In vivo transfer of cholesteryl esters from high density lipoproteins to very low density lipoproteins in man.

The fate of cholesteryl esters in high density lipoprotein (HDL) was studied to determine whether the transfer of esterified cholesterol from HDL to other plasma lipoproteins occurred to a significant extent in man. HDL cholesteryl ester, labelled in vitro with [3H] cholesterol, was injected into human subjects. Labelling of cholesteryl esters in very low density (VLDL) occurred rapidly and by 3 h, the esterified cholesterol in VLDL reached peak specific radioactivity. The removal rate of cholesteryl esters from HDL appeared to be exponential and of the order of 0.2/h; calculation of the apparent flux was about 150 mg/h which approximates reported values for total cholesterol esterification in human plasma in vivo. The rapid rate of labelling of VLDL from HDL suggests that the transfer of HDL cholesteryl esters to VLDL may represent a significant pathway for the disposal of HDL cholesterol.     

VLDL-bound lipoprotein lipase facilitates the cholesteryl ester transfer protein-mediated transfer of cholesteryl esters from HDL to VLDL

In recent years, it has been established that lipoprotein lipase (LPL) is partly associated with circulating lipoproteins. This report describes the effects of physiological amounts of very low density lipoprotein (VLDL)-bound LPL on the cholesteryl ester transfer protein (CETP)-mediated cholesteryl ester transfer (CET) from high density lipoprotein (HDL) to VLDL. Three patients with severe LPL deficiency exhibited a strong decrease in net mass CET that was more than 80% lower than that of common hypertriglyceridemic subjects. Recombination experiments showed that this was due to an abnormal behavior of the VLDL fraction. Replacement of the latter by normal VLDL totally normalized net mass CET. We therefore prepared VLDL containing controlled amounts of bound LPL that we used as CE acceptors in experiments involving unidirectional radioisotopic CET measurements. These were carried out either in the absence or in the presence of inhibitors of LPL lipolytic activity. When LPL-induced lipolysis was totally blocked, the stimulating effect of the enzyme on the CETP-dependent CET was only reduced by about 50%, showing that it did not entirely result from its lipolytic action. These data were dependent upon neither the type of LPL inhibitor (E600 or THL) nor the source of CETP (delipidated plasma or partially purified CETP). Thus, in addition to the well-known stimulating effect of LPL-dependent lipolysis on CET, our work demonstrates that physiological amounts of VLDL-bound LPL may facilitate CET through a mechanism partially independent of its lipolytic activity.     

Human plasma cholesteryl ester transfer protein enhances the transfer of cholesteryl ester from high density lipoproteins into cultured HepG2 cells

The role of human plasma cholesteryl ester transfer protein (CETP) in the cellular uptake of high density lipoprotein (HDL) cholesteryl ester (CE) was studied in a liver tumor cell line (HepG2). When HepG2 cells were incubated with [3H]cholesteryl ester-labeled HDL3 in the presence of increasing concentrations of CETP there was a progressive increase in cell-associated radioactivity to levels that were 2.8 times control. The CETP-dependent uptake of HDL-CE was found to be saturated by increasing concentrations of both CETP and HDL. The CETP-dependent uptake of CE radioactivity increased continuously during an 18-h incubation. In contrast to the effect on cholesteryl ester, CETP failed to enhance HDL protein cell association or degradation. Enhanced uptake of HDL cholesteryl ester was shown for the d greater than 1.21 g/ml fraction of human plasma, partially purified CETP, and CETP purified to homogeneity, but not for the d greater than 1.21 g/ml fraction of rat plasma which lacks cholesteryl ester transfer activity. HDL cholesteryl ester entering the cell under the influence of CETP was largely degraded to free cholesterol by a process inhibitable by chloroquine. CETP enhanced uptake of HDL [3H]CE in cultured smooth muscle cells and to a lesser extent in fibroblasts but did not significantly influence uptake in endothelial cells or J774 macrophages. These experiments show that, in addition to its known role in enhancing the exchange of CE between lipoproteins, plasma CETP can facilitate the in vitro selective transfer of CE from HDL into certain cells.     

Mechanism of the cholesteryl ester transfer protein-mediated uptake of high density lipoprotein cholesteryl esters by Hep G2 cells

Plasma cholesteryl ester transfer protein (CETP) mediates the transfer of cholesteryl esters (CE) between lipoproteins and was reported to also directly mediate the uptake of high density lipoprotein (HDL) CE by human Hep G2 cells and fibroblasts. The present study investigates that uptake and its relationship to a pathway for "selective uptake" of HDL CE that does not require CETP. HDL3 labeled in both the CE and apoprotein moieties was incubated with Hep G2 cells. During 4-h incubations, CE tracer was selectively taken up from doubly labeled HDL3 in excess of apoA-I tracer, and added CETP did not modify that uptake. However, during 18-20-h incubations, CETP stimulated the uptake of CE tracer more than 4-fold without modifying the uptake of apoA-I tracer. This suggested that secreted products, perhaps lipoproteins, might be required for the CETP effect. Four inhibitors of lipoprotein uptake via low density lipoprotein (LDL) receptors (heparin, monensin, an antibody against the LDL receptor, and antibodies against the receptor binding domains of apoB and apoE) effectively blocked the CETP stimulation of CE tracer uptake. Heparin caused an increase in CE tracer in a d less than 1.063 g/ml fraction of the medium that more than accounted for the heparin blockade of CETP-stimulated CE uptake. CETP did not affect the uptake of doubly labeled HDL3 by human fibroblasts, even at twice plasma levels of activity, and heparin did not modify uptake of HDL3 tracers. Thus the CETP effect on Hep G2 cells can be accounted for by transfer of HDL CE to secreted lipoproteins which are then retaken up, and there is no evidence for a direct effect of CETP on cellular uptake of HDL CE.     

Familial cholesteryl ester transfer protein deficiency is associated with triglyceride-rich low density lipoproteins containing cholesteryl esters of probable intracellular origin

The net transfer of core lipids between lipoproteins is facilitated by cholesteryl ester transfer protein (CETP). We have recently documented CETP deficiency in a family with hyperalphalipoproteinemia, due to a CETP gene splicing defect. The purpose of the present study was to characterize the plasma lipoproteins within the low density lipoprotein (LDL) density range and also the cholesteryl ester fatty acid distribution amongst lipoproteins in CETP-deficient subjects. In CETP deficiency, the conventional LDL density range contained both an apoE-rich enlarged high density lipoprotein (HDL) (resembling HDLc), and also apoB-containing lipoproteins. Native gradient gel electrophoresis revealed clear speciation of LDL subclasses, including a distinct population larger in size than normal LDL. Anti-apoB affinity-purified LDL from the CETP-deficient subjects were shown to contain an elevated triglyceride to cholesteryl ester ratio, and also a high ratio of cholesteryl oleate to cholesteryl linoleate, compared to their own HDL or to LDL from normal subjects. Addition of purified CETP to CETP-deficient plasma results in equilibration of very low density lipoprotein (VLDL) cholesteryl esters with those of HDL. These data suggest that, in CETP-deficient humans, the cholesteryl esters of VLDL and its catabolic product, LDL, originate predominantly from intracellular acyl-CoA:cholesterol acyltransferase (ACAT). The CETP plays a role in the normal formation of LDL, removing triglyceride and transferring LCAT-derived cholesteryl esters into LDL precursors.     

Lipid transfer protein-catalyzed exchange of cholesteryl ester between high density lipoproteins and apoB-containing lipoproteins

Low density lipoproteins (LDL), lipoprotein (a)(Lp(a)), and lipoprotein(a) after removal of the a-protein (Lp(a-)) were compared with respect to their ability to accept cholesteryl ester from high density lipoproteins (HDL). The incubations were performed at constant concentrations of HDL and various concentrations of either LDL, Lp(a), or Lp(a-). Lp(a) exchanged cholesteryl ester with HDL, but at a rate that was only 48.5 +/- 3.8% of the exchange rate found in the presence of autologous LDL. Cleavage of the apo(a) from Lp(a) resulted in Lp(a-), an LDL-like particle, with characteristics of cholesteryl ester exchange very similar to LDL.     

Plasma phospholipid transfer protein enhances transfer and exchange of phospholipids between very low density lipoproteins and high density lipoproteins during lipolysis

In order to determine the effects of a plasma phospholipid transfer protein on the transfer of phospholipids from very low density lipoproteins (VLDL) to high density lipoproteins (HDL) during lipolysis, biosynthetically labeled rat 32P-labeled VLDL was incubated with human HDL3 and bovine milk lipoprotein lipase (LPL) in the presence of the plasma d greater than 1.21 g/ml fraction or a partially purified human plasma phospholipid transfer protein (PTP). The addition of either the PTP or the d greater than 1.21 g/ml fraction resulted in a 2- to 3-fold stimulation of the transfer of phospholipid radioactivity from VLDL into HDL during lipolysis. In the absence of LPL, the PTP caused a less marked stimulation of transfer of phospholipid radioactivity. Both the d greater than 1.21 g/ml fraction and the PTP enhanced the transfer of VLDL phospholipid mass into HDL, but the percentage transfer of phospholipid radioactivity was greater than that of phospholipid mass, suggesting stimulation of both transfer and exchange processes. Stimulation of phospholipid exchange was confirmed in experiments where PTP was found to augment transfer of [14C]phosphatidylcholine radioactivity from HDL to VLDL during lipolysis. In experiments performed with human VLDL and human HDL3, both the d greater than 1.21 g/ml fraction and the PTP were found to stimulate phospholipid mass transfer from VLDL into HDL during lipolysis. Analysis of HDL by non-denaturing polyacrylamide gradient gel electrophoresis showed that enhanced lipid transfer was associated with only a slight increase in particle size, suggesting incorporation of lipid by formation of new HDL particles. In conclusion, the plasma d greater than 1.21 g/ml fraction and a plasma PTP enhance the net transfer of VLDL phospholipids into HDL and also exchange of the phospholipids of VLDL and HDL. Both the transfer and exchange activities of PTP are stimulated by lipolysis.     

Microemulsions of cholesteryl oleate and dimyristoylphosphatidylcholine: a model for cholesteryl ester rich very low density lipoproteins

This study describes the preparation, purification, and characterization of a cholesteryl oleate/dimyristoylphosphatidylcholine microemulsion as a model for the interaction of lipid domains in cholesteryl ester rich very low density lipoproteins. These lipids were chosen specifically because their thermal transitions were distinct from each other, and their differences in chemical structure permitted the motion(s) of each lipid component to be monitored independently by 13C nuclear magnetic resonance (NMR). The model particles were formed by cosonication of cholesteryl oleate and dimyristoylphosphatidylcholine in a 4:1 molar ratio for 45 min at 55-60 degrees C (above both lipid phase transition temperatures). The crude microemulsion was fractionated by low-speed centrifugation and Sepharose CL-2B chromatography. Microemulsion particles which eluted from the column at a volume similar to that of cholesteryl ester rich very low density lipoproteins had high cholesteryl ester:phospholipid ratios (2.5:1----6:1). Electron micrographs of negatively stained particles showed them to be large spheres devoid of multilamellar or unilamellar vesicle structures. Particle size calculated from a simple compositional model correlated well with sizes determined by electron microscopy (500-1000 A) for various column fractions. Differential scanning calorimetry studies of the microemulsion revealed two thermal transitions for the model particles, at 31.0 and 46.6 degrees C, which were tentatively assigned to the surface phospholipid and core cholesteryl ester domains, respectively. These assignments were confirmed by 13C NMR which demonstrated that, at temperatures near the lower thermotropic transition, only resonances derived from carbon atoms of dimyristoylphosphatidylcholine (DMPC) were observable. As the temperature was raised to 38.6 degrees C, resonances from the olefinic carbons in the cholesteryl ester acyl chain appeared in the spectrum. At 46.6 degrees C, the center of the higher temperature endotherm, resonances from both the steroid ring and remaining acyl chain carbons of cholesteryl oleate became observable in the spectrum. Further increases in temperature did not result in the appearance of new resonances; however, those that were present narrowed and increased in intensity. The elevation in transition temperature for DMPC in these particles (31 degrees C) as compared to that for DMPC in small unilamellar (18 degrees C) and large multilamellar (23 degrees C) vesicles suggested a stabilization of the phospholipid monolayer, possibly by interaction with the nonpolar core lipids.(ABSTRACT TRUNCATED AT 400 WORDS)     

Triacylglycerol increase in plasma very low density lipoproteins in cyclophosphamide-treated rabbit: relationship with cholesteryl ester transfer activity

We have studied the cholesteryl ester transfer between HDL and VLDL in cyclophosphamide-treated rabbits, in order to explain the abnormal cholesteryl ester partition between these two lipoprotein classes. The hypertriglyceridemia caused by treatment with the drug was associated with cholesteryl ester- and triacylglycerol-rich VLDL and with HDL poor in esterified cholesterol but relatively enriched in triacylglycerol. These two lipoprotein classes were characterized by their chemical composition and by gel filtration chromatography. VLDL particles were slightly larger in size, compared with controls. Different transfer combinations were envisaged between these abnormal lipoproteins and control ones. The transfer study involved the plasma fraction of d greater than 1.21 g/ml containing the cholesteryl ester transfer protein (CETP). It appeared that the chemical composition of lipoproteins was responsible for the level of cholesteryl ester transfer between lipoproteins. Actually, when the cholesteryl ester acceptor lipoproteins (VLDL) were enriched in triacylglycerol, the transfer was enhanced. Therefore, the effect of lipolysis on the transfer has also been explored. Lipoprotein lipase seemed to enhance the transfer of cholesteryl ester from HDL to VLDL when these lipoproteins were normal, but an important decline was obtained when triacylglycerol-rich VLDL were lipolyzed. This study defines the relationship between lipoprotein chemical composition and transfer activity of cholesteryl ester from HDL to VLDL.     

In vitro incorporation of radiolabeled cholesteryl esters into high and low density lipoproteins

We have developed and validated a method for in vitro incorporation of radiolabeled cholesteryl esters into low density (LDL) and high density lipoproteins (HDL). Radiolabeled cholesteryl esters dissolved in absolute ethanol were mixed with LDL or HDL in the presence of lipoprotein-deficient serum (LPDS) as a source of core lipid transfer activity. The efficiency of incorporation was dependent on: a) the core lipid transfer activity and quantity of LPDS, b) the mass of added radiolabeled cholesteryl esters, c) the length of incubation, and d) the amount of acceptor lipoprotein cholesterol. The tracer incorporation was documented by repeat density gradient ultracentrifugation, agarose gel electrophoresis, and precipitation with heparin-MnCl2. The radiolabeling conditions did not affect the following properties of the lipoproteins: 1) chemical composition, 2) electrophoretic mobility on agarose gels, 3) hydrated density, 4) distribution of apoproteins on SDS gels, 5) plasma clearance rates, and 6) immunoprecipitability of HDL apoproteins A-I and A-II. Rat HDL containing radiolabeled cholesteryl esters incorporated in vitro had plasma disappearance rates identical to HDL radiolabeled in vivo.     

Lipoprotein lipase. Mechanism of formation of triglyceride-rich remnant particles from very low density lipoproteins and chylomicrons.

The catalytic rate of membrane-supported lipoprotein lipase has been determined for chylomicron and very low density lipoprotein substrates during the formation of triglyceride-depleted ("remnant") particles. Both lipoprotein species and their generated remnant products were competitive substrates for lipase activity. Remnant formation from each species was associated with decreasing kc but an unchanged apparent Km. This finding was confirmed from the rate of plot of total triglyceride catabolism by lipase at low substrate concentrations. When compared with the major very low density lipoprotein fraction (Sf 100-400), a fraction isolated from plasma with a lower flotation rate (Sf 40-100) had a lipid composition and decreased kc compatible with this representing a physiological remnant particle.     

Role of cholesteryl ester transfer protein in selective uptake of high density lipoprotein cholesteryl esters by adipocytes

Previous reports attributed cholesteryl ester transfer protein (CETP)-mediated HDL cholesteryl ester (CE) selective uptake to the CETP-mediated transfer of CE from HDL to newly secreted apolipoprotein B-containing lipoproteins, which are then internalized by the LDL receptor (LDL-R). CETP has also been implicated in the remodeling of HDL, which renders it a better substrate for selective uptake by scavenger receptor class B type I (SR-BI). However, CETP-mediated selective uptake of HDL3-derived CE was not diminished in LDL-R null adipocytes, SR-BI null adipocytes, or in the presence of the receptor-associated protein. We found that monensin treatment or energy depletion of the SW872 liposarcoma cells with 2-deoxyglucose and NaN3 had no effect on CETP-mediated selective uptake, demonstrating that endocytosis is not required. This is supported by data indicating that CETP transfers CE into a compartment from which it can be extracted by unlabeled HDL. CETP could also mediate the selective uptake of HDL3-derived triacylglycerol (TG) and phospholipid (PL). The CETP-specific kinetics for TG and CE uptake were similar, and both reached saturation at approximately 5 microg/ml HDL. In contrast, CETP-specific PL uptake did not attain saturation at 5 microg/ml HDL and was approximately 6-fold greater than the uptake of CE. We propose two possible mechanisms to account for the role of CETP in selective uptake.     

Apparent inhibition of cholesteryl esters exchange by high density lipoproteins

The kinetics of the exchange of Cholesteryl esters between low density lipoproteins (LDL) and high density lipoproteins (HDL) stimulated by lipoprotein depleted plasma has been studied in vitro. The results indicate that the exchange is inhibited with the increase of HDL present in the assay, although the limiting factor is not the absolute concentration of HDL, since in a simultaneous LDL increase, the exchange augments proportionally to the total cholesteryl esters pool. Implications regarding overall metabolism of body cholesterol are discussed.     

Mechanism of the phospholipid transfer protein-mediated transfer of phospholipids from model lipid vesicles to high density lipoproteins

To study the effects of the phospholipid transfer protein (PLTP) on the thermodynamic parameters governing the transfer of phospholipids (PL) from single bilayer vesicles (SBV) to high density lipoprotein (HDL), we performed transfer measurements at various temperatures between 4 and 65 degrees C, using a pyrenylphosphatidylcholine (Pyr-PC) as probe. The proportion of excimer (E) to monomer (M) fluorescence of a pyrenyl moiety constitutes a direct measure of its local concentration. The transfers of Pyr-PC were monitored by following the decrease of E/M. The data were used to calculate the rate constants K(+1) for the transfer from SBV to HDL and to generate the corresponding Arrhenius plots. The equilibrium constants, K(eq), for the same reactions were also determined and used to generate Van't Hoff plots. From these data, we calculated the thermodynamic parameters for both the whole transfer reaction and the transition state. Both K(+1) and K(eq) values clearly varied with temperature. PLTP induced very similar decreases in the free energy for the whole reaction (DeltaG) and in that for the transition state (DeltaG(#)). At 37 degrees C, the decreases were of 0.37 and 0.29 kcal/mol, respectively. We studied the thermal denaturation of PLTP between 37 and 65 degrees C, and the effects of denatured PLTP samples on the PL transfer reaction were then determined. In all cases, the changes of DeltaG remained comparable to those of DeltaG(#). Thus the essential action of PLTP is to facilitate the first step of the reaction, which can be considered as the desorption of PL molecules from the surface of donor particles.     

Lipoprotein lipase releases esterified oxylipins from very low-density lipoproteins

We previously demonstrated that defects in lipoprotein metabolism alter the distribution of oxygenated polyunsaturated fatty acids (PUFAs) in lipoprotein particles. If these oxidation products are released by lipoprotein lipase (LpL), then their delivery to peripheral tissues with bulk lipids could influence cellular function. Using 26-week-old normolipidemic and hyperlipidemic Zucker rats, we measured PUFA alcohols, epoxides, diols, ketones, and triols (i.e. oxylipins) in esterified and non-esterified fractions of whole plasma, VLDL, and LpL-generated VLDL-lipolysates. Whole plasma, VLDL, and lipolysate oxylipin profiles were distinct and altered by hyperlipidemia. While >90% of the whole plasma oxylipins were esterified, the fraction of each oxylipin class in the VLDL varied: 46% of alcohols, 30% of epoxides, 19% of diols, <10% of ketones, and <1% triols. Whole plasma was dominated by arachidonate alcohols, while the linoleate alcohols, epoxides, and ketones showed an increased prevalence in VLDL. LpL-mediated VLDL lipolysis of PUFA alcohols, diols and ketones was detected and the relative abundance of oxygenated linoleates was enhanced in the lipolysates, relative to their corresponding VLDL. In summary esterified oxylipins were seen to be LpL substrates with heterogeneous distributions among lipoprotein classes. Moreover, oxylipin distributions are changes within the context of obesity-associated dyslipidemia. These results support the notion that the VLDL–LpL axis may facilitate the delivery of plasma oxylipins to the periphery. The physiological implications of these findings are yet to be elucidated; however, these molecules are plausible indicators of systemic oxidative stress, and could report this status to the peripheral tissues.     

Regulation of the selective uptake of cholesteryl esters from high density lipoproteins by sphingomyelin

Although sphingomyelin (SM) is a major phospholipid in lipoproteins as well as in the membrane rafts where the scavenger receptor class B type I (SR-BI) is localized, its possible role in the selective uptake of cholesteryl ester (CE) by the SR-BI-mediated pathway is unknown. We investigated the effect of SM in lipoproteins and cell membranes on the selective uptake in three different cell lines: SR-BI-transfected CHO cells, hepatocytes (HepG2), and adrenocortical cells (Y1BS1). Incorporation of SM into recombinant high density lipoprotein (rHDL) containing labeled CE resulted in up to 50% inhibition of the selective uptake of CE in all three cell lines. This inhibition was completely reversed by treatment of rHDL with sphingomyelinase (SMase). Selective uptake from plasma HDL was activated by 22-72% after treatment of HDL with SMase. In addition, pretreatment of the cells with SMase resulted in stimulation of CE uptake from rHDL by CHO and Y1BS1, although not by HepG2. Incorporation of ceramide into rHDL resulted in up to 2-fold stimulation of CE uptake, although pretreatment of cells with egg ceramide had no significant effect. These results show that SM and ceramide in the lipoproteins and the cell membranes regulate the SR-BI-mediated selective uptake of CE, possibly by interacting with the sterol ring or with SR-BI itself.     

Interactions of high density lipoproteins with very low and low density lipoproteins during lipolysis

Interactions of high density lipoproteins (HDL) with very low (VLDL) and low (LDL) density lipoproteins were investigated during in vitro lipolysis in the presence of limited free fatty acid acceptor. Previous studies had shown that lipid products accumulating on lipoproteins under these conditions promote the formation of physical complexes between apolipoprotein B-containing particles (Biochim. Biophys. Acta, 1987. 919: 97-110). The presence of increasing concentrations of HDL or delipidated HDL progressively diminished VLDL-LDL complex formation. At the same time, association of HDL-derived apolipoprotein (apo) A-I with both VLDL and LDL could be demonstrated by autoradiography of gradient gel electrophoretic blots, immunoblotting, and apolipoprotein analyses of reisolated lipoproteins. The LDL increased in buoyancy and particle diameter, and became enriched in glycerides relative to cholesterol. Both HDL2 and HDL3 increased in particle diameter, buoyancy, and relative glyceride content, and small amounts of apoA-I appeared in newly formed particles of less than 75 A diameter. Association of apoA-I with VLDL or LDL could be reproduced by addition of lipid extracts of lipolyzed VLDL or purified free fatty acids in the absence of lipolysis, and was progressively inhibited by the presence of increasing amounts of albumin. We conclude that lipolysis products promote multiple interactions at the surface of triglyceride-rich lipoproteins undergoing lipolysis, including physical complex formation with other lipoprotein particles and transfers of lipids and apolipoproteins. These processes may facilitate remodeling of lipoproteins in the course of their intravascular metabolism.     

An apolipoprotein preferentially enriched in cholesteryl ester-rich very low density lipoproteins

Phosphatidyl glycerol is present in lamellar bodies and in the material obtained by alveolar wash representing 12.3 and 11.5%, respectively, of the total phospholipid phosphorus. Lung microsomes catalyze the formation of phosphatidyl glycerol from the known precursors, L-glycerol 3-phosphate and CDP-diglyceride. The rate of [¹⁴C]L-glycerol 3-phosphate incorporation into phosphatidyl glycerol was 30% higher in microsomes as compared to mitochondria. The addition of mercuric chloride inhibited the synthesis of phosphatidyl glycerol, and stimulated the incorporation into another as yet incompletely identified lipid. After pulse labeling of microsomal phosphatidyl glycerol $\text{in vitro}$, further incubation of microsomes with lamellar bodies or alveolar wash resulted in nearly quantitative appearance of label in surfactant.     

Enhanced cholesteryl ester transfer protein activities and abnormalities of high density lipoproteins in familial hypercholesterolemia.

Cholesteryl ester transfer protein may play a role in the cholesteryl ester metabolism between high density lipoproteins (HDL) and apolipoprotein B-containing lipoproteins. To investigate relationship between HDL and cholesteryl ester transfer protein (CETP) activity in the development of atherosclerosis, the present study has focused on CETP activity in the patients with familial hypercholesterolemia (GH). HDL-C and HDL-C/apo A-I mass ratio in heterozygous FH were lower than those in normolipidemic controls. There was a 2-fold increase in total CETP activity in incubated FH serum compared with normolipidemic controls. Assays for CETP activity in the lipoprotein deficient serum (d greater than 1.215 g/ml) were carried out by measuring the transfer of radioactive cholesteryl ester from HDL (1.125 less than d less than 1.21 g/ml) to LDL (1.019 less than d less than 1.060 g/ml). CETP activities in heterozygous FH (79 +/- 4 nmol/ml/h) was significantly higher than those in normolipidemic controls (54 +/- 6 nmol/ml/h). The increased total cholesteryl ester transfer mainly results from increased CETP activity in the d greater than 1.215 g/ml, possibly reflecting an increase in CETP mass in serum. Increased CETP activity in the d greater than 1.215 g/ml was correlated positively with IDL-cholesterol/triglyceride mass ratio (r = 0.496, p less than 0.01), and negatively with HDL-cholesterol/apo A-I mass ratio (r = -0.334, p less than 0.05). These results indicate that the enhanced CETP activities may contribute to increase risk for developing atherosclerosis in FH by changing the distribution of cholesteryl ester in serum lipoproteins.     

Chylomicron remnant cholesteryl esters as the major constituent of very low density lipoproteins in plasma of cholesterol-fed rabbits.

Feeding rabbits 500 mg of cholesterol daily for 4 to 15 days greatly increased the concentration of esterified cholesterol in lipoproteins of d less than 1.006 g/ml. The origin of hypercholesterolemic very low density lipoproteins was investigated by monitoring the degradation of labeled lymph chyomicrons administered to normal and cholesterol-fed rabbits. Chylomicrons were labeled in vivo by feeding either 1) [3H]cholesterol and [14C]oleic acid or 2) [14C]cholesterol and [3H]retinyl acetate. After intravenous injection of labeled chylomicrons to recipient rabbits, [14C]triglyceride hydrolysis was equally rapid in normal and cholesterol-fed animals. Normal rabbits rapidly removed from plasma both labeled cholesteryl and retinyl esters, whereas cholesterol-fed rabbits retained nearly 50% of doubly labeled remnants in plasma 25 min after chylomicron injection. Ultracentrifugal separation of plasma into subfractions of very low density lipoproteins showed that chylomicron remnants in cholesterol-fed animals are found among all subclasses of very low density lipoproteins. Analysis of cholesteryl ester specific activity-time curves for the very low density lipoproteins subfraction from hypercholesterolemic plasma showed that nearly all esterified cholesterol in large very low density lipoproteins and approximately 30% of esterified cholesterol in small very low density lipoproteins was derived from chylomicron degradation. Apparently, nearly two-thirds of the esterified cholesterol in total very low density lipoproteins from moderately hypercholesterolemic rabbits is of dietary origin.