Resistance to LDL oxidative modifications of an N-terminal apolipoprotein B epitope

Apolipoprotein CI overexpression is not a relevant strategy to block cholesteryl ester transfer protein (CETP) activity in CETP transgenic mice

ApoCI (apolipoprotein CI) is a potent inhibitor of plasma CETP [CE (cholesteryl ester) transfer protein]. The relevance of apoCI overexpression as a method for CETP blockade in vivo was addressed in the present study in CETPTg/apoCITg mice (mice expressing both human CETP and apoCI). Despite a significant reduction in specific CETP activity in CETPTg/apoCITg mice compared with CETPTg mice [transgenic mouse to human CETP; 46.8+/-11.1 versus 101.8+/-25.7 pmol x h(-1).(mug of plasma CETP)(-1) respectively; P<0.05], apoCI overexpression increased both the CETP mass concentration (3-fold increase; P<0.05) and the hepatic CETP mRNA level (4-fold increase, P<0.005), leading to an increase in total plasma CE transfer activity (by 39%, P<0.05). The ratio of apoB-containing lipoprotein to HDL (high-density lipoprotein) CE was 10-fold higher in CETPTg/apoCITg mice than in apoCITg mice (P<0.0005). It is proposed that the increased CETP expression in CETPTg/apoCITg mice is a direct consequence of liver X receptor activation in response to the accumulation of cholesterol-rich apoB-containing lipoproteins. In support of the latter view, hepatic mRNA levels of other liver X receptor-responsive genes [ABCG5 (ATP-binding cassette transporter GS) and SREBP-1c (sterol-regulatory-binding protein-1c)] were higher in CETPTg/apoCITg mice compared with CETPTg mice. In conclusion, overexpression of apoCI, while producing a significant inhibitory effect on specific CETP activity, does not represent a suitable method for decreasing total CE transfer activity in CETPTg/apoCITg mice, owing to an hyperlipidaemia-mediated effect on CETP gene expression.

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.

Selective resistance of LDL core lipids to iron-mediated oxidation. Implications for the biological properties of iron-oxidized LDL

Although the nature and consequences of oxidative changes in the chemical constituents of low density lipoproteins (LDLs) have been extensively examined, the physical dynamics of LDL oxidation and the influence of physical organization on the biological effects of oxidized LDLs have remained relatively unexplored. To address these issues, in the present studies we monitored surface- and core-specific peroxidative stress relative to temporal changes in conjugated dienes (CDs), particle charge (an index of oxidative protein modification), and LDL-macrophage interactions. Peroxidative stress in LDL surface and core compartments was evaluated with the site-specific, oxidation-labile fluorescent probes parinaric acid (PnA) and PnA cholesteryl ester (PnCE), respectively. When oxidation was initiated by Cu2+, oxidative loss of the core probe (PnCE) closely followed that of the surface probe (PnA), as indicated by the time to 50% probe depletion (t1/2; 15.5 +/- 7.8 and 30.4 +/- 12 minutes for PnA and PnCE, respectively). Both probes were more resistant in LDL exposed to Fe3+ (t1/2, 53.2 +/- 8.1 and 346.7 +/- 155.4 minutes), although core probe resistance was much greater with this oxidant (PnCE t1/2/PnA t1/2 5.8 vs 2.0 for Cu2+). Despite differences in the rate and extent of oxidative changes in Cu(2+)- versus Fe(3+)-exposed LDLs, PnCE loss occurred in close correspondence with CD formation and appeared to precede changes in particle charge under both conditions. Exposure of LDLs to hemin, a lipophilic Fe(3+)-containing porphyrin that becomes incorporated into the LDL particle, resulted in rapid loss of PnCE and simultaneous changes in particle, charge, even at concentrations that yielded increases in CDs and thiobarbituric acid-reactive substances similar to those obtained with free Fe3+. These results suggest that oxidation of the LDL hydrophobic core occurs in conjunction with accelerated formation of CDs and may be essential for LDL protein modification. In accordance with the known effects of oxidative protein modifications on LDL receptor recognition, exposure of LDLs to Cu2+ and hemin but not Fe3+ produced particles that were readily processed by macrophages. Thus, the physical site of oxidative injury appears to be a critical determinant of the chemical and biological properties of LDLs, particularly when oxidized by Fe3+.

Competitive enzyme-linked immunosorbent assay of the human cholesteryl ester transfer protein (CETP)

The present report describes the first competitive enzyme-linked immunosorbent assay (ELISA) for the cholesteryl ester transfer protein (CETP), an enzyme playing an important role in lipoprotein metabolism. This assay was developed with well-characterized TP1 anti-CETP monoclonal antibodies. The sensitivity of the ELISA assay was comparable with the sensitivity of the previously described radioimmunoassays since 1 ng of CETP per microwell of the immunoplate could be detected. Intra- and inter-assay coefficients of variation were 4% and 6%, respectively. This enzyme immunoassay provides a specific, sensitive and reproducible method for measuring CETP concentrations in various biological samples. Within normolipidemic subjects, the mean (+/- S.D.) of the plasma CETP concentration was 2.77 (+/- 0.59) micrograms/ml with a range of 1.87 to 4.23 micrograms/ml. When plasmas were supplemented with fatty acid-free albumin, the positive correlation observed between plasma CETP mass and CETP activity was improved, suggesting that plasma non-esterified fatty acids could play a role in modulating the activity of the cholesteryl ester transfer protein. When applied to the study of the binding of CETP to lipoprotein substrates, the enzyme immunoassay revealed that the experimental protocol used to separate lipoprotein fractions can have a great influence on the plasma distribution of CETP.

Composition and immunoreactivity of serum low density lipoproteins (LDL) before and after LDL-apheresis on dextran sulfate-cellulose columns

The changes in low density lipoprotein (LDL) composition and immunoreactivity occurring after LDL-apheresis on dextran sulfate-cellulose columns (DSC) were investigated in 4 hypercholesterolemic patients. After apheretic treatment, serum levels of total cholesterol, triglycerides and apolipoprotein B (apo B) were decreased by 63, 80 and 65%, respectively, whereas the high density lipoprotein (HDL)-cholesterol remained unchanged. At the end of apheresis, LDL contained less triglycerides, more phospholipids and apo E and the ratio of LDL core lipid components, cholesteryl esters and triglycerides, to LDL surface lipid components, unesterified cholesterol and phospholipids was significantly lower. The post-apheretic LDL were characterized by the presence of subfractions slightly larger than those observed in the pre-apheretic LDL. The modifications of the composition and size of LDL after apheresis were accompanied by a relative increase in the immunoreactivity of 4G3 epitope, an apo B epitope located near the LDL-receptor binding site, with no change in the affinity of 1D1, an apo B epitope located in the amino-terminal region of the molecule. The changes in LDL composition, size and immunoreactivity following apheresis, suggest that postapheresis LDL could contain newly synthesized LDL, different from mature LDL. Thus, LDL-apheresis treatment could provide the opportunity to study the structural change of LDL during intravascular metabolism.

Immunoreactivity of apo B towards monoclonal antibodies that inhibit the LDL-receptor interaction: effects of LDL oxidation

We studied the immunochemical stability of the epitopes for six monoclonal antibodies to human apolipoprotein B-100 upon Cu(2+)-mediated (20 microM) oxidation of LDL. The antibodies used in this study, some of which are known to interfere with the interaction of LDL with their cellular receptors, recognize epitopes in the amino terminal region (Mb 19), in the middle part (6B, 2A, 7A, and 9A) and near aa 3500 (Mb 47) of native apo B. All antibodies except one (7A) recognized native and oxidized LDL (OxLDL) equally well; the immunoreactivity of the epitope for Ab 7A was markedly reduced upon LDL oxidation. Since antibodies 2A, 7A, 9A, and Mb 47 inhibit the LDL-receptor interaction and OxLDL poorly interact in vitro with the LDL receptor we conclude that: (1) various epitopes for monoclonal antibodies against native apo B are spared upon LDL oxidation; and (2) the epitopes for antibodies 2A, 9A, and Mb 47 do not define a unique domain of apo B directly involved in the binding of LDL to their receptor.