Role of lipid transfers in the formation of a subpopulation of small high density lipoproteins

Differential impact of plasma triglycerides on HDL-cholesterol and HDL-apo A-I in a large cohort

OBJECTIVES

To examine the relationship between plasma triglycerides (TG) to HDL-cholesterol (HDL-C) or HDL apo A-I.

DESIGN AND METHODS

Bivariate and multiple linear regression analyses in a large cohort of 1886 subjects.

RESULTS

Higher plasma TG levels were associated with lower concentrations of both HDL-C and HDL-apo A-I. However, the HDL-C/HDL-apo A-I ratio was inversely correlated with plasma TG indicating that the overall composition of the HDL changed as plasma TG changed. Plasma TG levels contributed to 15.9% of the variance of the HDL-C/HDL-apo A-I ratio, whereas gender, HDL-TG, LDL-TG, body mass index and plasma apo B levels represented between 0.15% and 2.21% of this variance.

CONCLUSIONS

These results indicate that increasing levels of plasma TG result in greater reduction in HDL-C levels than in HDL-apo A-I and this might explain, at least in part, the differences that have been observed in the magnitude of the association of HDL-C versus HDL-apo A-I with the risk of cardiovascular disease.

Formation of high density lipoproteins containing both apolipoprotein A-I and A-II in the rabbit

Human plasma HDLs are classified on the basis of apolipoprotein composition into those that contain apolipoprotein A-I (apoA-I) without apoA-II [(A-I)HDL] and those containing apoA-I and apoA-II [(A-I/A-II)HDL]. ApoA-I enters the plasma as a component of discoidal particles, which are remodeled into spherical (A-I)HDL by LCAT. ApoA-II is secreted into the plasma either in the lipid-free form or as a component of discoidal high density lipoproteins containing apoA-II without apoA-I [(A-II)HDL]. As discoidal (A-II)HDL are poor substrates for LCAT, they are not converted into spherical (A-II)HDL. This study investigates the fate of apoA-II when it enters the plasma. Lipid-free apoA-II and apoA-II-containing discoidal reconstituted HDL [(A-II)rHDL] were injected intravenously into New Zealand White rabbits, a species that is deficient in apoA-II. In both cases, the apoA-II was rapidly and quantitatively incorporated into spherical (A-I)HDL to form spherical (A-I/A-II)HDL. These particles were comparable in size and composition to the (A-I/A-II)HDL in human plasma. Injection of lipid-free apoA-II and discoidal (A-II)rHDL was also accompanied by triglyceride enrichment of the endogenous (A-I)HDL and VLDL as well as the newly formed (A-I/A-II)HDL. We conclude that, irrespective of the form in which apoA-II enters the plasma, it is rapidly incorporated into spherical HDLs that also contain apoA-I to form (A-I/A-II)HDL.

Pharmacological modulation of cholesteryl ester transfer protein, a new therapeutic target in atherogenic dyslipidemia

In mediating the transfer of cholesteryl esters (CE) from antiatherogenic high density lipoprotein (HDL) to proatherogenic apolipoprotein (apo)-B-containing lipoprotein particles (including very low density lipoprotein [VLDL], VLDL remnants, intermediate density lipoprotein [IDL], and low density lipoprotein [LDL]), the CE transfer protein (CETP) plays a critical role not only in the reverse cholesterol transport (RCT) pathway but also in the intravascular remodeling and recycling of HDL particles. Dyslipidemic states associated with premature atherosclerotic disease and high cardiovascular risk are characterized by a disequilibrium due to an excess of circulating concentrations of atherogenic lipoproteins relative to those of atheroprotective HDL, thereby favoring arterial cholesterol deposition and enhanced atherogenesis. In such states, CETP activity is elevated and contributes significantly to the cholesterol burden in atherogenic apoB-containing lipoproteins. In reducing the numbers of acceptor particles for HDL-derived CE, both statins (VLDL, VLDL remnants, IDL, and LDL) and fibrates (primarily VLDL and VLDL remnants) act to attenuate potentially proatherogenic CETP activity in dyslipidemic states; simultaneously, CE are preferentially retained in HDL and thereby contribute to elevation in HDL-cholesterol content. Mutations in the CETP gene associated with CETP deficiency are characterized by high HDL-cholesterol levels (>60 mg/dL) and reduced cardiovascular risk. Such findings are consistent with studies of pharmacologically mediated inhibition of CETP in the rabbit, which argue strongly in favor of CETP inhibition as a valid therapeutic approach to delay atherogenesis. Consequently, new organic inhibitors of CETP are under development and present a potent tool for elevation of HDL in dyslipidemias involving low HDL levels and premature coronary artery disease, such as the dyslipidemia of type II diabetes and the metabolic syndrome. The results of clinical trials to evaluate the impact of CETP inhibition on premature atherosclerosis are eagerly awaited.

Induction of the phospholipid transfer protein gene accounts for the high density lipoprotein enlargement in mice treated with fenofibrate

Fibrate treatment in mice is known to modulate high density lipoprotein (HDL) metabolism by regulating apolipoprotein (apo)AI and apoAII gene expression. In addition to alterations in plasma HDL levels, fibrates induce the emergence of large, cholesteryl ester-rich HDL in treated transgenic mice expressing human apoAI (HuAITg). The mechanisms of these changes may not be restricted to the modulation of apolipoprotein gene expression, and the aim of the present study was to determine whether the expression of factors known to affect HDL metabolism (i.e. phospholipid transfer protein (PLTP), lecithin:cholesterol acyltransferase, and hepatic lipase) are modified in fenofibrate-treated mice. Significant rises in plasma PLTP activity were observed after 2 weeks of fenofibrate treatment in both wild-type and HuAITg mice. Simultaneously, hepatic PLTP mRNA levels increased in a dose-dependent fashion. In contrast to PLTP, lecithin:cholesterol acyltransferase mRNA levels in HuAITg mice were not significantly modified by fenofibrate despite a significant decrease in plasma cholesterol esterification activity. Fenofibrate did not induce any change in hepatic lipase activity. Fenofibrate significantly increased HDL size, an effect that was more pronounced in HuAITg mice than in wild-type mice. This effect in wild-type mice was completely abolished in PLTP-deficient mice. Finally, fenofibrate treatment did not influence PLTP activity or hepatic mRNA in peroxisome proliferator-activated receptor-alpha-deficient mice. It is concluded that 1) fenofibrate treatment increases plasma phospholipid transfer activity as the result of up-regulation of PLTP gene expression through a peroxisome proliferator-activated receptor-alpha-dependent mechanism, and 2) increased plasma PLTP levels account for the marked enlargement of HDL in fenofibrate-treated mice.

Net mass transfer of plasma cholesteryl esters and lipid transfer proteins in normolipidemic patients with peripheral vascular disease

The role of plasma cholesteryl ester transfer and lipid transfer proteins in atherosclerosis is unclear. Recent data suggest both antiatherogenic and atherogenic properties for cholesteryl ester transfer protein (CETP). The overall effect of CETP on atherosclerosis may thus vary depending on individual lipid metabolism. To test whether lipid transfer parameters are of importance even in patients without major lipid risk factors for atherosclerosis, CETP mass and activity, net mass transfer of cholesteryl esters between endogenous lipoproteins (CET), and phospholipid transfer protein (PLTP) activity were determined in plasma from 18 normolipidemic male patients with peripheral vascular disease and 21 controls. Furthermore, lecithin: cholesterol acyltransferase (LCAT) activity was tested. The results show that CETP mass, CETP activity, and LCAT activity are not different between patients and controls. However, specific CETP activity (CETP activity/CETP mass) is lower in the patients (P < .02). On the contrary, higher CET is observed in patients' plasma (P < .001). Increased plasma PLTP activity (P = .052) is demonstrable in the patients. If the data of all subjects are combined, CET correlates positively with triglycerides ([TG], r = .45, P < .001) and with PLTP activity (r = .32, P < .05) but negatively with specific CETP activity (r = -.37 P < .05). CET and specific CETP activity remain significantly different in TG-matched patients and controls and are more strongly interrelated (r = -.71, P < .001), suggesting a higher and selective influence of lipid transfer inhibitor(s) on CET and CETP activity in the patients. CET allows the best discrimination between patients and controls in univariate and multivariate analysis. Eighty-eight percent of the subjects are correctly classified by CET as a single parameter. The results suggest that increased CET in the patients may reflect atherogenic alterations in TG metabolism and/or in lipid transfer protein activities despite normal fasting lipoprotein levels.

High-density lipoprotein subclasses and apolipoprotein A-I

Epidemiological and clinical studies showing an association between decreased concentrations of high-density lipoprotein (HDL) cholesterol and increased risk of premature coronary artery disease have generated interest in the mechanism through which HDL prevents atherosclerosis. Recognition of the importance of apolipoproteins (apo(s)) has led to the separation of HDL into subpopulations according to their apolipoprotein composition. It is now recognised that HDL comprises at least two types of apo A-I-containing lipoproteins: LpA-I:A-II containing both apo A-I and apo A-II and LpA-I containing apo A-I but not apo A-II. A majority of studies support the fact that LpA-I is more effective than LpA-I:A-II in promoting cellular cholesterol efflux, the first step in reverse cholesterol transport. Studies in transgenic animals have revealed that the gene transfer of human apo A-I in mice and rabbits increases plasma apo A-I and HDL cholesterol levels and particularly apo A-I-rich HDL particle concentrations, leading to inhibition of the development of dietary or genetically induced atherosclerosis. On the other hand, gene transfer of apo A-II in mice gives conflicting results. The conclusions of some experiments indicate either an atherogenic, or a poorly anti-atherogenic, or even a strongly anti-atherogenic role for apo A-II and for apo A-II-rich HDL lipoproteins. Although these experimental results have been obtained in animals, they confirm previous studies obtained in human clinical studies, indicating that apo A-I-rich HDL (tested as LpA-I in clinical studies) are generally strong plasma markers of atherosclerosis protection while the clinical significance of apo A-I + apo A-II HDL (tested as LpA-I:A-II in clinical studies) is more controversial. The introduction of immunological methods to measure LpA-I and LpA-I:A-II levels in blood make large-scale studies feasible to confirm the clinical significance of these HDL particles.

Abnormal capacity to induce cholesterol efflux and a new LpA-I pre-beta particle in type 2 diabetic patients

In this study, we first characterized the lipoprotein components of serum samples obtained from a group of well-controlled diabetic patients and from healthy subjects in fasting and postprandial states. We then explored some aspects of reverse cholesterol transport in the same population. Patients showed high levels of fasting triglycerides, postprandial triglyceride responses and LpC-III levels (3.18+/-0.86 vs 2.17+/-0.54 mg/dl, P < 0.001). There were also positive correlations between LpC-III and fasting triglycerides (r = 0.82, P < 0.001), total triglyceride area (r = 0.75, P < 0.001) and incremental triglyceride area (r = 0.54, P < 0.001). HDL-C and apo A-I were significantly decreased in diabetic patients due to a selective reduction in LpA-I subfraction, whose antiatherogenic role is generally accepted (37.4+/-8.0 vs 49.2+/-12.5 mg/dl, P < 0.001). In addition, HDL from patients proved to be triglyceride enriched and cholesteryl ester depleted, alterations which were further amplified in the postprandial state. The molar ratio HDL-C/apo A-I + apo A-II, already defined as a predictor of apo A-I fractional catabolic rate, was significantly diminished in the patient group (15.1+/-2.2 vs 20.8+/-3.3, P < 0.001), thus suggesting an accelerated catabolism of apo A-I. For the first time, we describe here the presence of a small apo A-I-containing particle, isolated by two-dimensional electrophoresis and characterized by immunoblotting, only in samples from diabetic patients. This particle that we named pre-beta0, has an apparent molecular weight of 40 kDa. As regards the capacity of serum samples to promote cholesterol efflux from [3H]cholesterol-labeled Fu5AH rat hepatoma cells, patient samples were found to induce significantly lower cholesterol efflux than controls only in the postprandial state (21.2+/-3.3 vs 23.8+/-1.8%, P = 0.012). The presence of pre-beta0 in samples from diabetic patients might therefore be associated to an altered capacity of these serum samples to promote cellular cholesterol efflux. Overall, these abnormalities may contribute to a delay in the reverse cholesterol transport pathway in type 2 diabetic patients.

Heterogeneity of high-density lipoprotein particles and insulin output during oral glucose tolerance test in men with coronary artery disease

We compared the high-density lipoprotein (HDL) composition and particle heterogeneity in 60 nonobese (normal body mass index, BMI) men suffering from coronary artery disease (CAD) with normolipemia and normoinsulinemia with lower and higher insulin output during the oral glucose tolerance test (silent hyperinsulinemia). The apolipoprotein apoAI, apoAII, and apoE levels were higher in the high insulin response (HI) group than in low insulin response (LI) group. The ratio of apoAI versus total protein and the ratio of apoAI versus total cholesterol were increased in HI compared with LI. The lipid components in HDL were higher in LI than in HI, while for HDL2 they were higher in HI. The fractioning of HDL by gradient gel electrophoresis revealed a different pattern of HDL particles in both groups. The larger particles, HDL2b and HDL2a (mean particle diameters 10.6 and 9.2 nm, respectively), occur more frequently in HI patients (up to 60%) than in LI patients, whereas the smaller particles, HDL3a and HDL3b (mean particle diameters 8.6 and 7.8 nm, respectively), predominate in LI patients. Our results demonstrate that even in the normoglycemic, normocholesterolemic CAD patients, a high insulin output observed during the oral glucose tolerance test may be connected with a different HDL particle pattern, which suggests changes in the reverse cholesterol transport.

Triglycerides are major determinants of cholesterol esterification/transfer and HDL remodeling in human plasma

Lecithin:cholesterol acyltransferase (LCAT) and cholesteryl ester transfer protein (CETP) are responsible for the esterification of cell-derived cholesterol and for the transfer of newly synthesized cholesteryl esters (CE) from HDL to apoB-containing lipoproteins in human plasma. LCAT and CETP are also crucial factors in HDL remodeling, a process by which HDL particles with a high capacity for cell cholesterol uptake are generated in plasma. In the present study, cholesterol esterification and transfer were evaluated in 60 patients with isolated hypercholesterolemia (HC, n = 20) and isolated (HTG, n = 20) or mixed hypertriglyceridemia (MHTG, n = 20) and in 20 normolipidemic healthy individuals (NL). Cholesterol esterification rate (CER) and net CE transfer rate (CETR) were measured in whole plasma. LCAT and CETP concentrations were determined by specific immunoassays. HDL remodeling was analyzed by monitoring changes in HDL particle size distribution during incubation of whole plasma at 37 degrees C. Mean CER and CETR were 48% and 73% higher, respectively, in hypertriglyceridemic (HTG + MHTG) versus normotriglyceridemic individuals. HDL remodeling was also significantly accelerated in plasma from hypertriglyceridemic patients. Strong positive correlations were found in the total sample between plasma and VLDL triglyceride levels and CER (r = .722 and r = .642, respectively), CETR (r = .510 and r = .491, respectively), and HDL remodeling (r = .625 and r = .620, respectively). No differences in plasma LCAT and CETP concentrations were found among the various groups except for a tendency toward higher CETP levels in hypercholesterolemic patients (+51% in MHTG and +20% in HC) versus control subjects (NL). By stepwise regression analysis, VLDL triglyceride level was the sole significant predictor of CER and CETR and contributed significantly together with baseline HDL particle distribution to HDL remodeling. These results indicate that plasma triglyceride level is a major factor in the regulation of cholesterol esterification/transfer and HDL remodeling in human plasma, whereas LCAT/CETP concentrations play a minor role in the modulation of reverse cholesterol transport.

Different effects of subclasses of HDL containing apoA-I but not apoA-II (LpA-I) on cholesterol esterification in plasma and net cholesterol efflux from foam cells

We investigated the effects of subclasses of plasma LpA-I (HDL containing apoA-I but not apoA-II) on cholesterol esterification in plasma and net cholesterol efflux from foam cells. LpA-I was composed of particles of three diameters: large (11.1 nm; Lg-LpA-I), medium (8.8 nm; Md-LpA-I), and small (7.7 nm; Sm-LpA-I). Plasma concentrations of LpA-I were positively correlated only with the level of Lg-LpA-I. Plasma concentrations of Lg-LpA-I were inversely correlated with the rate of cholesterol esterification in plasma and VLDL- and LDL-depleted plasma. Plasma concentrations of Md-LpA-I and Sm-LpA-I did not correlate with the rate of cholesterol esterification in plasma or VLDL- and LDL-depleted plasma. When macrophage foam cells were incubated with Md- and Sm-LpA-I, cellular cholesterol mass was reduced by approximately 70%. In contrast, the cellular cholesterol-reducing capacity of Lg-LpA-I was negligible. Lg-LpA-I inhibited net cholesterol removal from foam cells that was mediated by Md- and Sm-LpA-I and cholesteryl ester production with these particles. These results suggest that Md- and Sm-LpA-I may actively participate in cellular cholesterol removal and cholesterol esterification in plasma and HDL, while Lg-LpA-I may regulate these functions of Md- and Sm-LpA-I.

Cycling of apolipoprotein A-I between lipid-associated and lipid-free pools

It has been shown previously that the reduction in particle size of HDL which follows incubation with the cholesteryl ester transfer protein (CETP) plus very-low-density lipoproteins (VLDL) or low-density lipoproteins (LDL) is accompanied by the dissociation of lipid-free apolipoprotein A-I (apo A-I) from HDL. In the present study, we demonstrate that this dissociation of apo A-I is reversible in a process dependent on the activity of lecithin:cholesterol acyltransferase (LCAT). The lipoprotein fraction (d < 1.21 g/ml) of human plasma was mixed with CETP and incubated under conditions such that the HDL decreased in size and there was a dissociation of about 30% of the apo A-I. Following this incubation, the d < 1.21 g/ml fraction was reisolated, supplemented with lipid-free apo A-I and reincubated in the presence and absence of LCAT, either as a component of lipoprotein-deficient plasma or as purified enzyme. In the absence of LCAT, HDL size did not increase and there was no incorporation of lipid-free apo A-I into the HDL density range. In contrast, when LCAT was present, the particle size of HDL increased and lipid-free apo A-I was incorporated into the HDL such that the HDL apo A-I content was comparable to that of the original, unmodified particles. The incorporation of lipid-free apo A-I into the HDL density range was dependent on both the presence of pre-existing HDL and an increase in their size. Thus, just as a reduction in HDL size is accompanied by the dissociation of lipid-free apo A-I, we have now shown that a subsequent increase in HDL size is accompanied by the reincorporation of lipid-free apo A-I into the particle.

Binding of etiopurpurin to human plasma proteins. Delivery in cremophor EL and dimethyl sulphoxide. III

Binding of the photosensitizer etiopurpurin (ET2) to human plasma was assessed, using conditions that would yield a high percentage of ET2 in the form of LDL-bound monomers which may favor photosensitizer tumor localization. Two delivery systems, Cremophor EL (CRM) and dimethyl sulphoxide (DMSO), were used. The binding of ET2 to CRM-modified lipoproteins was compared to the binding of the dye to the native proteins using delivery in DMSO. Plasma-bound monomers and unbound high density aggregates were shown to coexist. The density and rate of formation of the dye aggregates were correlated. The aggregates formed by delivery in DMSO could be partially converted into plasma-bound monomeric ET2. There was no mode-delivery-effect upon the distribution of monomeric ET2 among the plasma proteins. 70% of monomeric ET2 was bound to LDL and most of the remainder to HDL. In delivery in DMSO the yield of LDL-bound dye monomers (up to 30% of added ET2) increased with decreasing concentration of ET2 in the delivery solution and with increasing time of incubation (< or = 48 hr). Long incubation also induced changes in the densities of LDL and HDL. The yields of LDL-bound monomers (up to 40%) increased with increasing concentration of CRM-bound ET2. High yields of LDL-bound monomers were obtained using both modes of delivery. Although the aggregates associated with the two modes of delivery had different properties. The change in lipoprotein composition might be involved in the conversion of aggregates into plasma-bound monomers.

Cholesteryl ester transfer protein: its role in plasma lipid transport

1. The cholesteryl ester transfer protein (CETP) is a hydrophobic glycoprotein which acts in plasma to redistribute cholesteryl esters and triglyceride between plasma lipoproteins. 2. CETP also plays an important role in determining the composition and particle size distribution of high density lipoproteins (HDL). 3. Activity of CETP may be regulated in four ways: By factors which influence the concentration of CETP in plasma; by the activity of CETP inhibitor proteins; by variations in the concentrations and compositions of donor and acceptor lipoproteins and by factors which influence the interaction of CETP with plasma lipoproteins. 4. The mechanism of action of CETP is uncertain. Two models have been proposed: (i) a shuttle model in which CETP physically transports lipids between lipoprotein particles and (ii) a ternary complex model in which CETP forms a bridge between two lipoprotein particles, enabling them to exchange lipids. 5. Evidence is accumulating that CETP may be a pro-atherogenic factor.

Cholesteryl ester transfer activity in lipoprotein lipase deficiency and other primary hypertriglyceridemias

Cholesteryl ester transfer protein (CETP) activity was measured in d > 1.21 g/ml plasma from hypertriglyceridemic patients and compared with normolipidemic subjects. The assay consisted in measuring the specific transfer of [3H]cholesteryl oleate from a prelabelled, apo E-poor HDL fraction to VLDL after incubation at 37 degrees C in the presence of the d > 1.21 g/ml plasma sample: the lipoproteins were then separated by precipitation with dextran sulfate/Mg2+ solution. Increasing the volume of d > 1.21 g/ml plasma or purified human CETP in the assay produced linear responses in measured activity, whereas, either during incubation at 4 degrees C or in the presence of rat plasma instead of human plasma, the transfer of [3H]cholesteryl oleate to VLDL was not stimulated. Thus, the assay reflects changes in CETP in the sample and appears to be suitable for measuring CETP activity in d > 1.21 g/ml plasma. CETP activity was very similar in the two groups of normolipidemic subjects considered: adolescents (203 +/- 11 nmol esterified cholesterol transferred per 8 h/ml plasma) and adults (215 +/- 5). Patients were grouped into lipoprotein-lipase (LPL)-deficient and non-LPL-deficient according to their enzyme activity in postheparin plasma. CETP activity was highly increased in LPL-deficient, severe hyperchylomicronemic patients (430 +/- 42) and was directly correlated with VLDL levels in the non-LPL-deficient individuals. Marked differences were observed in the lipid composition of HDL and apolipoprotein A-I levels among patients and controls. In the control group, CETP activity was correlated only with HDL-triglyceride and HDL-triglyceride/apo A-I mass ratio, which is compatible with the physiological role of CETP in transferring triglyceride to HDL from other lipoprotein particles. When all hypertriglyceridemic patients were considered together, CETP activity was inversely correlated with apo A-I and HDL-cholesterol, whereas it was directly correlated with HDL-triglyceride/HDL-cholesterol and HDL-triglyceride/apo A-I mass ratios. The results indicate that the enhanced CETP activity associated with hypertriglyceridemia contributes to the compositional change of HDL, which in turn may be responsible for the reduction of HDL levels in this condition.

Influence of plasma cholesteryl ester transfer activity on the LDL and HDL distribution profiles in normolipidemic subjects

The relations of cholesteryl ester transfer protein (CETP) activity to the distribution of low density lipoproteins (LDLs) and high density lipoproteins (HDLs) were investigated in fasting plasma samples from 27 normolipidemic subjects. LDL and HDL subfractions were separated by electrophoresis on 20-160 g/L and 40-300 g/L polyacrylamide gradient gels, respectively. Subjects were subdivided into two groups according to their LDL pattern. Monodisperse patterns were characterized by the presence of a single LDL band, whereas polydisperse patterns were characterized by the presence of several LDL bands of different sizes. To investigate the influence of lipid transfers on LDL patterns, total plasma was incubated at 37 degrees C in the absence of lecithin:cholesterol acyltransferase (LCAT) activity. The incubation induced a progressive transformation of polydisperse patterns into monodisperse patterns. Under the same conditions, initially monodisperse patterns remained unchanged. Measurements of the rate of radiolabeled cholesteryl esters transferred from HDL3s to very low density lipoproteins (VLDLs) and LDLs revealed that subjects with a monodisperse LDL pattern presented a significantly higher plasma CETP activity than subjects with a polydisperse LDL pattern (301 +/- 85%/hr per milliliter versus 216 +/- 47%/hr per milliliter, respectively; p < 0.02). In addition, when total plasma was incubated for 24 hours at 37 degrees C in the absence of LCAT activity, the relative mass of cholesteryl esters transferred from HDLs to apolipoprotein B-containing lipoproteins was greater in plasma with monodisperse LDL than in plasma with polydisperse LDL (0.23 +/- 0.06 versus 0.17 +/- 0.06, respectively; p < 0.02). These results indicated that in normolipidemic plasma, CETP could play an important role in determining the size distribution of LDL particles. The analysis of lipoprotein cholesterol distribution in the two groups of subjects sustained this hypothesis. Indeed, HDL cholesterol levels, the HDL:VLDL+LDL cholesterol ratio, and the esterified cholesterol:triglyceride ratio in HDL were significantly lower in plasma with the monodisperse LDL pattern than in plasma with the polydisperse LDL pattern (p < 0.01, p < 0.01, and p < 0.02, respectively). Plasma LCAT activity did not differ in the two groups. Plasma CETP activity correlated positively with the level of HDL3b (r = 0.542, p < 0.01) in the entire study population. Whereas plasma LCAT activity correlated negatively with the level of HDL2b (r = -0.455, p < 0.05) and positively with the levels of HDL2a (r = 0.475, p < 0.05) and HDL3a (r = 0.485, p < 0.05), no significant relation was observed with the level of HDL3b.(ABSTRACT TRUNCATED AT 400 WORDS)

Binding of drugs to human plasma proteins, exemplified by Sn(IV)-etiopurpurin dichloride delivered in cremophor and DMSO

1. The mode-delivery-effect upon the binding of Sn(IV)-etiopurpurin dichloride (SnET2) in human plasma has been studied by ultracentrifugation, combined with absorption and fluorescence spectroscopy. SnET2 was delivered to plasma either in Cremophore EL (CRM) or in dimethyl sulfoxide (DMSO). To facilitate interpretation, optical, conductivity and aggregation properties of SnET2 were obtained for various solutions. 2. The second order rate constant for the aggregation of SnET2 monomers seemed to be remarkably small, of the order of 10(3) M-1 min-1. 3. SnET2 was bound as monomeric entities. Such entities had environmental-sensitive fluorescent properties dependent on the type of protein or solvent (DMSO, CRM, H2O) with which they interacted. 4. SnET2 showed saturable binding with high density subfraction(s) of high density lipoproteins and with one or more high density proteins. Complete or substantial saturation was achieved at the SnET2 level of 3.5 micrograms/ml. Such binding might be mediated by apolipoprotein D and alpha 1-acid glycoprotein. 5. There was little effect of SnET2 concentrations (3.5-35 micrograms SnET2/ml) upon the plasma binding of SnET2, irrespective of the mode of delivery. 6. The percentages of SnET2 bound to low density lipoproteins (LDL), high density lipoproteins (HDL), and high density proteins (HDP) were 10, 70 and 20%, respectively, for delivery in DMSO. The value for LDL also includes binding with very low density lipoproteins (VLDL). For delivery in CRM the corresponding values were 20, 50 and 30%. Apparently, CRM interacted with HDL entities and reduced their affinity for SnET2. 7. The distribution pattern of SnET2 among lipoproteins reflects interactions with apoproteins and/or with surface phospholipids rather than with core lipid constituents of lipoproteins. 8. Conductivity measurements showed that SnET2 was partly an ionic entity in water. 9. The plasma binding of SnET2 is compared with the corresponding binding of other drugs, both tetrapyrroles and nontetrapyrroles.

Gemfibrozil increases apolipoprotein A-I and cholesterol concentrations in human peripheral lymph

Peripheral lymph lipoproteins were studied in four hyperlipidaemic men before and after 6 weeks of treatment with gemfibrozil, a drug which is known to increase the fractional catabolic rate of very low density lipoproteins (VLDL) by raising lipoprotein lipase activity in peripheral tissues. Decreases in plasma triglycerides of 18-60% (mean, 45%) were accompanied by increases in lymph apolipoprotein (apo) A-I concentration of 30-108% (mean, 66%; P < 0.01), and in lymph cholesterol concentration of 35-100% (mean, 59%; P < 0.05). The additional lymph cholesterol was distributed over a broad range of lipoprotein particle sizes. Effects on plasma apo A-I concentration (mean, +7%) and plasma total cholesterol concentration (-7%) were not statistically significant. No changes were observed in four untreated control subjects. These findings are compatible with the hypothesis that lipolysis of VLDL at the blood-endothelium interface increases the transfer of apo A-I from plasma to interstitial fluids, and thereby promotes cholesterol efflux from cells.

High density lipoprotein metabolism in the horse (Equus caballus)

1. Apolipoprotein A-I dependent lecithin:cholesterol acyl transferase (LCAT) activity was identified in equine lipoprotein deficient plasma (LPDP). 2. LCAT activity showed no breed or sex variation, and was unaltered postprandially. 3. There was no significant cholesteryl ester transfer activity in equine LPDP. 4. Hydrophobic interaction chromatography on phenyl sepharose failed to unmask transfer activity or identify an inhibitor of cholesteryl ester transfer. 5. In 12 Shetland ponies, plasma high density lipoprotein (HDL) concentrations were positively correlated with those of triglyceride, but not with the activities of LCAT, lipoprotein lipase or hepatic lipase.

HDLs and alimentary lipemia. Studies in men with previous myocardial infarction at a young age

The plasma concentration, particle size, and chemical composition of high density lipoproteins (HDLs) are associated with the metabolism of triglyceride-rich lipoproteins (TGRLs). During alimentary lipemia there is active exchange of lipids and apolipoproteins between HDL and apolipoprotein B-containing lipoproteins. Whereas HDL has been assigned a protective role against the development of atherosclerosis, alimentary lipemia has been proposed to represent a potentially atherogenic state. We examined plasma HDL concentration, particle size, and composition and their relations to postprandial TGRLs in 32 postinfarction patients and 10 healthy control subjects after intake of a standardized oral fat load of a mixed-meal type. All patients had undergone coronary angiographies in connection with the myocardial infarction and around 5 years thereafter. The plasma HDL cholesterol concentration decreased significantly in response to the oral fat load, particularly in hypertriglyceridemic patients, with a concomitant increase of HDL triglycerides. A limited and reversible yet consistent increase of HDL particle size (1-2%) was seen 6 hours after intake of the oral fat load on nondenaturing gradient gel electrophoresis (GGE) in both patients and control subjects. Virtually no changes in the plasma concentration of HDL GGE subclasses, lipoproteins containing apolipoprotein A-I but no apolipoprotein A-II (LpA-I), or lipoproteins containing both apolipoproteins A-I and A-II (LpA-I:A-II) were induced in the postprandial state despite massive increases of large very low density lipoprotein (VLDL) and large chylomicron remnant levels (determined as apolipoproteins B-100 and B-48 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis). Strong inverse correlations with fasting plasma HDL cholesterol and the larger HDL GGE subspecies were found for large postprandial VLDL and large chylomicron remnants, whereas the corresponding relations for small VLDL and small chylomicron remnants were weaker. The relations of both large and small VLDL and chylomicron remnants to HDL cholesterol were confined to subjects in the lower fasting plasma HDL cholesterol range (< 1.2 mmol/l). None of the HDL parameters measured, either in the fasting or in the postprandial state (HDL cholesterol, HDL triglycerides, HDL GGE subclasses, LpA-I, and LpA-I:A-II), were related to the development of coronary atherosclerosis, whereas the postprandial plasma levels of small chylomicron remnants, which showed weak negative correlations with HDL, related positively to the progression of coronary atherosclerosis.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of gammalinolenic acid on the subfractions of plasma high density lipoprotein of the rabbit

The effect of dietary supplementation with evening primrose oil (containing 70% gammalinolenic acid) on the concentration of plasma lipids and lipoproteins of the New Zealand White rabbit was investigated. No significant changes were observed in the concentrations of plasma cholesterol or triglycerides during the treatment, although an increase in high density lipoprotein (HDL) cholesterol (P < 0.01) was observed at 4 weeks of evening primrose oil intake and 2 weeks after withdrawal. However, when HDL subpopulations were resolved by gradient gel electrophoresis, major alterations were observed in the distribution of HDL subfractions. These included an increase in HDL2b (P < 0.001) and HDL3c (P < 0.001) and the appearance of very large particles of HDL. These findings suggest that supplementation of diets with n-6 fatty acids may be effective in the long-term prevention of atherosclerosis.

Changes in particle size of high density lipoproteins during incubation with very low density lipoproteins, cholesteryl ester transfer protein and lipoprotein lipase

Previous reports have produced conflicting views of the effects of lipoprotein lipase (LPL) on the particle size distribution of high density lipoproteins (HDL). In this study we have investigated the changes in particle size of HDL promoted by the interaction of LPL, the cholesteryl ester transfer protein (CETP) and very low density lipoproteins (VLDL). When the plasma fraction of d less than 1.21 g/ml (containing all lipoprotein fractions) was incubated for 24 h with bovine milk LPL alone or with CETP alone, there was relatively little change in the particle size distribution of HDL. When both LPL and CETP were added to the lipoprotein mixture, there was a substantial reduction in the particle size of HDL. This reduction in HDL particle size was found to be a direct function of the concentration of CETP. It was also influenced by the concentrations of VLDL and LPL, although in these cases the relationships were complex. When mixtures of the plasma fraction of d = 1.006-1.21 g/ml (this fraction includes low density lipoproteins and HDL but not VLDL) were supplemented with both LPL and CETP and incubated in the presence of varying concentrations of added VLDL, there was a progressive increase in the conversion of HDL into very small HDL particles of radius 3.7 nm as the concentration of VLDL triacylglycerol increased up to about 400 nmol/nml. However, further increases in the concentration of VLDL were accompanied by a progressive reduction in the formation of small HDL particles until, at higher VLDL concentrations, the effect was all but abolished. There was a similar enhancement in the formation of small HDL when LPL was added at low but not at high concentrations. These findings are consistent with the existence of two opposing processes. On the one hand there is likely to be a synergism between CETP and the non-esterified fatty acids (NEFA) released by LPL; this will favour a reduction in HDL particle size. On the other hand, the transfer of lipolysis products from VLDL to HDL may mask any such particle size reduction. The fact that the reduction in HDL particle size promoted by LPL, CETP and VLDL was found to be all but abolished by adding fatty acid-poor albumin to the incubation mixture is consistent with the proposition that NEFA are involved in the process. It also suggests, however, that the phenomenon may have little if any physiological significance.

Human apolipoprotein A-I forms small high density lipoprotein particles in rats in vivo

The effects of injection of purified human or rat apolipoprotein (apo) A-I (1.7 mg/100 g body weight) on the size and composition of rat high density lipoprotein (HDL) particles have been investigated. The injection of human apo A-I results in the formation (over a period of 3 to 6 h) of a population of smaller HDL particles resembling human HDL3. This population of smaller particles contains human apo A-I and rat apo A-IV but lacks rat apo A-I and rat apo E. Small HDL3-like particles are not detected in rat plasma following the injection of rat apo A-I. Associated with the injection of either human or rat apo A-I is a gradual increase of plasma cholesterol levels of 20 to 50% (over 24 h) and the appearance of larger HDL particles. The results suggest that the smaller HDL particles in human plasma compared to rat plasma are not simply due to the action of lipid modifying enzymes or lipid transfer proteins but a specific property of human apo A-I.

High density lipoprotein particle size distribution in cord blood

Cord blood from 73 full term healthy newborns and blood from adults were analysed for the protein content of high density lipoprotein subclasses separated by gradient gel electrophoresis. Cholesterol and triglyceride concentrations of very low (VLDL), low (LDL) and high (HDL) density lipoproteins were also analysed and newborns had lower concentrations of cholesterol and triglycerides in VLDL, LDL and HDL (p less than 0.001) than adults. The HDL3c subclass, comprising the smallest particles of the HDL particle spectrum, was the major component for newborns and the minor one for adults and was the only lipoprotein fraction with a higher concentration in cord than in adult blood. No sex differences were present for any of the lipoprotein levels of the newborns. Serum cholesterol concentrations were positively correlated to HDL2b (r = 0.49, p less than 0.001) and HDL2a levels (0.42, p less than 0.001), correlations confined to the cholesterol contents of HDL (r = 0.72 and r = 0.67 respectively, both p less than 0.001). Serum triglycerides were inversely correlated to HDL2b and HDL2a levels in male newborns only (r = 0.38 and r = 0.34 respectively, both p less than 0.05). Irrespective of sex, gestational age and birthweight the newborns had 2 typical HDL subclass distributions, characterized by high or low levels of HDL2b and HDL2a. The newborns with high HDL2b and HDL2a levels also had low VLDL lipid levels and high HDL cholesterol concentrations.

Evidence that reverse cholesterol transport is stimulated by lipolysis of triglyceride-rich lipoproteins

The hypothesis that reverse cholesterol transport by high density lipoprotein (HDL) is augmented by lipolysis of triglyceride-rich lipoproteins received support from experiments in rabbits whose tissue cholesterol had been pre-labeled with [3H]cholesterol several weeks earlier. When lipolysis was stimulated by intravenous heparin (which releases lipoprotein lipase from vascular endothelium), reciprocal changes in plasma triglyceride and HDL cholesterol concentrations were accompanied by a rise in the specific radioactivity of HDL cholesterol, indicative of increased transfer of cholesterol into HDL from slowly exchanging cholesterol pools in extra-hepatic tissues.

Reverse cholesterol transport: physiology and pharmacology

Reverse cholesterol transport identifies a series of metabolic events resulting in the transport of cholesterol from peripheral tissues to the liver and plays a major role in maintaining cholesterol homeostasis in the body. 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 seems to be of great promise in the prevention and treatment of arterial disease. Only few drugs are now known that can modify the activity of the various factors involved in the process. Clofibrate reduces cholesterol esterification, but the newer fibric acids are generally ineffective as anion-exchange resins. Probucol directly increases the activity and mass of cholesteryl ester transfer protein, thus possibly improving the physiological process of cholesterol removal from tissues. The few available data on the effects of drugs on reverse cholesterol transport should stimulate the search for new agents specifically stimulating this antiatherogenic process.

Effects of various non-esterified fatty acids on the particle size redistribution of high density lipoproteins induced by the human cholesteryl ester transfer protein

The effects of various non-esterified fatty acids on the CETP-mediated particle size redistribution of HDL were studied by incubating HDL3 and CETP for 24 h at 37 degrees C in the absence or in the presence of either saturated, monounsaturated or polyunsaturated non-esterified fatty acids. In the absence of non-esterified fatty acids, CETP induced a redistribution of the initial population of HDL3 (Stokes' radius 4.3 nm) by promoting the appearance of one larger (Stokes' radius 4.8 nm) and two smaller (Stokes' radii 3.9 and 3.7 nm) HDL subpopulations. Whereas the non-esterified fatty acids alone did not modify the HDL3 distribution profile, they were able to alter markedly the capacity of CETP to induce the particle size redistribution of HDL. All the saturated fatty acids with at least 10 carbons were able to increase the formation of the very small sized particles (Stokes' radius 3.7 nm) in a concentration dependent manner, the medium chain fatty acids (12 and 14 carbons) being the best activators. The potential effect of non-esterified fatty acids was also influenced by the presence of double bonds in their monomeric carbon chain. While at low concentrations of non-esterified fatty acids (0.1 mmol/l) the enhancement of the formation of very small HDL particles appeared to be greater with oleic and linoleic acids than with stearic acid, at higher concentrations (0.4 mmol/l), oleic, linoleic and arachidonic acids decreased the formation of the 3.7 nm radius particles. The inhibition of the process at high concentrations of unsaturated fatty acids was linked to the degree of unsaturation of their carbon chain, arachidonic acid being the strongest inhibitor. The present study has demonstrated that non-esterified fatty acids can modulate the particle size redistribution of HDL3 mediated by the cholesteryl ester transfer protein even in the absence of any other lipoprotein classes. The effect of non-esterified fatty acid is dependent on both the length and the degree of unsaturation of their monomeric carbon chain.

Interactions between model triacylglycerol-rich lipoproteins and high-density lipoproteins in rat, rabbit and man

There are inverse relationships between HDL cholesterol and plasma triacylglycerol concentrations in normal and in hypertriglyceridemic individuals. To investigate the interactions between triacylglycerol-rich lipid particles and HDL, a lipid emulsion model of the triacylglycerol-rich lipoproteins was prepared. When emulsion particles were incubated with rat high-density lipoproteins (HDL) in the presence of lipid transfer activity (d greater than 1.21 g/ml fractions) from rabbit or human plasma there was a rapid bi-directional exchange of cholesteryl oleate (CO) and phospholipid (PL) labels between lighter and heavier fractions of HDL and emulsion particles. The transfers of CO and PL labels between both light and heavy fractions of HDL and the emulsion particles were increased with increasing amounts of emulsion added to the incubations. Incubation with the d greater than 1.21 g/ml fraction from rat plasma resulted in only a small exchange of CO whereas PL exchange was similar to rabbit and human plasma. Retinyl palmitate label was not transferred from emulsion particles to the HDL fractions even in the presence of lipid transfer activity from rabbit or human plasma. The present study shows that the transfer protein-mediated exchanges of surface and core lipids between HDL and the triacylglycerol-rich lipoproteins are affected by the quantity of triacylglycerol-rich particles in the system. This mechanism may contribute to the inverse relationships between plasma triacylglycerol concentrations and HDL concentrations in normal and hypertriglyceridemic individuals.

Sodium oleate dissociates the heteroexchange of cholesteryl esters and triacylglycerol between HDL and triacylglycerol-rich lipoproteins

Mixtures of human high-density lipoproteins (HDL) and triacylglycerol-rich lipoproteins (TGRL) have been incubated in the presence of partially pure cholesteryl ester transfer protein (CETP). There were net mass transfers of cholesteryl ester from HDL to TGRL and of triacylglycerol from TGRL to HDL which were accompanied by the formation of minor subpopulations of small HDL particles. When the mixture of HDL, TGRL and CETP was supplemented with fatty acid-poor bovine serum albumin (40 mg/ml) there was a 7% reduction in the transfer of cholesteryl esters out of HDL (P less than 0.05) and a 14% increase in the transfer of triacylglycerol into HDL (P less than 0.05); there was also a reduction in the formation of very small HDL particles. In contrast, when the mixture of HDL, TGRL and CETP was supplemented with 0.16 mM sodium oleate the transfer of cholesteryl esters out of HDL was increased by 31% (P less than 0.001) and the transfer of triacylglycerol into HDL was decreased by 25% (P less than 0.01); under these conditions the formation of very small HDL particles was enhanced. It has been concluded that in the presence of sodium oleate, there is a dissociation of the CETP-mediated heteroexchange of cholesteryl esters and triacylglycerol between HDL and TGRL.

The activity of cholesteryl ester transfer protein is decreased in hypothyroidism: a possible contribution to alterations in high-density lipoproteins

The activity of cholesteryl ester transfer protein is instrumental in the distribution of cholesteryl ester between lipoproteins in plasma. We measured the activity of cholesteryl ester transfer protein in plasma, designated cholesteryl ester transfer activity, as the rate of cholesteryl ester transfer between exogenous radiolabelled low-density and high-density lipoproteins. The effect of hypothyroidism on cholesteryl ester transfer activity was investigated in 13 athyreotic patients who were studied in the hypothyroid condition and in the euthyroid state, after they had received triiodothyronine supplementation for 33 to 67 days. During hypothyroidism plasma total cholesterol, very-low- plus low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, plasma triacylglycerol, apolipoprotein A1 and B were significantly higher than in the euthyroid state. Cholesteryl ester transfer activity was 15% lower during hypothyroidism (P less than 0.02), and an effect of treatment duration was observed. The changes in high-density lipoprotein total cholesterol (P less than 0.02), free cholesterol (P less than 0.001), triacylglycerol (P less than 0.05) and the free cholesterol/cholesteryl ester molar ratio in high-density lipoproteins (P less than 0.01) were inversely-related to the changes in cholesteryl ester transfer activity. We concluded that thyroid hormone is involved in the regulation of cholesteryl ester transfer protein activity, and that cholesteryl ester transfer protein activity may play a role in the alterations in high-density lipoprotein lipids observed in hypothyroidism.

In vitro remodelling of plasma lipoproteins in whole plasma by lipoprotein lipase in primary and secondary hypertriglyceridaemia

In patients with familial lipoprotein lipase deficiency (FLPL-d) and glycogen storage disease type I (GSD-I), hypertriglyceridaemia (1445 +/- 247 and 1082 +/- 312 mg dl-1, n = 5 per group) was associated primarily with reduced extrahepatic lipoprotein lipase (LPL) activity (0.33 +/- 0.33 and 1.69 +/- 0.38 mumol FFA ml-1 h-1) when compared with controls (4.83 +/- 0.90). Hypercholesterolaemia was characterized by elevated LDL cholesterol (191 +/- 30 and 344 +/- 34 vs. 115 +/- 5 mg dl-1 in controls P less than 0.01) and low HDL cholesterol (12 +/- 2 and 22 +/- 2 vs. 56 +/- 3 in controls, P less than 0.001). In order to ascertain the role of LPL in the interconversion and remodelling of lipoproteins in these disorders, we analysed lipid and lipoprotein profiles before and following in vitro incubation of patient plasma with purified milk LPL (EC 3.1.1.34) for 6 h at 37 degrees C. The efficiency of exogenous LPL in vitro was demonstrated by the extent of hydrolysis of chylomicrons and of VLDL-TG in both groups. Concomitant with the disappearance of TG-rich lipoprotein particles, a consistent per cent increment of IDL (99.2 +/- 30.8 and 43.9 +/- 70.5), LDL (152.8 +/- 36.2 and 137.0 +/- 36.1) and of HDL2 (144.8 +/- 29.4 and 99.8 +/- 18.7) was observed in both groups of patients. The enhancement of the latter fractions contrasted with the decline of HDL3 mass concentration (25.4 +/- 7.7 and 51.4 +/- 5.8%), suggesting that a major shift of HDL3----HDL2 occurs following in vitro lipolysis by LDL. Simultaneous compositional and morphological changes of individual lipoprotein particles were noted, confirming the dynamic movement and exchange of neutral lipids and proteins. Specificity of LPL results was demonstrated by experiments in which incubation of the whole plasma at 37 degrees C without exogenous lipolytic enzyme did not cause any substantial changes. The present study, therefore, demonstrates a correction of the major lipoprotein abnormalities associated with FLPL-d and GSD-I by exogenous LPL. No substantial difference was noted between primary (FLPL-d) and secondary (GSD-I) hyperlipidaemias. These studies allow us to conclude that a simple in vitro system, utilizing an exogenous source of LPL and plasma from patients, may serve as a suitable model for the study of the metabolic relationships of lipoproteins. However, in view of the fact that the extent of lipolysis achieved in vitro did not differ between FLPL-d and GSD-I, it may not be able to separate primary from secondary hyperlipaemias.

The interaction of cholesteryl ester transfer protein and unesterified fatty acids promotes a reduction in the particle size of high-density lipoproteins

Purified human cholesteryl ester transfer protein (CETP) has been found, under certain conditions, to promote changes to the particle size distribution of high-density lipoproteins (HDL) which are comparable to those attributed to a putative HDL conversion factor. When preparations of either the conversion factor or CETP are incubated with HDL3 in the presence of very-low-density lipoproteins (VLDL) or low-density lipoproteins (LDL), the HDL3 are converted to very small particles. The possibility that the conversion factor may be identical to CETP was supported by two observations: (1) CETP was found to be the main protein constituent of preparations of the conversion factor and (2) an antibody to CETP not only abolished the cholesteryl ester transfer activity of the conversion factor preparations but also inhibited changes to HDL particle size. In additional studies, the changes to HDL particle size promoted by purified CETP were inhibited by the presence of fatty-acid-free bovine serum albumin; by contrast, albumin had no effect on the cholesteryl ester transfer activity of the CETP. The possibility that albumin may inhibit changes to HDL particle size by removing unesterified fatty acids from either the lipoproteins or CETP was tested by adding exogenous unesterified fatty acids to the incubations. In incubations of HDL with either VLDL or LDL, sodium oleate had no effect on HDL particle size. However, when CETP was also present in the incubation mixtures the capacity of CETP to reduce the particle size of HDL was greatly enhanced by the addition of sodium oleate. It is concluded that the changes in HDL particle size which were previously attributed to an HDL conversion factor can be explained in terms of the interacting effects of CETP and unesterified fatty acids.

Lipoprotein lipase prevents the hepatic lipase-induced reduction in particle size of high density lipoproteins during incubation of human plasma

Human plasma lipoproteins or human whole plasma have been incubated in vitro with canine hepatic lipase (HL) and bovine milk lipoprotein lipase (LPL) to determine the effects of lipases on the particle size distribution of HDL. Confirming previous reports, HL preferentially hydrolysed high density lipoprotein (HDL) triacylglycerol while LPL hydrolysed predominantly very low density lipoprotein (VLDL) triacylglycerol; however, neither lipase altered HDL particle size unless both VLDL and cholesteryl ester transfer protein (CETP) were present. Under these conditions HL promoted marked reduction in HDL particle size in a process dependent on the concentration of VLDL triacylglycerol while LPL was virtually without effect. When both LPL and HL were included in the same incubation, however, LPL prevented the effects of HL. These results are consistent with a proposition that HL has a direct effect on HDL particle size in a process which is dependent on concurrent lipid transfers between HDL and VLDL and that LPL reduces the effect of HL by reducing the concentration of VLDL triacylglycerol.

Synergistic effects of lipid transfers and hepatic lipase in the formation of very small high-density lipoproteins during incubation of human plasma

Studies have been performed to determine the involvement of very-low-density lipoproteins (VLDL), cholesteryl ester transfer protein (CETP) and hepatic lipase (HL) in the formation of very small HDL particles. Human whole plasma has been incubated for 6 h at 37 degrees C in the absence and in the presence of various additions. There was minimal formation of very small HDL in incubations of non-supplemented plasma or in plasma supplemented with either VLDL, CETP or HL alone; nor were small HDL prominent after incubating plasma supplemented with mixtures of VLDL plus CETP, VLDL plus HL or CETP plus HL. By contrast, when plasma was supplemented with a mixture containing all three of VLDL, CETP and HL, incubation resulted in an almost total conversion of the HDL fraction into very small particles of radius 3.7 nm. The appearance of these very small HDL was independent of activity of lecithin: cholesterol acyltransferase. It was, however, dependent on both duration of incubation and on the concentrations of the added VLDL, CETP and HL. The effects of these incubations was also assessed in terms of changes to the concentration and distribution of lipid constituents across the lipoprotein spectrum. It was found that not only did lipid transfers and HL exhibit a marked synergism in promoting a reduction in HDL particle size but also that HL, although deficient in intrinsic transfer activity, enhanced the CETP-mediated transfers of cholesteryl esters from HDL to other lipoprotein fractions.

Apolipoprotein AIMilano. Partial lecithin:cholesterol acyltransferase deficiency due to low levels of a functional enzyme

The cholesterol esterification process was analyzed in 19 carriers of the apolipoprotein AIMilano (AIM) variant and in 19 age-sex matched controls by measuring lecithin:cholesterol acyltransferase (LCAT) mass, activity (i.e., cholesterol esterification with a standard proteoliposome substrate) and cholesterol esterification rate (i.e., cholesterol esterification in the presence of the endogenous substrate). The AIM subjects had lower LCAT mass (3.30 +/- 0.85 micrograms/ml), activity (71.1 +/- 36.4 nmol/ml per h) and cholesterol esterification rate (23.6 +/- 12.5 nmol/ml per h) compared to controls (5.22 +/- 0.74 micrograms/ml, 121.6 +/- 54.6 nmol/ml per h and 53.6 +/- 29.9 nmol/ml per h, respectively). The specific LCAT activity, i.e., LCAT activity per microgram of LCAT, was similar in the two groups, indicating that the LCAT protein in the AIM carriers is structurally and functionally normal. However, the specific cholesterol esterification rate was 23% lower in the AIM subjects (8.03 +/- 6.01 nmol/h per microgram) compared to controls (10.49 +/- 5.86 nmol/h per microgram; P less than 0.05). The capacity of HDL3, purified from both AIM and control plasma, to act as substrates for cholesterol esterification was similar, thus suggesting that other mechanism(s) may be in play. Carriers with a relative abundance of abnormal, small HDL3b particles had the most altered cholesterol esterification pattern. Upon evaluating all AIM subjects, a complex relationship between HDL structure, plasma lipid-lipoprotein levels and cholesterol esterification emerged, making the AIMilano condition a unique model for the study of the mechanisms regulating the cholesterol esterification-transfer process in man.

Effect of lipid transfer protein on plasma lipids, apolipoproteins and metabolism of high-density lipoprotein cholesteryl ester in the rat

The role of human plasma lipid transfer protein (LTP) in lipoprotein metabolism was studied in the rat, a species without endogenous cholesteryl ester and triacylglycerol transfer activity. Partially purified human LTP was injected intravenously into rats. The plasma activity was between 1.5- and 4-fold that of human plasma during the experiments. 6 h after the injection of LTP, a significant increase in serum apoB, and no significant changes in serum total cholesterol, free cholesterol, triacylglycerols, apoA-I, apoE, or apoA-IV were noted. Cholesterol was increased in very-low density and low-density lipoproteins (VLDL and LDL) and decreased in large-sized apoE-rich HDL. ApoA-I-containing particles with a size smaller than in normal rats were present in serum of LTP-treated rats. The mean diameter of HDL particles decreased and apoE, normally present on large-sized HDL, was present on smaller sized particles. The metabolic fate of cholesteryl ester, originally associated with HDL, was studied by injection of [3H]cholesteryl linoleyl ether-labelled apoA-I-rich HDL in the absence and in the presence of LTP. The disappearance of [3H]cholesteryl linoleyl ether, injected as part of apoA-I-rich HDL, from serum was increased in the LTP-treated rats; the t1/2 changed from 3.9 to 2.2 h, resulting in an increased accumulation of [3H]cholesteryl linoleyl ether in the liver. This can be explained by the redistribution of HDL [3H]cholesteryl linoleyl ether to VLDL and LDL in the presence of LTP, leading to the combined contribution of VLDL, LDL and HDL to the hepatic uptake. The present findings show profound effects of LTP on the chemical composition of HDL subspecies, the size of HDL and on the plasma turnover and hepatic uptake of cholesteryl esters originally present in apo A-I-rich HDL.

Conversion of apolipoprotein-specific high-density lipoprotein populations during incubation of human plasma

Incubation studies were performed on plasma obtained from subjects selected for relatively low levels of high-density lipoprotein cholesterol (HDL-C) (no greater than 30 mg/dl) and particle size distributions enriched in the HDL3 subclass. Incubation (12 h, 37 degrees C) of plasma in the presence or absence of lecithin: cholesterol acyltransferase activity produces marked alteration in size profiles of both major apolipoprotein-specific HDL3 populations (HDL3(AI w AII), HDL3 species containing both apolipoprotein A-I and apolipoprotein A-II, and HDL3(AI w/o AII), HDL3 species containing apolipoprotein A-I) as isolated by immunoaffinity chromatography. In the presence or absence of lecithin: cholesterol acyltransferase activity, plasma incubation results in a shift of HDL3(AI w AII) species (initial mean sizes of major components, approx. 8.8 and 8.0 nm) predominantly to larger particles (mean size, 9.8 nm). A less prominent shift to smaller particles (mean size, 7.8 nm) accompanies the conversion to larger particles only when the enzyme is active. Combined shifts to larger (mean size, 9.8 nm) and smaller (mean size, 7.4 nm) particles are observed for HDL3(AI w/o AII) particles (mean size, 8.3 nm) also only in the presence of enzyme activity. However, in the absence of enzyme activity, HDL3(AI w/o AII) species, unlike the HDL3(AI w AII) species, are converted to smaller (mean size 7.4 nm) rather than to larger particles. Like native HDL2b(AI w/o AII) particles, the larger HDL3(AI w/o AII) conversion products exhibit a protein moiety with molecular weight equivalent to four apolipoprotein A-I molecules per particle; small HDL3(AI w/o AII) products are comprised predominantly of particles with two apolipoprotein A-I per particle. Incubation-induced conversion of HDL3 particles in the presence of lecithin: cholesterol acyltransferase activity is associated with increased binding of both apolipoprotein-specific HDL populations to low-density lipoproteins (LDL). The present studies indicate that, in the absence of lecithin: cholesterol acyltransferase activity, the two HDL3 populations follow different conversion pathways, possibly due to apolipoprotein-specific activities of lipid transfer protein or conversion protein in plasma. Our studies also suggest that lecithin: cholesterol acyltransferase activity may play a role in the origins of large HDL2b(AI w/o AII) species in human plasma by participating in the conversion of HDL3(AI w/o AII) particles, initially with three apolipoprotein A-I, to larger particles with four apolipoprotein A-I per particle.

Apolipoprotein A-I inhibits transformation of high density lipoprotein subpopulations during incubation of human plasma

We have examined the effect of added apolipoprotein A-I (apoA-I) on the changes in high density lipoprotein (HDL) particle size that occur when human plasma is incubated in vitro. In the absence of added apoA-I, incubation of plasma at 37 degrees C resulted in a dramatic increase in HDL particle size. When these incubations contained an inhibitor of LCAT, an additional population of smaller HDL particles was formed. These changes in particle size were even more pronounced when the incubations were supplemented with an artificial triglyceride emulsion, Intralipid. All these changes in HDL particle size were markedly inhibited when incubations were supplemented with apoA-I. Even when the amount of added apoA-I was as little as 4.5% of the endogenous apolipoprotein there was an obvious inhibition of the changes in HDL particle size. The presence of added apoA-I sufficient to increase the plasma concentration by 18% virtually abolished the changes in HDL particle size. This effect did not relate to an inhibition of cholesterol esterification, nor did it appear to depend on an incorporation of the added apoA-I into the HDL.