Specificity of lipid transfer protein for molecular species of cholesteryl ester

ApoF knockdown increases cholesteryl ester transfer to LDL and impairs cholesterol clearance in fat-fed hamsters

Cholesteryl ester transfer protein (CETP) regulates intravascular lipoprotein metabolism. In vitro studies indicate that ApoF alters CETP function by inhibiting its activity with LDL. To explore in vivo the complexities driving ApoF's effects on CETP, we developed a siRNA-based hamster model of ApoF knockdown. In both male and female hamsters on chow- or fat-fed diets, we measured lipoprotein levels and composition, determined CETP-mediated transfer of cholesteryl esters (CEs) between lipoproteins, and quantified reverse cholesterol transport (RCT). We found that apoF knockdown in chow-fed hamsters had no effect on lipoprotein levels or composition, but these ApoF-deficient lipoproteins supported 50-100% higher LDL CETP activity in vitro. ApoF knockdown in fat-fed male hamsters created a phenotype in which endogenous CETP-mediated CE transfer from HDL to LDL increased up to 2-fold, LDL cholesterol increased 40%, HDL declined 25%, LDL and HDL lipid compositions were altered, and hepatic LDLR gene expression was decreased. Diet-induced hypercholesterolemia obscured this phenotype on occasion. In fat-fed female hamsters, ApoF knockdown caused similar but smaller changes in plasma CETP activity and LDL cholesterol. Notably, ApoF knockdown impaired HDL RCT in fat-fed hamsters but increased sterol excretion in chow-fed animals. These in vivo data validate in vitro findings that ApoF regulates lipid transfer to LDL. The consequences of ApoF knockdown on lipoproteins and sterol excretion depend on the underlying lipid status. By minimizing the transfer of HDL-derived CE to LDL, ApoF helps control LDL cholesterol levels when LDL clearance mechanisms are limiting.

Effects of eicosapentaenoic acid and docosahexaenoic acid on low-density lipoprotein cholesterol and other lipids: a review

In this exploratory, hypothesis-generating literature review, we evaluated potentially differential effects of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) on low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), triglycerides (TG), and non-HDL-C in published studies of ω-3 fatty acid supplementation or prescription ω-3 fatty acid ethyl esters. Placebo-adjusted changes in mean lipid parameters were compared in randomized, controlled trials in subjects treated for ≥ 4 weeks with DHA or EPA. Of 22 studies identified, 6 compared DHA with EPA directly, 12 studied DHA alone (including 14 DHA-treated groups), and 4 examined EPA alone. In studies directly comparing EPA with DHA, a net increase in LDL-C of 3.3% was observed with DHA (DHA: +2.6%; EPA: -0.7%). In such head-to-head comparative studies, DHA treatment was associated with a net decrease in TG by 6.8% (DHA: -22.4%; EPA: -15.6%); a net increase in non-HDL-C by 1.7% (DHA: -1.2%; EPA -2.9%); and a net increase in HDL-C by 5.9% (DHA: +7.3%; EPA: +1.4%). Increases in LDL-C were also observed in 71% of DHA-alone groups [with demonstrated statistical significance (P < .05) in 67% (8 of 12) DHA-alone studies] but not in any EPA-alone studies. Changes in LDL-C significantly correlated with baseline TG for DHA-treated groups. The range of HDL-C increases documented in DHA-alone vs EPA-alone studies further supports the fact that HDL-C is increased more substantially by DHA than EPA. In total, these findings suggest that DHA-containing supplements or therapies were associated with more significant increases in LDL-C and HDL-C than were EPA-containing supplements or therapies. Future prospective, randomized trials are warranted to confirm these preliminary findings, determine the potential effects of these fatty acids on other clinical outcomes, and evaluate the generalizability of the data to larger and more heterogeneous patient populations.

High throughput quantification of cholesterol and cholesteryl ester by electrospray ionization tandem mass spectrometry (ESI-MS/MS)

Analysis of free cholesterol (FC) is not well suited for electrospray ionization (ESI); however, cholesteryl ester (CE) form ammonium adducts in positive ion mode and generate a fragment ion of m/z 369 upon collision-induced fragmentation. In order to allow parallel analysis of FC and CE using ESI tandem mass spectrometry (ESI-MS/MS), we developed an acetyl chloride derivatization method to convert FC to cholesteryl acetate (CE 2:0). Derivatization conditions were chosen to provide a quantitative conversion of FC to CE 2:0 without transesterification of naturally occurring CE species. FC and CE were analyzed by direct flow injection analysis using a fragment of m/z 369 in a combination of selected reaction monitoring (SRM) and precursor ion scan for FC and CE, respectively. Quantification was achieved using deuterated D(7)-FC and CE 17:0/CE 22:0 as internal standards as well as calibration lines generated by addition of FC and naturally occurring CE species to the respective sample matrix. The developed assay showed a precision and detection limit sufficient for routine analysis. A run time of 1.3 min and automated data analysis allow high throughput analysis. Loading of human skin fibroblast and monocyte derived macrophages with stable isotope labeled FC showed a potential application of this method in metabolism studies. Together with existing mass spectrometry methodologies for lipid analysis, the present methodology will provide a useful tool for clinical and biochemical studies and expands the lipid spectrum that can be analyzed from one lipid sample on a single instrumental platform.

Variations in serum cholesteryl ester transfer and phospholipid transfer activities in healthy women and men consuming diets enriched in lauric, palmitic or oleic acids

Cholesteryl ester transfer protein (CETP) and phospholipid transfer protein (PLTP) activities were measured in sera from 32 normolipidemic women and men consuming diets enriched in lauric, palmitic, or oleic acids. Serum CETP activity, measured as the rate of radiolabeled cholesteryl esters transferred from HDL toward serum apo B-containing lipoproteins, was higher with the palmitic acid diet (25.1+/-2.5%) than with the lauric acid (23.7+/-2.4%) and the oleic acid (24.0+/-2.7%) diets (P = 0.0028 and 0.0283, respectively). CETP mass concentrations, as measured with an enzyme-linked immunosorbent assay were increased after the lauric acid diet (2.57+/-0.63 mg/l) and the palmitic acid diet (2.49+/-0.64 mg/l) as compared with the oleic acid diet (2.34+/-0.45 mg/l) (P = 0.0035 and 0.0249, respectively). In contrast with CETP, serum PLTP activity, as measured as the rate of radiolabeled phosphatidylcholine transferred from liposomes toward serum HDL, was significantly higher with the lauric acid diet (23.5+/2.6%) than with the palmitic acid diet (22.5+/-2.5%) (P = 0.0013), while no significant differences were noted when comparing the saturated diets versus the oleic acid diet (23.0+/-2.3%). No significant alterations in the mean apparent diameter of LDL, and in the relative proportions of individual HDL subpopulations were observed from one dietary period to another. Nevertheless, lipid transfer activities correlated significantly with the relative abundance of HDL2b, HDL2a, HDL3b, and HDL3c, with opposite tendencies being observed for cholesteryl ester transfer and phospholipid transfer activities. In general, serum CETP activity correlated negatively with HDL cholesterol, but positively with triglyceride concentrations after the dietary interventions, and the relations with serum lipids were just the opposite for PLTP activity. In addition, CETP and PLTP activities correlated negatively when subjects consumed the standardized diets (P < 0.05 in all cases), but not when subjects consumed their habitual diet. It is concluded that serum lipid transfer activities in normolipidemic subjects can be significantly affected by the fatty acid content of the diet, with differential effects on CETP and PLTP activities.

Inhibition of cholesteryl ester transfer protein by apolipoproteins, lipopolysaccharides, and cholesteryl sulfate

Cholesteryl ester transfer protein (CETP) mediates the exchange of cholesteryl esters and triglycerides between lipoproteins in the plasma. In studies dealing with the mechanism of CETP-mediated lipid transfer, we have examined the effects of several classes of biomolecules, including apolipoproteins and related synthetic peptides, cholesteryl sulfate, and lipopolysaccharides. In all cases, the molecules were inhibitory and their effects were associated with modifications of either HDL, LDL, or both. However, the probable mechanisms were distinct for each class of inhibitor. Inhibition of lipid transfer activity by apolipoprotein A-I was correlated with an increase in the apolipoprotein A-I content of HDL but not LDL, whereas the primary effect of cholesteryl sulfate was associated with modification of LDL, and only modest alteration of HDL. Lipopolysaccharides were found to modify the size and charge properties of both LDL and HDL over the same concentration ranges that affected CETP activity, but might also interact directly with CETP. It is suggested from the present studies that a variety of biomolecules that can interact with lipoproteins under natural or pathological situations have the potential to modify CETP activity, which in turn could affect normal lipoprotein composition and distribution.

Different effects of palmitic and stearic acid-enriched diets on serum lipids and lipoproteins and plasma cholesteryl ester transfer protein activity in healthy young women

The effects of palmitic and stearic acid-enriched diets on serum lipids, lipoproteins, apolipoproteins (apo) A-I and B, and plasma cholesteryl ester transfer protein (CETP) activity were examined in 12 healthy young women. Subjects followed the two experimental diets for 4 weeks according to a randomized crossover design. Both experimental diet periods were preceded by consumption of a baseline diet for 2 weeks. The diets provided 37% of total energy intake (E%) as fat, and differed only with respect to fatty acid composition. There was a substitution of 5E% of palmitic acid or stearic acid in the experimental diets for 5E% of monounsaturated fatty acids in the baseline diet. After the palmitic acid diet, serum total and high-density lipoprotein (HDL) cholesterol and apo A-I concentrations were higher (8%, P = .015, 9%, P = .040, and 11%,P = .011, respectively) and mean serum low-density lipoprotein (LDL) cholesterol concentration tended to be higher (8%, P = .077) as compared with values after the stearic acid diet. Plasma CETP activity increased in the palmitic acid diet as compared with the stearic acid diet (12%, P = .006). In conclusion, palmitic acid and stearic acid-enriched diets had different effects on serum lipids and lipoproteins and also on plasma CETP activity in young healthy women.

Cholesterol-loading of peripheral tissues alters the interconversion of high density lipoprotein subfractions in rabbits

High density lipoprotein (HDL) has been implicated in the process of reverse cholesterol transport,by which surplus cholesterol is removed from peripheral tissues and transported to the liver for excretion. It has been suggested that some subfractions of HDL may have a particular role in this process, though the underlying mechanism remains unclear. The present study was aimed at investigating the role of specific subfractions of HDL in reverse cholesterol transport. The interconversion of HDL subfractions in normal and cholesterol-loaded rabbits was studied in vivo. Rabbit HDL was separated by heparin-Sepharose affinity chromatography into six subfractions (HDL(I)-HDL(VI)), which were progressively enriched with apolipoprotein E (apo E), and varied in diameter and composition. Total HDL and its subfractions were individually labelled with 14C sucrose and injected in the rabbits. When rabbits which were not acutely loaded with [3H]cholesterol were injected with 14C-HDL(I), 70% of the label remained in this fraction while less than 5% was recovered in HDL(VI), containing the largest particles and those most enriched in apo E. No label was detectable in the liver of these animals. In rabbits which had received a prior loading of cholesterol, an average of only 18.3% of the 14C label was present in HDL(I) while approx. 40% of the label was recovered in HDL(VI). On average, 5.1% of the total 14C injected in these rabbits was recovered in the liver. It is concluded that two alternative routes for reverse cholesterol transport may be operative. While a continuous cholesterol-clearance route may be provided by particles of HDL of intermediate size, another route may be operative for clearance of excess cholesterol loaded into peripheral endothelial cells.

Plasma cholesteryl ester transfer protein activity is increased when trans-elaidic acid is substituted for cis-oleic acid in the diet

The trans isomer of oleic acid (elaidic acid) increases low density lipoprotein (LDL) cholesterol and decreases high density lipoprotein (HDL) cholesterol in man. One possible mechanism for this effect is that trans fatty acids increase plasma cholesteryl ester transfer protein (CETP) activity. We examined the effect of dietary trans fatty acids on activity of this protein in plasma from 27 men in a double blind crossover comparison. The background diet, containing 15% energy as fat from dairy products, meat, bread and cereals, was supplemented with oleic or elaidic acid providing a further 20% energy. The elaidic supplement provided about 6% energy as trans fatty acid. Activity of CETP in plasma was significantly higher (P < 0.001) after the elaidic acid-rich diet (23.95 +/- 1.26%) compared with the diet enriched with oleic acid (19.61 +/- 0.89%). A significant correlation between the change in plasma trans 18:1 fatty acids and the change in plasma CETP activity (r = 0.58, P < 0.002) was independent of changes in LDL-cholesterol. The increase in CETP activity was in turn significantly correlated with a fall in HDL-cholesterol among subjects during the elaidic acid-rich period (r = -0.57, P < 0.01). We have shown that CETP demonstrates substrate specificity and that the increase in activity with dietary trans fatty acids may contribute to a more atherogenic lipoprotein profile.

Drug control of reverse cholesterol transport

Reverse cholesterol transport identifies a series of metabolic events resulting in the transport of excess cholesterol from peripheral tissues to the liver. High-density lipoproteins (HDL) are the vehicle of cholesterol in this reverse transport, a function believed to explain the inverse correlation between plasma HDL levels and atherosclerosis. An attempt to stimulate, by the use of drugs, this transport process may hold promise in the prevention and treatment of arterial disease. Among the agents affecting lipoprotein metabolism, only probucol exerts significant effects on reverse cholesterol transport, by stimulating the activity of the cholesteryl ester transfer protein and, consequently, altering HDL subfraction composition/distribution. Another approach to the stimulation of reverse cholesterol transport consists of raising plasma HDL levels; studies in animals, either by exogenous supplementation or by endogenous overexpression, have shown a consistent benefit in terms of atherosclerosis regression and/or non-progression. Thus, it is time to consider different future treatments of atherosclerosis, combining the classical lipid-lowering treatments with innovative methods to promote cholesterol removal from the arterial wall.

Docosahexaenoic and eicosapentaenoic acids in plasma phospholipids are divergently associated with high density lipoprotein in humans

The effect of fish oil rich in eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids on serum lipoprotein concentrations is not clear, and it is not known whether EPA and DHA are similarly related to serum lipid or lipoprotein levels. We conducted a randomized, 10-week, dietary supplementation trial in which the effects of 6 g per day of 85% EPA and DHA were compared with 6 g per day of corn oil in 156 men and women. Multivariate analyses were used to assess independent relations between plasma phospholipid EPA and DHA and serum lipoprotein levels. In the fish oil group triglycerides fell 21% (p less than 0.001) and high density lipoprotein cholesterol (HDL-C) rose 3.8% (p less than 0.05). In the corn oil group triglycerides did not change, but HDL-C rose 6.1% (p less than 0.01). Compared with fish oil, apolipoprotein A-I (apo A-I) rose 5.1% after corn oil intake (p less than 0.05). Plasma EPA and DHA levels rose after fish oil intake and fell after corn oil intake (all p less than 0.001). The change (delta) in EPA was inversely correlated with delta triglycerides (p = 0.035) and positively correlated with delta HDL-C and delta apo A-I (both p less than 0.001) in the multivariate analyses. In contrast, delta DHA was not correlated with delta triglycerides but was inversely correlated with delta HDL-C and delta apo A-I (both p less than 0.001). Standardizing for DHA removed the difference in apo A-I levels between groups. This study suggests that EPA and DHA are divergently associated with HDL, possibly through different mechanisms.

Estimation of HDL cholesteryl ester kinetic parameters in the cebus monkey, an animal species with high plasma cholesteryl ester transfer activity

We studied the kinetic parameters of high density lipoprotein (HDL) cholesteryl esters in the cebus monkey, an animal species with high plasma cholesteryl ester transfer activity. HDL were radiolabeled with cholesteryl [1-14C]oleate and intravenously administered to 4 cebus monkeys. The calculated fractional catabolic rate (FCR) of the HDL cholesteryl esters was 0.081 +/- 0.002 (mean +/- SD) h-1 and the calculated residence time was 12.3 +/- 0.3 h. The production or disposal rate of plasma HDL cholesteryl esters was 34.3 +/- 4.5 mumol/h. The radiolabeled cholesteryl esters were rapidly transferred from the HDL to the very low and low density lipoproteins (VLDL + LDL) and the amount of tracer in the VLDL + LDL had already reached a maximum at 3.5 +/- 0.7 h after tracer administration. The estimated fraction of VLDL + LDL cholesteryl esters derived from the HDL was 0.77 +/- 0.06. We also used radiolabeled [1,2-3H(N)]cholesteryl palmityl ether to trace HDL cholesteryl esters, but the ether tracer was more slowly cleared from the plasma and less readily transferred between plasma lipoproteins than the ester tracer.

Interaction of lipid transfer protein with plasma lipoproteins and cell membranes

The hydrophobic lipid components of lipoproteins, cholesteryl ester and triglyceride, are transferred between all lipoproteins by a specific plasma glycoprotein, termed lipid transfer protein (LTP). LTP facilitates lipid transfer by an exchange process in which cholesteryl ester and triglyceride compete for transfer. Thus, LTP promotes remodeling of the lipoprotein structure, and plays an important role in the intravascular metabolism of these particles and in the lipoprotein-dependent pathways of cholesterol clearance from cells. The properties of LTP, its mechanisms of action, its roles in lipoprotein metabolism, and its modes of regulation are reviewed along with recent data that suggest a possible role for this protein in directly modifying cellular lipid composition.

An improvement of Barter's method for assaying plasma cholesterol ester transfer activity: experimental and clinical applications

The use of a discontinuous density gradient and of a vertical rotor to separate plasma lipoproteins are modifications of Barter's described method for assaying cholesteryl ester transfer activity (CETA) in plasma. The original feature of our approach is the fast preparation of the labeled substrate by a physiologic-like process, which renders the assay easy and suitable for measurement of this activity in both man and animals.

Neutral lipid transfer and lipolysis convert high molecular weight LDL from cholesterol-fed nonhuman primates towards normal: a molecular analysis

In cynomolgus monkeys (Macaca fascicularis) fed an atherogenic diet, large, cholesterol ester-rich LDL (Mr greater than 3.5.10(6] are found at the same time that the plasma triacylglycerol levels are low. We studied whether the presence of higher concentrations of triacylglycerol-rich lipoproteins (VLDL) during in vitro incubations would allows depletion from LDL of cholesterol ester and a decreased LDL molecular weight. Three high Mr LDL (Mr = (3.7-4.8).10(6)), rich in cholesterol ester (50 +/- 1.4% by weight), were isolated from three animals by zonal ultracentrifugation, and were then incubated with human VLDL at 37 degrees C for 18 h in lipoprotein-deficient human plasma containing neutral lipid transfer activity. After incubation, modified LDL (M-LDL) was isolated by zonal ultracentrifugation. M-LDL was triacylglycerol-rich (36 +/- 5% by weight) and cholesterol ester-poor (20 +/- 3%), and cholesterol ester had transferred into VLDL. Purified lipoprotein lipase was added to the M-LDL, and triacylglycerol was hydrolyzed. The size of the post-lipolysis M-LDL (Mp-LDL) particles became smaller (mean diameters of 253 A and 228 A for two native LDLs and 215 A and 193 A for Mp-LDL, respectively). Both analytical and zonal ultracentrifugation showed Mp-LDL to be more dense than native LDL. Estimated molecular weights for Mp-LDL were 40%-50% less than that of the original LDL, and fell within the molecular weight range for normal human and monkey LDL. Lipid exchanges, but not apoprotein transfers, were responsible for LDL remodelling, as supported by three separate methods of analysis. Cholesterol ester losses accounted for about two-thirds of the molecular weight decrease. These in vitro results suggest that cholesterol ester enrichment of apoprotein B lipoprotein particles can be reversed by providing adequate levels of VLDL in the presence of neutral lipid transfer processes and lipolytic activity.