Effect of unesterified cholesterol on the activity of cholesteryl ester transfer protein

Prediction of the influences of missense mutations on cholesteryl ester transfer protein structure

The structure of human plasma cholesteryl ester transfer protein (CETP) was mapped in silico by a search of the structural effects of missense mutations in the CETP gene. Sixteen deleterious substitutions were chosen among 54 known missense mutations and further ranked by stability change score into six structural and ten functional mutations with large and small stability changes, respectively. A cluster of eight mutations in a central region spanning residues 184-296 with exclusively destabilizing effects was evident. Moreover, the mutations were differently distributed between ordered and highly fluctuating regions. Putative cholesterol-binding regions, mostly unique for CETP in a whole CETP-including protein family, were identified. Three of six structural mutations influence cholesteryl ester and phosphatidylcholine binding by CETP. The local partially disordered structure of some putative cholesterol-binding regions is suggested to be differently influenced by cholesterol binding. This may underlie the impairment of the local ordering effect of cholesterol by the L261R substitution. Also, cholesterol may competitively inhibit cholesteryl ester binding to the CETP molecule, with triglyceride binding being largely undisturbed. This analysis may contribute to the ongoing design and mechanistic studies of new CETP inhibitors.

Reversible flow of cholesteryl ester between high-density lipoproteins and triacylglycerol-rich particles is modulated by the fatty acid composition and concentration of triacylglycerols

We determined the influence of fasting (FAST) and feeding (FED) on cholesteryl ester (CE) flow between high-density lipoproteins (HDL) and plasma apoB-lipoprotein and triacylglycerol (TG)-rich emulsions (EM) prepared with TG-fatty acids (FAs). TG-FAs of varying chain lengths and degrees of unsaturation were tested in the presence of a plasma fraction at d > 1.21 g/mL as the source of CE transfer protein. The transfer of CE from HDL to FED was greater than to FAST TG-rich acceptor lipoproteins, 18% and 14%, respectively. However, percent CE transfer from HDL to apoB-containing lipoproteins was similar for FED and FAST HDL. The CE transfer from HDL to EM depended on the EM TG-FA chain length. Furthermore, the chain length of the monounsaturated TG-containing EM showed a significant positive correlation of the CE transfer from HDL to EM (r = 0.81, P < 0.0001) and a negative correlation from EM to HDL (r = -041, P = 0.0088). Regarding the degree of EM TG-FAs unsaturation, among EMs containing C18, the CE transfer was lower from HDL to C18:2 compared to C18:1 and C18:3, 17.7%, 20.7%, and 20%, respectively. However, the CE transfer from EMs to HDL was higher to C18:2 than to C18:1 and C18:3, 83.7%, 51.2%, and 46.3%, respectively. Thus, the EM FA composition was found to be the rate-limiting factor regulating the transfer of CE from HDL. Consequently, the net transfer of CE between HDL and TG-rich particles depends on the specific arrangement of the TG acyl chains in the lipoprotein particle core.

Composition, structure and substrate properties of reconstituted discoidal HDL with apolipoprotein A-I and cholesteryl ester

To investigate the influence of lipid unsaturation and neutral lipid on the maturation of high density lipoproteins, the discoidal complexes of apoA-I, phosphatidylcholine and cholesteryl ester (CE) were prepared. Saturated dipalmitoylphosphatidylcholine (DPPC) and unsaturated palmitoyllinoleoylphosphatidylcholine (PLPC), palmitoyloleoylphosphatidylcholine (POPC), and fluorescent probe cholesteryl 1-pyrenedecanoate (CPD) that forms in a diffusion- and concentration-dependent manner short-lived dimer of unexcited and excited molecules (excimer) were used. The apoA-I/DPPC/CPD complexes were heterogeneous by size, composition and probe location. CPD molecules incorporated more efficiently into larger complexes and accumulated in a central part of the discs. The apoA-I/POPC(PLPC)/CPD were also heterogeneous, however, probe molecules distributed preferentially into smaller complexes and accumulated at disc periphery. The kinetics of CPD transfer by recombinant cholesteryl ester transfer protein (CETP) to human plasma LDL is well described by two-exponential decay, the fast component with a shorter transfer time being more populated in PLPC compared to DPPC complexes. The presence of CE molecules in discoidal HDL results in particle heterogeneity. ApoA-I influences the CETP activity modulating the properties of apolipoprotein-phospholipid interface. This may include CE molecules accumulation in the boundary lipid in unsaturated phosphatidylcholine and cluster formation in the bulk bilayer in saturated phosphatidylcholine.

Effect of the nonenzymatic glycosylation of high density lipoprotein-3 on the cholesterol ester transfer protein activity

This study examines the relationship between high density lipoprotein-3 (HDL-3) glycation and cholesteryl ester transfer mediated by cholesteryl ester transfer protein (CETP). HDL-3 were glycated with various glucose concentrations (0-200 mM) for 3 d at 37 degrees C with sodium cyanoborohydride as reducing agent and antioxidants. About 47% of the lysine residues were glycated in the presence of 200 mM glucose, resulting in an increase in the cholesterol ester (CE) transfer of about 30%. Apparent kinetic parameters [expressed as maximal transfer (appT(max)) and CE concentration at half of T(max)(appK(H))] of CETP activity with glycated HDL-3 showed conflicting and paradoxical data: an increase in CETP activity associated with a decrease of CETP affinity. These alterations were not due to a change in HDL-3 lipid and protein composition nor to a peroxidative process but were associated with an increase in HDL-3 electronegativity and a decrease of HDL-3 fluidity. This study suggests that glycation modifies the apolipoprotein's conformation and solvation which are major determinants of interfacial properties of HDL-3. These modifications in turn affect CETP reactivity.

Effect of dietary omega-3 eicosapentaenoic acid supplements on cholesteryl ester transfer from HDL in cholesterol-fed rabbits

We investigated the reactivities of cholesteryl ester transfer protein (CETP) in Japanese white rabbits fed either a low-cholesterol diet containing 0.1% cholesterol (Control group) or a diet containing 0.1% cholesterol plus 17.5% omega-3 eicosapentaenoic acid (omega-3 20:5, EPA) of 4.5% (w/w) total lipid (EPA group) for 6 weeks. The plasma total and LDL cholesterol levels and aortic cholesterol content were all significantly higher in the EPA group than in the control group. The aortic cholesterol content significantly correlated with LDL cholesterol (r = 0.81). HDL cholesterol levels tended to be lower in the EPA group compared with control group, which was not statistically significant. The plasma VLDL cholesterol levels did not differ significantly between the groups. In addition, no significant differences were observed in the plasma CETP activity or lecithin:cholesterol acyltransferase (LCAT) activity between the groups. However, the cholesteryl ester (CE) mass transfer from fractionated HDL in the EPA group to excess VLDL and/or LDL as acceptors by purified CETP increased significantly compared with the control group, even if the acceptors were fractionated from either the EPA or the control group. Fatty acid analyses of CE showed that the omega-3 18:3, 20:4 or omega-3 20:5 fatty acid acyl groups in CE of HDL were significantly more transferred to apo B-containing lipoproteins compared with the 14:0,16:0, 18:0, 18:1 or 18:2 fatty acid acyl groups in CE of HDL during the incubation period. The amount of CE in HDL containing omega-3 18:3 and omega-3 20:5 fatty acid acyl groups was greater, while the amount of CE containing 18:2 fatty acid acyl groups was smaller in the EPA group than in the control group. These results show that although CETP itself did not change, the transfer of CE in HDL to apo B-containing lipoproteins by CETP increased in the rabbits fed a diet containing EPA as the HDL is modified by the diet, which may partly explain why atherogenicity was thus found to progress in the rabbits fed a cholesterol plus EPA diet.

Effects of charged lipids on the interaction of cholesteryl ester transfer protein with lipid microemulsions

This study reports the effects of charged lipids on the transfer of cholesterlyl-1 pyrene decanoate (Py-CE) between apolipoprotein-free microemulsion particles mediated by cholesteryl ester transfer protein (CETP). The surface charge characteristics of microemulsion particles composed of cholesteryl oleate and egg yolk phosphatidylcholine were altered by incorporating phosphatidyl-serine, oleate or stearylamine into the phosphatidylcholine that forms the surface monolayer of the particle. The transfer of Py-CE was measured continuously by following the decrease in excimer fluorescence that accompanies the transfer of the probe from donor to acceptor particles [Rajaram, Chan and Sawyer (1994) Biochem. J. 304, 423-430]. The inclusion of 20 mol% phosphatidylserine relative to the phospholipid in the surface monolayer of the emulsion caused a 64% decrease in the first-order rate constant describing the transfer. An increase in ionic strength caused a partial reversal of this effects, indicating that electrostatic factors are only partially responsible for the interaction with lipid. Complete inhibition of transfer was observed when 10 mol% sodium oleate was incorporated into the surface monolayer. The incorporation of stearylamine into the emulsion caused a 32% increase in the transfer rate. The binding of CETP to the different emulsion surfaces was also examined using a surface plasmon resonance biosensor. The presence of negatively charged lipid (phosphatidylserine or oleic acid) decreased the rate of association of CETP with the emulsion without a significant change in the dissociation rate constant. The presence of the positively charged lipid stearylamine increased the rate of association of CETP with the lipid surface. It is concluded that a negative surface charge on the monolayer decreases the rate of transfer by decreasing the affinity of CETP for these particles.

Physical and kinetic characterization of recombinant human cholesteryl ester transfer protein

Cholesteryl ester transfer protein (CETP) mediates the exchange of triglycerides (TGs), cholesteryl esters (CEs) and phospholipids (PLs) between lipoproteins in the plasma. In order to better understand the lipid transfer process, we have used recombinant human CETP expressed in cultured mammalian cells, purified to homogeneity by immunoaffinity chromatography. Purified recombinant CETP had a weight-average relative molecular mass (MW) of 69561, determined by sedimentation equilibrium, and a specific absorption coefficient of 0.83 litre.g-1.cm-1. The corresponding hydrodynamic diameter (Dh) of the protein, determined by dynamic light scattering, was 14 nm, which is nearly twice the expected value for a spheroidal protein of this molecular mass. These data suggest that CETP has a non-spheroidal shape in solution. The secondary structure of CETP was estimated by CD to contain 32% alpha-helix, 35% beta-sheet, 17% turn and 16% random coil. Like the natural protein from plasma, the recombinant protein consisted of several glycoforms that could be only partially deglycosylated using N-glycosidase F. Organic extraction of CETP followed by TLC showed that CE, unesterified cholesterol (UC), PL, TG and fatty acids (FA) were associated with the pure protein. Quantitative analyses verified that each mol of CETP contained 1.0 mol of cholesterol, 0.5 mol of TG and 1.3 mol of PL. CETP mediated the transfer of CE, TG, PL, and UC between lipoproteins, or between protein-free liposomes. In dual-label transfer experiments, the transfer rates for CE or TG from HDL to LDL were found to be proportional to the initial concentrations of the respective ligands in the donor HDL particles. Kinetic analysis of CE transfer was consistent with a carrier mechanism, having a Km of 700 nM for LDL particles and of 2000 nM for HDL particles, and a kcat of 2 s-1. The Km values were thus in the low range of the normal physiological concentration for each substrate. The carrier mechanism was verified independently for CE, TG, PL and UC in 'half-reaction' experiments.