Small HDL particles containing two apoA-I molecules are precursors in vivo to medium and large HDL particles containing three and four apoA-I molecules in nonhuman primates

Distinct Differences in Lipoprotein Particle Number Evaluation between GP-HPLC and NMR: Analysis in Dyslipidemic Patients Administered a Selective PPARα Modulator, Pemafibrate

Aim: We established a method to evaluate the lipid concentrations, size and particle numbers (PNs) of lipoprotein subclasses by gel permeation chromatography (GP-HPLC). Nuclear magnetic resonance (NMR) is widely used to analyze these parameters of lipoprotein subclasses, but differences of the two methods are unknown. Current study compared the PNs of each lipoprotein subclass measured by GP-HPLC and NMR, and assessed the effect of a selective PPARα modulator, pemafibrate.

Methods: Lipoprotein profiles of 212 patients with dyslipidemia who participated in the phase 2 clinical trial of a selective PPARα modulator, pemafibrate, were analyzed by two methods, GP-HPLC and NMR, which were performed with LipoSEARCH (Skylight Biotech) and LipoProfile 3 (LabCorp), respectively. GP-HPLC evaluated the PNs of 18 subclasses, consisting of CM, VLDL1-5, LDL1-6, and HDL1-6. NMR evaluated the PNs of 9 subclasses, consisting of large VLDL & CM, medium VLDL, small VLDL, IDL, large LDL, small LDL, large HDL, medium HDL and small HDL.

Results: Three major classes, total CM&VLDL, total LDL and total HDL were obtained by grouping of corresponding subclasses in both methods and PNs of these classes analyzed by GP-HPLC were correlated positively with those by NMR. The correlation coefficients in total CM&VLDL, total LDL and total HDL between GP-HPLC and NMR was 0.658, 0.863 and 0.798 (all p ＜0.0001), respectively. The PNs of total CM&VLDL, total LDL and total HDL analyzed by GP-HPLC was 249.5±51.7nM, 1,679±359 nM and 13,273±1,564 nM, respectively, while those by NMR was 124.6±41.8 nM, 1,514±386 nM and 31,161±4,839 nM, respectively. A marked difference in the PNs between the two methods was demonstrated especially in total HDL.

The number of apolipoprotein (Apo) B molecule per one ApoB-containing lipoprotein particle, total CM&VLDL plus total LDL, was 1.10±0.05 by GP-HPLC, while 1.32±0.18 by NMR. The number of ApoA-I per one HDL particle was 3.40±0.17 by GP-HPLC, but only 1.46±0.15 by NMR, much less than reported previously.

From the phase 2 clinical trial, randomizing 212 patients to pemafibrate 0.025-0.2 mg BID, fenofibrate 100 mg QD, or placebo groups, pemafibrate reduced the PNs of CM, large VLDL1-VLDL3 and medium VLDL4, but not small VLDL5 by GP-HPLC. It significantly decreased the PNs of smaller LDL and larger HDL particles, but increased those of larger LDL and smaller HDL particles. In contrast, NMR showed marked variations in the effect of pemafibrate on lipoprotein PNs, and no significant size-dependent changes.

Conclusions: GP-HPLC evaluates the lipoprotein PNs more accurately than NMR and can be used for assessing the effects of lipid-lowering drugs on lipoprotein subclasses.

Effect of fasting and feeding on apolipoprotein A-I kinetics in preβ1-HDL, α-HDL, and triglyceride-rich lipoproteins

The aim of this study was to compare the kinetics of apolipoprotein (apo)A-I during fed and fasted states in humans, and to determine to what extent the intestine contributes to apoA-I production. A stable isotope study was conducted to determine the kinetics of apoA-I in preβ₁ high-density lipoprotein (HDL) and α-HDL. Six healthy male subjects received a constant intravenous infusion of ²H₃-leucine for 14 h. Subjects in the fed group also received small hourly meals. Blood samples were collected hourly during tracer infusion and then daily for 4 days. Tracer enrichments were measured by mass spectrometry and then fitted to a compartmental model using asymptotic plateau of very-low-density lipoprotein (VLDL) apoB100 and triglyceride-rich lipoprotein (TRL) apoB48 as estimates of hepatic and intestinal precursor pools, respectively. The clearance rate of preβ₁-HDL-apoA-I was lower in fed individuals compared with fasted subjects (p < 0.05). No other differences in apoA-I production or clearance rates were observed between the groups. No significant correlation was observed between plasma apoC-III concentrations and apoA-I kinetic data. In contrast, HDL-apoC-III was inversely correlated with the conversion of α-HDL to preβ₁-HDL. Total apoA-I synthesis was not significantly increased in fed subjects. Hepatic production was not significantly different between the fed group (17.17 ± 2.75 mg/kg/day) and the fasted group (18.67 ± 1.69 mg/kg/day). Increase in intestinal apoA-I secretion in fed subjects was 2.20 ± 0.61 mg/kg/day. The HDL-apoA-I kinetics were similar in the fasted and fed groups, with 13% of the total apoA-I originating from the intestine with feeding.

Effects of α-linolenic acid intake on blood lipid profiles：a systematic review and meta-analysis of randomized controlled trials

To investigate the effect of ALA intake on blood lipid profiles, including triglycerides (TG), total cholesterol (TC), high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), very-low-density lipoprotein (VLDL-C) and ratio of TC to HDL-C. We systematically searched randomized controlled trials of ALA intervention on PubMed, Embase, Cochrane library and related references up to March 2018. The final values were calculated as weighted mean difference (WMD) by using a random effects model. Subgroup analysis and meta-regression were used to explore the source of heterogeneity. Generalized least square was performed for dose-response analysis. Forty-seven studies with 1305 individuals in the ALA arm and 1325 individuals in the control arm were identified. Compared with control group, dietary intake of ALA significantly reduced the concentrations of TG (WMD -0.101 mmol/L; 95% CI: -0.158 to -0.044 mmol/L; P = 0.001), TC (WMD -0.140 mmol/L; 95% CI: -0.224 to -0.056 mmol/L; P = 0.001), LDL-C (WMD -0.131 mmol/L; 95% CI: -0.191 to -0.071 mmol/L; P < 0.001), VLDL-C (WMD -0.121 mmol/L; 95% CI: -0.170 to -0.073 mmol/L; P < 0.001), TC/HDL-C ratio (WMD -0.165 mmol/L; 95% CI: -0.317 to -0.013 mmol/L; P = 0.033) and LDL-C/HDL-C ratio (WMD -0.158 mmol/L; 95% CI: -0.291 to -0.025 mmol/L; P = 0.02). There is no effect of ALA intake on HDL-C (WMD 0.008 mmol/L; 95% CI: -0.018 to 0.034 mmol/L; P = 0.541). Dose-response analysis indicated that 1 g per day increment of ALA was associated with a 0.0016 mmol/L, 0.0071 mmol/L, 0.0015 and 0.0061 mmol/L reduction in TG (95% CI: -0.0029 to -0.0002 mmol/L), TC (95% CI: -0.0085 to -0.0058 mmol/L), HDL-C (95% CI: -0.0020 to -0.0011 mmol/L) and LDL-C (95% CI: -0.0073 to -0.0049 mmol/L) levels, respectively. The effects of ALA intake on TG, TC and LDL-C concentrations were more obvious among Asian participants, and also more obvious on patients with hyperlipidemia or hyperglycemia compared to healthy individuals. Dietary ALA intervention improves blood lipid profiles by decreasing levels of TG, TC, LDL and VLDL-C. Our findings add to the evidence that increasing ALA intake could potentially prevent risk of cardiovascular diseases.

Dietary unsaturated fat increases HDL metabolic pathways involving apoE favorable to reverse cholesterol transport

BACKGROUND

HDL that contains apolipoprotein E (apoE) is a subspecies especially active in steps in reverse cholesterol transport, a process that brings cholesterol from peripheral cells to the liver. Here, we studied the effect of dietary unsaturated fat compared with carbohydrate on the metabolism of HDL containing apoE.

METHODS

We enrolled 9 adults who were overweight or obese and had below-average HDL-cholesterol in a crossover study of a high-fat diet, primarily unsaturated, and a low-fat, high-carbohydrate diet. A metabolic tracer study was performed after each diet period.

RESULTS

Dietary fat increased the secretion, metabolism, and clearance of HDL subspecies containing apoE. Dietary fat increased the rate of clearance of large cholesterol-rich HDL containing apoE and increased their conversion to small HDL containing apoE, indicating selective cholesterol ester delivery to the liver. The high-unsaturated-fat diet did not affect the metabolism of HDL lacking apoE.

CONCLUSION

HDL containing apoE is a diet-responsive metabolic pathway that renders HDL more biologically active in reverse cholesterol transport. This may be a mechanism by which unsaturated fat protects against coronary heart disease. Protein-based HDL subspecies such as HDL containing apoE may be used to identify additional atheroprotective treatment targets not evident in the total HDL-cholesterol measurement.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01399632.

FUNDING

NIH and the National Center for Advancing Translational Science.

Modification by isolevuglandins, highly reactive γ-ketoaldehydes, deleteriously alters high-density lipoprotein structure and function

Cardiovascular disease risk depends on high-density lipoprotein (HDL) function, not HDL-cholesterol. Isolevuglandins (IsoLGs) are lipid dicarbonyls that react with lysine residues of proteins and phosphatidylethanolamine. IsoLG adducts are elevated in atherosclerosis. The consequences of IsoLG modification of HDL have not been studied. We hypothesized that IsoLG modification of apoA-I deleteriously alters HDL function. We determined the effect of IsoLG on HDL structure-function and whether pentylpyridoxamine (PPM), a dicarbonyl scavenger, can preserve HDL function. IsoLG adducts in HDL derived from patients with familial hypercholesterolemia (n = 10, 233.4 ± 158.3 ng/mg) were found to be significantly higher than in healthy controls (n = 7, 90.1 ± 33.4 pg/mg protein). Further, HDL exposed to myeloperoxidase had elevated IsoLG-lysine adducts (5.7 ng/mg protein) compared with unexposed HDL (0.5 ng/mg protein). Preincubation with PPM reduced IsoLG-lysine adducts by 67%, whereas its inactive analogue pentylpyridoxine did not. The addition of IsoLG produced apoA-I and apoA-II cross-links beginning at 0.3 molar eq of IsoLG/mol of apoA-I (0.3 eq), whereas succinylaldehyde and 4-hydroxynonenal required 10 and 30 eq. IsoLG increased HDL size, generating a subpopulation of 16-23 nm. 1 eq of IsoLG decreased HDL-mediated [³H]cholesterol efflux from macrophages via ABCA1, which corresponded to a decrease in HDL-apoA-I exchange from 47.4% to only 24.8%. This suggests that IsoLG inhibits apoA-I from disassociating from HDL to interact with ABCA1. The addition of 0.3 eq of IsoLG ablated HDL's ability to inhibit LPS-stimulated cytokine expression by macrophages and increased IL-1β expression by 3.5-fold. The structural-functional effects were partially rescued with PPM scavenging.

From High-Density Lipoprotein Cholesterol to Measurements of Function: Prospects for the Development of Tests for High-Density Lipoprotein Functionality in Cardiovascular Disease

The evidence is strong that biological functions contained in high-density lipoproteins (HDL) are antiatherogenic. These functions may track with HDL cholesterol or apolipoprotein A1 concentration to explain the strongly inverse risk curve for cardiovascular disease. Moreover, there are harmful as well as protective HDL subspecies in regard to cardiovascular disease, which could be responsible for paradoxical responses to HDL-directed treatments. Recent metabolic studies show that apolipoprotein A1-containing HDL is secreted into the circulation as mostly spherical cholesterol ester-rich lipoproteins that span the HDL size range. Most of the flux of apolipoprotein A1 HDL into and out of the circulation occurs in these spherical cholesterol-replete particles. Discoidal cholesterol-poor HDL comprises a minority of HDL secretion. We propose that much cholesterol in reverse cholesterol transport enters and exits medium and large size HDL without changing a size category, and its flux may be estimated provisionally from holoparticle clearance of cholesterol ester-rich HDL. An accurate framework for metabolism of HDL is essential to finding steady-state biomarkers that reflect HDL function in vivo. Whereas cholesterol efflux from cells to mainly discoidal HDL, mediated by ABCA1 (ATP-binding cassette transporter ABCA1), predicts cardiovascular disease, cholesterol transfers to spherical HDL also can be measured and may be relevant to protection against atherosclerosis. We propose several investigative paths on which human HDL biology may be investigated leading to convenient biomarkers of HDL quality and function having potential not only to improve risk prediction but also to more accurately target drug treatments.

Combined use of metformin and atorvastatin attenuates atherosclerosis in rabbits fed a high-cholesterol diet

Statins are widely used to reduce cardiovascular risk. Unfortunately, some patients still experience cardiovascular events though prescribed with high-intensity statins. Metformin, an anti-diabetic drug, was reported to possess anti-atherosclerotic effects. Therefore, the experiments were designed to evaluate whether combined use of metformin and atorvastatin can achieve additional benefits. In rabbits fed a high-cholesterol diet, we evaluated the effects of the combination therapy on atherosclerotic plaques, lipid profiles, blood glucose levels, liver and kidney functions. Effects of combination therapy on cholesterol efflux and the expression of related transporters were studied in vitro. Our results showed that the combination therapy induced a more significant decrease in atherosclerotic lesion area than atorvastatin without additional lipid-lowering effect. The combination therapy significantly increased the percentage of large high-density lipoprotein subfraction. The intravenous glucose tolerance test showed that atorvastatin-treated rabbits had an increased area under the curve for time-dependent glucose levels after a bolus injection of glucose, which was completely reversed by metformin treatment. In cultured macrophages, co-treatment with metformin and atorvastatin promoted cholesterol efflux and up-regulated expression of ATP-binding cassette transporters A1 and G1. Taken together, our results suggest that atorvastatin/metformin combination therapy may achieve additional anti-atherosclerotic benefits likely through increasing cholesterol efflux in macrophages.

Novel Pathways of Apolipoprotein A-I Metabolism in High-Density Lipoprotein of Different Sizes in Humans

OBJECTIVE

A prevailing concept is that high-density lipoprotein (HDL) is secreted into the systemic circulation as a small mainly discoidal particle, which expands progressively and becomes spherical by uptake and esterification of cellular cholesterol and then contracts by cholesterol ester delivery to the liver, a process known as reverse cholesterol transport, thought to be impaired in people with low HDL cholesterol (HDLc). This metabolic framework has not been established in humans.

APPROACH AND RESULTS

We studied the metabolism of apolipoprotein A-I in 4 standard HDL sizes by endogenous isotopic labeling in 6 overweight adults with low HDLc and in 6 adults with normal body weight with high plasma HDLc. Contrary to expectation, HDL was secreted into the circulation in its entire size distribution from very small to very large similarly in both groups. Very small (prebeta) HDL comprised only 8% of total apolipoprotein A-I secretion. Each HDL subfraction circulated mostly within its secreted size range for 1 to 4 days and then was cleared. Enlargement of very small and medium to large and very large HDL and generation of very small from medium HDL were minor metabolic pathways. Prebeta HDL was cleared slower, whereas medium, large, and very large HDL were cleared faster in the low HDLc group.

CONCLUSIONS

A new model is proposed from these results in which HDL is metabolized in plasma mainly within several discrete, stable sizes across the common range of HDLc concentrations.

MD simulations suggest important surface differences between reconstituted and circulating spherical HDL

Since spheroidal HDL particles (sHDL) are highly dynamic, molecular dynamics (MD) simulations are useful for obtaining structural models. Here we use MD to simulate sHDL with stoichiometries of reconstituted and circulating particles. The hydrophobic effect during simulations rapidly remodels discoidal HDL containing mixed lipids to sHDL containing a cholesteryl ester/triglyceride (CE/TG) core. We compare the results of simulations of previously characterized reconstituted sHDL particles containing two or three apoA-I created in the absence of phospholipid transfer protein (PLTP) with simulations of circulating human HDL containing two or three apoA-I without apoA-II. We find that circulating sHDL compared with reconstituted sHDL with the same number of apoA-I per particle contain approximately equal volumes of core lipid but significantly less surface lipid monolayers. We conclude that in vitro reconstituted sHDL particles contain kinetically trapped excess phospholipid and are less than ideal models for circulating sHDL particles. In the circulation, phospholipid transfer via PLTP decreases the ratio of phospholipid to apolipoprotein for all sHDL particles. Further, sHDL containing two or three apoA-I adapt to changes in surface area by condensation of common conformational motifs. These results represent an important step toward resolving the complicated issue of the protein and lipid stoichiometry of circulating HDL.

An apoA-I mimetic peptibody generates HDL-like particles and increases alpha-1 HDL subfraction in mice

The aim of this study is to investigate the capability of an apoA-I mimetic with multiple amphipathic helices to form HDL-like particles in vitro and in vivo. To generate multivalent helices and to track the peptide mimetic, we have constructed a peptibody by fusing two tandem repeats of 4F peptide to the C terminus of a murine IgG Fc fragment. The resultant peptidbody, mFc-2X4F, dose-dependently promoted cholesterol efflux in vitro, and the efflux potency was superior to monomeric 4F peptide. Like apoA-I, mFc-2X4F stabilized ABCA1 in J774A.1 and THP1 cells. The peptibody formed larger HDL particles when incubated with cultured cells compared with those by apoA-I. Interestingly, when administered to mice, mFc-2X4F increased both pre-β and α-1 HDL subfractions. The lipid-bound mFc-2X4F was mostly in the α-1 migrating subfraction. Most importantly, mFc-2X4F and apoA-I were found to coexist in the same HDL particles formed in vivo. These data suggest that the apoA-I mimetic peptibody is capable of mimicking apoA-I to generate HDL particles. The peptibody and apoA-I may work cooperatively to generate larger HDL particles in vivo, either at the cholesterol efflux stage and/or via fusion of HDL particles that were generated by the peptibody and apoA-I individually.

Selective delipidation of plasma HDL enhances reverse cholesterol transport in vivo

Uptake of cholesterol from peripheral cells by nascent small HDL circulating in plasma is necessary to prevent atherosclerosis. This process, termed reverse cholesterol transport, produces larger cholesterol-rich HDL that transfers its cholesterol to the liver facilitating excretion. Most HDL in plasma is cholesterol-rich. We demonstrate that treating plasma with a novel selective delipidation procedure converts large to small HDL [HDL-selectively delipidated (HDL-sdl)]. HDL-sdl contains several cholesterol-depleted species resembling small alpha, prebeta-1, and other prebeta forms. Selective delipidation markedly increases efficacy of plasma to stimulate ABCA1-mediated cholesterol transfer from monocytic cells to HDL. Plasma from African Green monkeys underwent selective HDL delipidation. The delipidated plasma was reinfused into five monkeys. Prebeta-1-like HDL had a plasma residence time of 8 +/- 6 h and was converted entirely to large alpha-HDL having residence times of 13-14 h. Small alpha-HDL was converted entirely to large alpha-HDL. These findings suggest that selective HDL delipidation activates reverse cholesterol transport, in vivo and in vitro. Treatment with delipidated plasma tended to reduce diet-induced aortic atherosclerosis in monkeys measured by intravascular ultrasound. These findings link the conversion of small to large HDL, in vivo, to improvement in atherosclerosis.

Initial interaction of apoA-I with ABCA1 impacts in vivo metabolic fate of nascent HDL

We investigated the in vivo metabolic fate of pre-beta HDL particles in human apolipoprotein A-I transgenic (hA-I (Tg)) mice. Pre-beta HDL tracers were assembled by incubation of [(125)I]tyramine cellobiose-labeled apolipoprotein A-I (apoA-I) with HEK293 cells expressing ABCA1. Radiolabeled pre-beta HDLs of increasing size (pre-beta1, -2, -3, and -4 HDLs) were isolated by fast-protein liquid chromatography and injected into hA-I (Tg)-recipient mice, after which plasma decay, in vivo remodeling, and tissue uptake were monitored. Pre-beta2, -3, and -4 had similar plasma die-away rates, whereas pre-beta1 HDL was removed 7-fold more rapidly. Radiolabel recovered in liver and kidney 24 h after tracer injection suggested increased (P < 0.001) liver and decreased kidney catabolism as pre-beta HDL size increased. In plasma, pre-beta1 and -2 were rapidly (<5 min) remodeled into larger HDLs, whereas pre-beta3 and -4 were remodeled into smaller HDLs. Pre-beta HDLs were similarly remodeled in vitro with control or LCAT-immunodepleted plasma, but not when incubated with phospholipid transfer protein knockout plasma. Our results suggest that initial interaction of apoA-I with ABCA1 imparts a unique conformation that partially determines the in vivo metabolic fate of apoA-I, resulting in increased liver and decreased kidney catabolism as pre-beta HDL particle size increases.

NMR analysis of lipoprotein particle size does not increase sensitivity to the effect of soy protein on CVD risk when compared with the traditional lipid profile

The traditional lipid profile compared with nuclear magnetic resonance (NMR) may underestimate the risk for cardiovascular disease and may explain some of the discrepancies in results between studies analyzing the salubrious effects of soy. Our purpose was to compare the traditional lipid profile with NMR quantification of the number of lipoprotein particles, subclasses, and diameters or sizes in 30 sedentary males, between 18 and 30 years of age, consuming 1 of the following 3 supplements daily for 28 days: milk protein (Milk), isoflavone-poor soy protein (Soy–), or isoflavone-rich soy protein (Soy+). The study used a double-blind, parallel-arm design with random assignment to 1 of the 3 protein supplement groups. Fasting EDTA blood samples were collected at baseline and after 28 days of supplementation and analyzed for the number and size of very low-density lipoprotein (VLDL), low-density lipoprotein (LDL), and high-density lipoprotein (HDL) particles, respectively. Fasting serum samples were analyzed for concentrations of total cholesterol (TC), LDL cholesterol (LDL-C), total HDL cholesterol (HDL-C), HDL2-C, HDL3-C, triglycerides (TGs), free fatty acids (FFAs), and glucose. Fasting heparin blood samples were collected at baseline and after supplementation and analyzed for apolipoproteins A-I, A-II, B, C-II, C-III, and E, as well as hepatic and lipoprotein lipase concentrations. HDL3-C increased by 47.2% after Soy+ supplementation and hepatic lipase decreased 19.2% after Soy– supplementation (p < 0.05). HDL-C and apolipoproteins A-I and A-II were found to increase in all 3 groups (p < 0.05). Results support that NMR analysis of lipoprotein particle number and size are not more sensitive to the effect of soy protein on CVD risk compared with the traditional lipid profile. Furthermore, the lack of isoflavones in soy protein seems to have a deleterious effect on hepatic lipase.

SR-BI-mediated selective lipid uptake segregates apoA-I and apoA-II catabolism

The HDL receptor scavenger receptor class B type I (SR-BI) binds HDL and mediates the selective uptake of cholesteryl ester. We previously showed that remnants, produced when human HDL(2) is catabolized in mice overexpressing SR-BI, become incrementally smaller, ultimately consisting of small alpha-migrating particles, distinct from pre-beta HDL. When mixed with mouse plasma, some remnant particles rapidly increase in size by associating with HDL without the mediation of cholesteryl ester transfer protein, LCAT, or phospholipid transfer protein. Here, we show that processing of HDL(2) by SR-BI-overexpressing mice resulted in the preferential loss of apolipoprotein A-II (apoA-II). Short-term processing generated two distinct, small alpha-migrating particles. One particle (8.0 nm diameter) contained apoA-I and apoA-II; the other particle (7.7 nm diameter) contained only apoA-I. With extensive SR-BI processing, only the 7.7 nm particle remained. Only the 8.0 nm remnants were able to associate with HDL. Compared with HDL(2), this remnant was more readily taken up by the liver than by the kidney. We conclude that SR-BI-generated HDL remnants consist of particles with or without apoA-II and that only those containing apoA-II associate with HDL in an enzyme-independent manner. Extensive SR-BI processing generates small apoA-II-depleted particles unable to reassociate with HDL and readily taken up by the liver. This represents a pathway by which apoA-I and apoA-II catabolism are segregated.

HDLs in apoA-I transgenic Abca1 knockout mice are remodeled normally in plasma but are hypercatabolized by the kidney

Patients homozygous for Tangier disease have a near absence of plasma HDL as a result of mutations in ABCA1 and hypercatabolize normal HDL particles. To determine the relationship between ABCA1 expression and HDL catabolism, we investigated intravascular remodeling, plasma clearance, and organ-specific uptake of HDL in mice expressing the human apolipoprotein A-I (apoA-I) transgene in the Abca1 knockout background. Small HDL particles (7.5 nm), radiolabeled with (125)I-tyramine cellobiose, were injected into recipient mice to quantify plasma turnover and the organ uptake of tracer. Small HDL tracer was remodeled to 8.2 nm diameter particles within 5 min in human apolipoprotein A-I transgenic (hA-I(Tg)) mice (control) and knockout mice. Decay of tracer from plasma was 1.6-fold more rapid in knockout mice (P < 0.05) and kidney uptake was twice that of controls, with no difference in liver uptake. We also observed 2-fold greater hepatic expression of ABCA1 protein in hA-I(Tg) mice compared with nontransgenic mice, suggesting that overexpression of human apoA-I stabilized hepatic ABCA1 protein in vivo. We conclude that ABCA1 is not required for in vivo remodeling of small HDLs to larger HDL subfractions and that the hypercatabolism of normal HDL particles in knockout mice is attributable to a selective catabolism of HDL apoA-I by the kidney.

Effects of peroxisome proliferator-activated receptor alpha/delta agonists on HDL-cholesterol in vervet monkeys

The objective of this study was to demonstrate the efficacy of a novel peroxisome proliferator-activated receptor (PPAR) agonist and known PPARalpha and PPARdelta agonists to increase HDL-cholesterol (HDL-C) in the St. Kitts vervet, a nonhuman primate model of atherosclerosis. Four groups (n = 6) were studied and each group was assigned one of the following "treatments": a) vehicle only (vehicle); b) the PPARdelta selective agonist GW501516 (GW); c) the PPARalpha/delta agonist T913659 (T659); and d) the PPARalpha agonist TriCor (fenofibrate). No statistically significant changes were seen in body weight, total plasma cholesterol, plasma triglycerides, VLDL-C, LDL-C, or apolipoprotein B (apoB) concentrations. Each of the PPARalpha and PPARdelta agonists investigated in this study increased plasma HDL-C, apoA-I, and apoA-II concentrations and increased HDL particle size in St. Kitts vervets. The maximum percentage increase in HDL-C from baseline for each group was as follows: vehicle, 5%; GW, 43%; T659, 43%; and fenofibrate, 20%. Treatment with GW and T659 resulted in an increase in medium-sized HDL particles, whereas fenofibrate showed increases in large HDL particles. These data provide additional evidence that PPARalpha and PPARdelta agonists (both mixed and selective) have beneficial effects on HDL-C in these experimental primates.

Prebeta high density lipoprotein has two metabolic fates in human apolipoprotein A-I transgenic mice

We compared the in vivo metabolism of prebeta HDL particles isolated by anti-human apolipoprotein A-I (apoA-I) immunoaffinity chromatography (LpA-I) in human apoA-I transgenic (hA-I Tg) mice with that of lipid-free apoA-I (LFA-I) and small LpA-I. After injection, prebeta LpA-I were removed from plasma more rapidly than were LFA-I and small LpA-I. Prebeta LpA-I and LFA-I were preferentially degraded by kidney compared with liver; small LpA-I were preferentially degraded by the liver. Five minutes after tracer injection, 99% of LFA-I in plasma was found to be associated with medium-sized (8.6 nm) HDL, whereas only 37% of prebeta tracer remodeled to medium-sized HDL. Injection of prebeta LpA-I doses into C57Bl/6 recipients resulted in a slower plasma decay compared with hA-I Tg recipients and a greater proportion (>60%) of the prebeta radiolabel that was associated with medium-sized HDL. Prebeta LpA-I contained one to four molecules of phosphatidylcholine per molecule of apoA-I, whereas LFA-I contained less than one. We conclude that prebeta LpA-I has two metabolic fates in vivo, rapid removal from plasma and catabolism by kidney or remodeling to medium-sized HDL, which we hypothesize is determined by the amount of lipid associated with the prebeta particle and the particle's ability to bind to medium-sized HDL.

Lipoprotein lipase and hepatic lipase: their relationship with HDL subspecies Lp(A-I) and Lp(A-I,A-II)

HDL subspecies Lp(A-I) and Lp(A-I,A-II) have different anti-atherogenic potentials. To determine the role of lipoprotein lipase (LPL) and hepatic lipase (HL) in regulating these particles, we measured these enzyme activities in 28 healthy subjects with well-controlled Type 1 diabetes, and studied their relationship with Lp(A-I) and Lp(A-I,A-II). LPL was positively correlated with the apolipoprotein A-I (apoA-I), cholesterol, and phospholipid mass in total Lp(A-I), and with the apoA-I in large Lp(A-I) (r >or= 0.58, P >or= 0.001). HL was negatively correlated with all the above Lp(A-I) parameters plus Lp(A-I) triglyceride (r >or= -0.53, P <or= 0.003). No correlation was detected between LPL and Lp(A-I,A-II). However, HL was inversely correlated with total Lp(A-I,A-II) phospholipid, and with large Lp(A-I,A-II) (r >or= 0.50, P <or= 0.006). Similar studies were performed with phospholipid transfer protein (PLTP). Only total Lp(A-I) triglyceride in women (not men) (r = 0.71, P = 0.009) was significantly correlated with PLTP activity. These observations indicate that LPL and HL play major roles in determining the level and composition of plasma Lp(A-I), particularly large Lp(A-I), but not with Lp(A-I,A-II) level. Furthermore, select correlations of LPL and/or HL with the apoA-I, cholesterol, and triglyceride of Lp(A-I) but not Lp(A-I,A-II) imply that the apoA-I and lipid of Lp(A-I) and Lp(A-I,A-II) are not fully equilibrated.

Metabolism of apoA-I as lipid-free protein or as component of discoidal and spherical reconstituted HDLs: studies in wild-type and hepatic lipase transgenic rabbits

OBJECTIVE

Apolipoprotein (apo)A-I exists in 3 forms in plasma: as lipid-free apoA-I, as a component of pre-beta-migrating discoidal high density lipoproteins (HDLs), and as a component of alpha-migrating spherical HDLs. This study investigates (1) the in vivo metabolism of apoA-I in each of these forms and (2) the effects of hepatic lipase (HL) on apoA-I metabolism.

METHODS AND RESULTS

Wild-type and HL transgenic rabbits were studied. When lipid-free (125)I-apoA-I and 125I-apoA-I in pre-beta-migrating discoidal reconstituted HDLs (rHDLs) were injected into wild-type rabbits, the label rapidly appeared in alpha-migrating particles and decayed with the same fractional catabolic rate (FCR) as when they were injected as a component of spherical rHDLs. Spherical rHDLs did not change in size when they were injected into wild-type rabbits but were reduced in size in HL transgenic rabbits. The FCR of apoA-I in HL transgenic rabbits was double that in wild-type rabbits.

CONCLUSIONS

In vivo, (1) lipid-free apoA-I rapidly incorporates into preexisting alpha-migrating particles, (2) pre-beta-migrating discoidal HDLs are rapidly converted into alpha-migrating HDLs, (3) the FCR of apoA-I is independent of the form in which it is introduced into plasma, and (4) HL reduces the size of alpha-migrating HDLs and increases the rate of catabolism of apoA-I.

The fate of HDL particles in vivo after SR-BI-mediated selective lipid uptake

Scavenger receptor class B type I (SR-BI) delivers cholesterol ester from HDL to cells via a selective uptake mechanism, whereby lipid is transferred from the core of the particle without concomitant degradation of the protein moiety. The precise metabolic fate of HDL particles after selective lipid uptake is not known. To characterize SR-BI-mediated HDL processing in vivo, we expressed high levels of this receptor in livers of apoA-I(-/-) mice by adenoviral vector gene transfer, and then injected the mice with a bolus of human HDL(2) traced with (125)I-dilactitol tyramine. HDL recovered from apoA-I(-/-) mice over-expressing SR-BI was significantly smaller than HDL recovered from control mice as measured by non-denaturing gel electrophoresis. When injected into C57BL/6 mice, these HDL "remnants" were rapidly converted to HDL(2)-sized lipoprotein particles, and were cleared from the plasma at a rate similar to HDL(2). In assays in cultured cells, HDL remnants did not stimulate ATP-binding cassette transporter A1-dependent cholesterol efflux. When mixed with mouse plasma ex vivo, HDL remnants rapidly converted to larger HDL particles. These studies identify a previously ill-defined pathway in HDL metabolism, whereby SR-BI generates small, dense HDL particles that are rapidly remodeled in plasma. This remodeling pathway may represent a process that is important in determining the rate of apoA-I catabolism and HDL-mediated reverse cholesterol transport.

Plasma factors controlling atrial natriuretic peptide (ANP) aggregation: role of lipoproteins

We have previously shown that human plasma atrial alpha-natriuretic peptide (alpha-hANP) sequestering is a protective phenomenon against amyloid aggregation. In the present work, the possible role of lipoproteins as alpha-hANP binding factors has been investigated in vitro using an experimental model, developed in our laboratory, that allows to work at physiological concentrations. This approach consists of gel filtration on Sephacryl S-300 HR of big alpha-[(125)I]hANP generated in phosphate buffered saline or in human normal plasma supplemented or not with lipoproteins. The results of these experiments indicate that high density lipoproteins (HDL) are responsible for the ANP binding phenomenon observed in vitro, while low density lipoproteins and very low density lipoproteins do not directly interact with ANP. Moreover, the HDL remodeling process occurring in vitro has been analyzed during plasma incubation by monitoring the redistribution of lipids and apolipoproteins among the HDL subclasses. The changes in HDL size and composition observed in incubated plasma were compared with the redistribution of endogenous and labeled big ANP. The obtained results revealed that both tend to follow the molecular rearrangement in plasma of apolipoprotein A-I containing particles and suggested that, among HDL species, the small particles are mainly involved in the ANP binding phenomenon. This hypothesis was further demonstrated by ligand blotting experiments that confirmed the existence of differences in the ability of HDL particles to bind alpha-[(125)I]hANP.

LpAI in HDL subfractions: serum levels in men and women with coronary heart disease and changes under hypolipemic therapy

BACKGROUND

It is generally accepted that different HDL subpopulations may vary in their antiatherogenic potential, and the identification and differentiation of individual HDL subclasses may be useful in documentation and understanding of metabolic changes of different HDL particle groups.

METHODS

In the present study, LpAI particles concentrations in HDL(2) and HDL(3) subfractions were determined in serum of 54 CHD patients (33 men and 21 women) and 46 control subjects (19 men and 27 women) with similar total cholesterol and HDL-cholesterol levels.

RESULTS

In CHD patients, both men and women, as compared to control subjects lower levels of LpAI subpopulations were found, however, the difference was much more predominant for LpAI-HDL(2) than for LpAI-HDL(3). The effect of hypolipidemic treatment on the distribution of LpAI subpopulations between HDL subfractions was investigated in 44 hyperlipidemic patients assigned to fenofibrate therapy and 43 patients assigned to simvastatin therapy. Fenofibrate did not change LpAI level but had an effect on LpAI particle distribution among HDL(2) and HDL(3) increasing LpAI concentration in HDL(2) and slightly decreasing LpAI concentration in HDL(3). Simvastatin led to an increase in LpAI-HDL(3) and did not change significantly LpAI-HDL(2) particle concentration.

CONCLUSION

Further studies are needed to evaluate the significance of different HDL subpopulations.

Stable isotope turnover of apolipoproteins of high-density lipoproteins in humans

Amino acid precursors labelled with stable isotopes have been successfully used to explore the metabolism of the apolipoproteins of HDL. Some methodological and mathematical modelling problems remain, mainly related to amino acid recycling in a plasma protein such as apolipoprotein A-I with a long residence time (the reciprocal of the fractional catabolic rate) of 4-5 days. Apolipoprotein A-I, apolipoprotein E, and apolipoprotein A-IV in triglyceride-rich lipoproteins (containing chylomicrons, VLDL, and remnants) exhibit more complex kinetics. The small amounts of apolipoprotein A-I and of apolipoprotein A-IV in the triglyceride-rich lipoproteins have a residence time similar to that of the apolipoprotein A-I of HDL. In contrast, the apolipoprotein E in triglyceride-rich lipoproteins has been found to have an average residence time of 0.11 days. Diets low in saturated fat and cholesterol, which lower HDL levels, do so by decreasing the secretion of apolipoprotein A-I, with apolipoprotein A-II kinetics unaffected. Individuals with impaired glucose tolerance have a decreased residence time of apolipoprotein A-I but no change in secretion rate or in apolipoprotein A-II kinetics. This suggests a link between insulin resistance and the risk of atherosclerosis. In heterozygous familial hypercholesterolemia, both the fractional catabolic rate and the secretion rate of apolipoprotein A-I are increased, resulting in no change in the plasma level. Stable isotope studies have strengthened the evidence that triglyceride enrichment of HDL increases its catabolism Laboratory.