.73 (mean +/- S.D.) nmol/ml/h and distributed to a wide range (60-450 nmol/ml/h). Using multiple regression analysis, we found significant positive correlations between CETP activity and LDL-cholesterol (P < 0.03), apolipoprotein (apo) E (P < 0.005) and LCAT activity (P < 0.001). CETP activities showed significant negative correlation with apo A-I (P < 0.03). However, CETP activity showed no significant correlation either with HDL cholesterol or with apo B. One-way layout analysis of variance showed that alcohol drinking and cigarette smoking significantly reduced CETP activity, but there was no significant association between CETP activity and body mass index. Although CETP activities were significantly higher in females than in males (P < 0.001), multiple regression analysis showed no correlation between CETP activity and age in either the males or the females. Our results suggest that CETP activity regulates the concentration of apo A-I and LDL-cholesterol, and that such activity may be influenced by gender, alcohol consumption and cigarette smoking.