Cholesterol esters selectively delivered in vivo by high-density-lipoprotein subclass LpA-I to rat liver are processed faster into bile acids than are LpA-I/A-II-derived cholesterol esters

Predictive value of different HDL particles for the protection against or risk of coronary heart disease

The inverse relationship between plasma HDL levels and the risk of developing coronary heart disease is well established. The underlying mechanisms of this relationship are poorly understood, largely because HDL consist of several functionally distinct subpopulations of particles that are continuously being interconverted from one to another. This review commences with an outline of what is known about the origins of individual HDL subpopulations, how their distribution is regulated, and describes strategies that are currently available for isolating them. We then summarise what is known about the functionality of specific HDL subpopulations, and how these findings might impact on cardiovascular risk. The final section highlights major gaps in existing knowledge of HDL functionality, and suggests how these deficiencies might be addressed. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Plasma factors required for human apolipoprotein A-II dimerization

Although plasma high-density lipoproteins (HDL) have been implicated in several cardioprotective pathways, the physiologic role of apolipoprotein (apo) A-II, the second most abundant of the HDL proteins, remains ambiguous. Human apo A-II is distinguished from most other species by a single cysteine (Cys6), which forms a disulfide bond with other cysteine-containing apos. In human plasma, nearly all apo A-II occurs as disulfide-linked homodimers of 17.4 kDa. Although dimerization is an important determinant of human apo A-II metabolism, its mechanism and the plasma and/or cellular sites of its dimerization are not known. Using SDS-PAGE and densitometry we investigated the kinetics of apo A-II dimerization and observed a slow (t(1/2) = approximately 10 days), second-order process in Tris-buffered saline. In 3 M guanidine hydrochloride, which disrupts apo A-II secondary structure and self-association, the rate was 3-fold slower. In contrast, lipid surfaces that promote apo A-II alpha-helix formation and lipophilic interaction profoundly enhanced the rate. Reassembled HDL increased the second-order rate constant k(2) by 7500-fold, unilamellar 1-palmitoyl-2-oleoylphosphatidylcholine vesicles increased k(2) 850-fold, and physiological concentrations of human serum albumin increased k(2) 220-fold. Thus, while dimerization of apo A-II in aqueous buffer is too slow to account for the high fraction of dimer found in plasma, lipids and proteins "catalyze" dimer formation, a process that could occur either intracellularly prior to secretion or in the plasma compartment following secretion. These data suggest that formation of disulfide links within or between polypeptide chains can be controlled, in part, by coexisting lipids and proteins.

ApoA-II modulates the association of HDL with class B scavenger receptors SR-BI and CD36

The class B scavenger receptors SR-BI and CD36 exhibit a broad ligand binding specificity. SR-BI is well characterized as a HDL receptor that mediates selective cholesteryl ester uptake from HDL. CD36, a receptor for oxidized LDL, also binds HDL and mediates selective cholesteryl ester uptake, although much less efficiently than SR-BI. Apolipoprotein A-II (apoA-II), the second most abundant HDL protein, is considered to be proatherogenic, but the underlying mechanisms are unclear. We previously showed that apoA-II modulates SR-BI-dependent binding and selective uptake of cholesteryl ester from reconstituted HDL. To investigate the effect of apoA-II in naturally occurring HDL on these processes, we compared HDL without apoA-II (from apoA-II null mice) with HDLs containing differing amounts of apoA-II (from C57BL/6 mice and transgenic mice expressing a mouse apoA-II transgene). The level of apoA-II in HDL was inversely correlated with HDL binding and selective cholesteryl ester uptake by both scavenger receptors, particularly CD36. Interestingly, for HDL lacking apoA-II, the efficiency with which CD36 mediated selective uptake reached a level similar to that of SR-BI. These results demonstrate that apoA-II exerts a marked effect on HDL binding and selective lipid uptake by the class B scavenger receptors and establishes a potentially important relationship between apoA-II and CD36.

Polymorphisms of mouse apolipoprotein A-II: seven alleles found among 41 inbred strains of mice

In mice, apolipoprotein A-II (apoA-II) associates to form amyloid fibrils in an age-associated manner. We determined the complete nucleotide sequences of the apoA-II gene (Apoa2) cDNA in 41 inbred strains of mice including Mus musculus domesticus (laboratory mouse), Mus musculus castaneus, Mus musculus molossinus, Mus musculus musculus and Mus spretus. Among these strains we identified 7 alleles (Apoa2a1, Apoa2a2, Apoa2b, Apoa2c, Apoa2d, Apoa2e and Apoa2f). Polymorphisms of nucleotides at 15 positions were detected and amino acid substitutions were found at 8 positions. Apoa2a1 was found in all mouse subspecies, but Apoa2b and Apoa2c were found only in Mus musculus domesticus. Two strains of Mus spretus have the unique alleles Apoa2e and Apoa2f which resemble Apoa2c. We confirmed that VICS in which we found severe amyloidosis here and other amyloidoneic strains in published reports have Apoa2c allele. We determined the plasma concentrations of total and HDL cholesterol in the strains of Mus musculus domesticus with the Apoa2a1, Apoa2b and Apoa2c alleles. Significantly higher concentrations of plasma cholesterol were observed in mouse strains with the Apoa2b allele. These findings provide fundamental data on mouse Apoa2 alleles. Furthermore, differences in these alleles likely have considerable influence on traits related to amyloidosis and lipid metabolism.

Scavenger receptor BI (SR-BI) mediates a higher selective cholesteryl ester uptake from LpA-I compared with LpA-I:A-II lipoprotein particles

Scavenger receptor class B, type I (SR-BI) mediates the selective uptake of high-density lipoprotein- (HDL-) associated cholesteryl esters (CE), i.e. lipid uptake independent from HDL holo-particle internalisation. This pathway contributes to the HDL-mediated CE delivery to the liver. From human plasma HDL, two major lipoprotein subfractions can be isolated: one contains apolipoprotein (apo) A-I and apo A-II (LpA-I:A-II) as dominant protein components, whereas in the other population apo A-II is absent (LpA-I). In this investigation the role of SR-BI in selective CE uptake from HDL, LpA-I and LpA-I:A-II was explored. HDL(3) (d=1.125-1.21 g/ml), LpA-I and LpA-I:A-II were isolated from human plasma and radiolabeled in the protein (125I) as well as in the CE moiety ([3H]). Baby hamster kidney (BHK) cells were stably transfected with the full-length human SR-BI cDNA and these cells demonstrate SR-BI expression in immunoblots. In contrast, no SR-BI protein was detectable in control BHK cells (vector). To investigate lipoprotein uptake, cells incubated (37 degrees C, 4 h) in medium containing radiolabeled HDL(3), LpA-I or LpA-I:A-II and finally cellular tracer content was determined. For both types of BHK cells, the rate of apparent lipoprotein particle uptake according to the lipid tracer ([3H]) was in substantial excess over that due to the protein tracer (125I) demonstrating selective CE uptake ([3H]-(125)I) from HDL(3), LpA-I and LpA-I:A-II. SR-BI expression increased cellular selective CE uptake from labeled HDL(3) up to 24-fold. In BHK cells without SR-BI expression, selective CE uptake was higher from LpA-I compared with LpA-I:A-II. Analogously, in BHK cells with SR-BI expression, the rate of selective CE uptake was higher from LpA-I compared with LpA-I:A-II. In summary, SR-BI significantly increases selective CE uptake from HDL(3), LpA-I and LpA-I:A-II. Concerning HDL subfractions, the rate of SR-BI-mediated selective CE uptake is greater from LpA-I compared with LpA-I:A-II and this result suggests that SR-BI preferentially facilitates the CE transfer from LpA-I lipoprotein particles to cells.

Fatty acid saturation of the diet and plasma lipid concentrations, lipoprotein particle concentrations, and cholesterol efflux capacity

BACKGROUND

The fatty acid content and saturation degree of the diet can modulate HDL composition and cholesterol efflux.

OBJECTIVE

We studied the modifications in plasma lipoprotein particles and serum capacity to stimulate cholesterol efflux induced by different fatty acids.

DESIGN

Seventeen women and 24 men followed in the same sequence 4 diets containing 35% of total energy as fat. The saturated fat diet contained 17% palm oil; the monounsaturated fat diet, 20.9% olive oil; the n-6 polyunsaturated fat diet, 12.5% sunflower oil; and the n-3 polyunsaturated fat diet, sunflower oil supplemented with 4-4.5 g fish oil/d. Each phase lasted 4-5 wk.

RESULTS

In both sexes, apolipoprotein (apo) A-I concentrations were significantly lower with unsaturated fat diets than with the saturated fat diet, but concentrations of lipoproteins containing only apo A-I (Lp A-I) were lower only in the men. Concentrations of lipoproteins containing both apo A-I and apo A-II (Lp A-I:A-II) were lower with both polyunsaturated fat diets in the women but significantly higher in the men. Lp E concentrations were significantly higher with the 2 polyunsaturated fat diets. Lp E non-B particle concentrations were not modified in the men but were significantly higher in the women in both polyunsaturated fat phases. Lp C-III concentrations were higher with the saturated fat diet only in the men. The serum samples taken after the n-3 polyunsaturated fat phase were the most efficient for extracting cellular cholesterol in both sexes.

CONCLUSIONS

The monounsaturated and polyunsaturated fat diets were healthier, producing a better lipid profile. The n-3 polyunsaturated fat diet increased the capacity of serum to promote the efflux of cholesterol from cells in culture.

Apolipoprotein A-II modulates the binding and selective lipid uptake of reconstituted high density lipoprotein by scavenger receptor BI

High density lipoprotein (HDL) represents a mixture of particles containing either apoA-I and apoA-II (LpA-I/A-II) or apoA-I without apoA-II (LpA-I). Differences in the function and metabolism of LpA-I and LpA-I/A-II have been reported, and studies in transgenic mice have suggested that apoA-II is pro-atherogenic in contrast to anti-atherogenic apoA-I. The molecular basis for these observations is unclear. The scavenger receptor BI (SR-BI) is an HDL receptor that plays a key role in HDL metabolism. In this study we investigated the abilities of apoA-I and apoA-II to mediate SR-BI-specific binding and selective uptake of cholesterol ester using reconstituted HDLs (rHDLs) that were homogeneous in size and apolipoprotein content. Particles were labeled in the protein (with (125)I) and in the lipid (with [(3)H]cholesterol ether) components and SR-BI-specific events were analyzed in SR-BI-transfected Chinese hamster ovary cells. At 1 microg/ml apolipoprotein, SR-BI-mediated cell association of palmitoyloleoylphosphatidylcholine-containing AI-rHDL was significantly greater (3-fold) than that of AI/AII-rHDL, with a lower K(d) and a higher B(max) for AI-rHDL as compared with AI/AII-rHDL. Unexpectedly, selective cholesterol ester uptake from AI/AII-rHDL was not compromised compared with AI-rHDL, despite decreased binding. The efficiency of selective cholesterol ester uptake in terms of SR-BI-associated rHDL was 4-5-fold greater for AI/AII-rHDL than AI-rHDL. These results are consistent with a two-step mechanism in which SR-BI binds ligand and then mediates selective cholesterol ester uptake with an efficiency dependent on the composition of the ligand. ApoA-II decreases binding but increases selective uptake. These findings show that apoA-II can exert a significant influence on selective cholesterol ester uptake by SR-BI and may consequently influence the metabolism and function of HDL, as well as the pathway of reverse cholesterol transport.

Studies with apolipoprotein A-II transgenic mice indicate a role for HDLs in adiposity and insulin resistance

Apolipoprotein A-II (apoA-II) is the second most abundant protein in HDLs. Genetic studies in humans have provided evidence of linkage of the apoA-II gene locus to plasma free fatty acid (FFA) levels and to type 2 diabetes, and transgenic mice overexpressing mouse apoA-II have elevated levels of both FFA and triglycerides. We now show that apoA-II promotes insulin resistance and has diverse effects on fat homeostasis. ApoA-II transgenic mice have increased adipose mass and higher plasma leptin levels than C57BL/6J control mice. Fasting glucose levels were similar between apoA-II transgenic and control mice, but plasma insulin levels were elevated approximately twofold in the apoA-II transgenic mice. Compared with control mice, apoA-II transgenic mice exhibited a delay in plasma clearance of a glucose bolus. Adipose tissue isolated from fasted apoA-II transgenic mice exhibited a 50% decrease in triglyceride hydrolysis compared with adipose tissue from control mice. This is consistent with a normal response of adipose tissue to the increased insulin levels in the apoA-II transgenic mice and may partially explain the increased fat deposition. Skeletal muscle isolated from fasted apoA-II transgenic mice exhibited reduced uptake of 2-deoxyglucose compared with muscles isolated from control mice. Our observations indicate that a primary disturbance in lipoprotein metabolism can result in several traits associated with insulin resistance, consistent with the hypothesis that insulin resistance and type 2 diabetes can, under certain circumstances, be related primarily to altered lipid metabolism rather than glucose metabolism.

Branched synthetic peptide constructs mimic cellular binding and efflux of apolipoprotein AI in reconstituted high density lipoproteins

This study investigates the suitability of the trimeric apolipoprotein (apo)AI(145-183) peptide that we recently described, to serve as a model to probe the relationship between apoAI structure and function. Three copies of the apoAI(145-183) unit, composed each of two amphipathic alpha-helical segments, were branched onto a covalent core matrix and the construct was recombined with phospholipids. A similar construct was made with the apoAI(102-140) peptide and used as a comparison with dimyristoylglycerophosphocholine (DMPC)-apoAI complexes. The DMPC-trimeric-apoAI(145-183) complexes had similar immunological reactivity with monoclonal antibodies directed against the 149-186 apoAI sequence (A44), suggesting that the A44 epitope is exposed similarly in both the synthetic peptide and the native apoAI complexes. The complexes generated with the trimeric-apoAI(145-183) bind specifically to HeLa cells with comparable affinity to the DMPC apoAI complexes; they are a good competitor for binding of apoAI to both HeLa cells and Fu5AH rat hepatoma cells; finally, these complexes promote cholesterol efflux from Fu5AH cells with an efficiency comparable with the apo AI/lipid complexes. To study LCAT activation by the trimeric apo AI(145-183) construct, complexes were prepared with dipalmitoylphosphatidylcholine (DPPC), cholesterol (C) and either the trimeric construct or apoAI. LCAT activation by the trimeric construct was much lower than by apo AI, possibly because the conformation of the trimeric 145-183 peptide in DPPC/C/peptide complexes does not mimic that of apoAI in the corresponding complexes. In comparison, the complexes generated with the multimeric apoAI(102-140) construct had a poor capacity to mimic the physico-chemical and biological properties of apoAI. The apoAI(102-140) construct had low affinity for lipid compared with the (145-183) construct. After association with lipids, it was a poor competitor of DMPC-apoAI complexes for cellular binding and had only limited capacity to promote cholesterol efflux. These results suggest trimeric constructs can serve as an appropriate models for apoAI, enabling further investigations and new experimental approaches to determine the structure-function relationship of apoAI.

Selective uptake of cholesteryl esters from high-density lipoprotein-derived LpA-I and LpA-I:A-II particles by hepatic cells in culture

Selective uptake of high-density lipoprotein (HDL)-associated cholesteryl esters (CE), i.e. lipid uptake independent of HDL particle uptake, delivers CE to the liver and steroidogenic tissues in vivo and in vitro. From human plasma HDL, two major subpopulations of particles can be isolated: one contains both apolipoprotein (apo) A-I and apo A-II (designated LpA-I:A-II) as dominant protein components, whereas in the other apo A-II is absent (LpA-I). In this study, selective CE uptake from LpA-I and LpA-I:A-II by cultured cells was investigated. LpA-I and LpA-I:A-II were isolated by immunoaffinity chromatography from human plasma high-density lipoprotein3 (HDL3, d = 1.125-1.21 g/ml) and both particles were radiolabeled in the protein (125I) as well as in the CE moiety ([3H]cholesteryl oleyl ether ([3H]CEt)). Several control experiments validated the labeling methodology applied. To investigate selective CE uptake, human Hep G2 hepatoma cells, human hepatocytes in primary culture and human skin fibroblasts were incubated in medium containing doubly radiolabeled LpA-I or LpA-I:A-II particles. Thereafter cellular tracer content was determined. For each cell type the rate of apparent lipoprotein particle uptake according to the lipid tracer ([3H]CEt) was in substantial excess over that due to the protein tracer (125I), demonstrating selective CE uptake from LpA-I as well as from LpA-I:A-II. This difference in uptake between [3H]CEt and 125I, i.e. the rate of apparent selective CE uptake, was significantly higher for LpA-I compared to LpA-I:A-II, and this was dose- as well as time-dependent. Thus in human hepatic cell and fibroblasts, CE are selectively taken up to a higher extent from LpA-I compared to LpA-I:A-II. These results may suggest that LpA-I particles of the human plasma HDL fraction may be those lipoproteins which more efficiently deliver CE to the liver via the selective uptake pathway whereas LpA-I:A-II may play a less important role.

Overexpression of apolipoprotein AII in transgenic mice converts high density lipoproteins to proinflammatory particles

Previous studies showed that transgenic mice overexpressing either apolipoprotein AI (apoAI) or apolipoprotein AII (apoAII), the major proteins of HDL, exhibited elevated levels of HDL cholesterol, but, whereas the apoAI-transgenic mice were protected against atherosclerosis, the apoAII-transgenic mice had increased lesion development. We now examine the basis for this striking functional heterogeneity. HDL from apoAI transgenics exhibited an enhanced ability to promote cholesterol efflux from macrophages, but HDL from apoAII transgenics and nontransgenics were not discernibly different in efflux studies. In contrast with HDL from nontransgenics and apoAI transgenics, HDL from the apoAII transgenics were unable to protect against LDL oxidation in a coculture model of the artery wall. Furthermore, HDL taken from apoAII-transgenic mice, but not HDL taken from either the apoAI transgenics or nontransgenic littermate controls, by itself stimulated lipid hydroperoxide formation in artery wall cells and induced monocyte transmigration, indicating that the apoAII-transgenic HDL were in fact proinflammatory. This loss in the ability of the apoAII-transgenic HDL to function as an antioxidant/antiinflammatory agent was associated with a decreased content of paraoxonase, an enzyme that protects against LDL oxidation. Reconstitution of the apoAII transgenic HDL with purified paraoxonase restored both paraoxonase activity and the ability to protect against LDL oxidation. We conclude that overexpression of apoAII converts HDL from an anti- to a proinflammatory particle and that paraoxonase plays a role in this transformation.

Increased selective uptake in vivo and in vitro of oxidized cholesteryl esters from high-density lipoprotein by rat liver parenchymal cells

Oxidation of low-density lipoprotein (LDL) leads initially to the formation of LDL-associated cholesteryl ester hydroperoxides (CEOOH). LDL-associated CEOOH can be transferred to high-density lipoprotein (HDL), and HDL-associated CEOOH are rapidly reduced to the corresponding hydroxides (CEOH) by an intrinsic peroxidase-like activity. We have now performed in vivo experiments to quantify the clearance rates and to identify the uptake sites of HDL-associated [3H]Ch18:2-OH in rats. Upon injection into rats, HDL-associated [3H]Ch18:2-OH is removed more rapidly from the circulation than HDL-associated [3H]Ch18:2. Two minutes after administration of [3H]Ch18:2-OH-HDL, 19.6 +/- 2.6% (S.E.M.; n = 4) of the label was taken up by the liver as compared with 2.4 +/- 0.25% (S.E.M.; n = 4) for [3H]Ch18:2-HDL. Organ distribution studies indicated that only the liver and adrenals exhibited preferential uptake of [3H]Ch18:2-OH as compared with [3H]Ch18:2, with the liver as the major site of uptake. A cell-separation procedure, employed 10 min after injection of [3H]Ch18:2-OH-HDL or [3H]Ch18:2-HDL, demonstrated that within the liver only parenchymal cells take up HDL-CE by the selective uptake pathway. Selective uptake by parenchymal cells of [3H]Ch18:2-OH was 3-fold higher than that of [3H]Ch18:2, while Kupffer and endothelial cell uptake of the lipid tracers reflected HDL holoparticle uptake (as analysed with iodinated versus cholesteryl ester-labelled HDL). The efficient uptake of [3H]Ch18:2-OH by parenchymal cells was coupled to a 3-fold increase in rate of radioactive bile acid secretion from [3H]Ch18:2-OH-HDL as compared with [3H]Ch18:2-HDL. In vitro studies with freshly isolated parenchymal cells showed that the association of [3H]Ch18:2-OH-HDL at 37 degrees C exceeded [3H]Ch18:2-HDL uptake almost 4-fold. Our results indicate that HDL-associated CEOH are efficiently and selectively removed from the blood circulation by the liver in vivo. The selective liver uptake is specifically exerted by parenchymal cells and coupled to a rapid biliary secretion pathway. The liver uptake and biliary secretion route may allow HDL to function as an efficient protection system against potentially atherogenic CEOOH.

The molecular structure of apolipoprotein A-II modulates the capacity of HDL to promote cell cholesterol efflux

The influence of apolipoprotein A-II (apoA-II) molecular structure on the capacity of high density lipoproteins (HDL) to promote cellular cholesterol efflux was investigated in cultured mouse peritoneal macrophages (MPM). Conversion by reduction and carboxamidomethylation of the naturally occurring dimeric apoA-II to its monomeric form in both native or reconstituted HDL did not change apolipoprotein secondary structure and lipoprotein size/composition. All particles containing monomeric apoA-II, i.e., native HDL3 or reconstituted HDL with or without apoA-I, showed a higher ability to promote cholesterol efflux originating from plasma membrane and intracellular stores, compared to particles containing dimeric apoA-II. These findings indicate that apolipoprotein molecular structure is a major determinant of HDL capacity to promote cholesterol efflux from cells.

Abnormal reverse cholesterol transport in controlled type II diabetic patients. Studies on fasting and postprandial LpA-I particles

The high incidence and prevalence of coronary heart disease in diabetes mellitus is clearly established. The usual lipid pattern found in type II diabetic patients is a moderate increase in fasting triglyceride levels associated with low HDL cholesterol levels. These abnormalities are further amplified in the postprandial state. To study the effect of these alterations on reverse cholesterol transport, we isolated lipoprotein containing apoA-I but not apoA-II (LpA-I) particles by immunoaffinity chromatography from the plasma of well-controlled type II diabetic patients and nondiabetic matched control subjects. Different parameters involved in this antiatherogenic pathway were measured in both fasting and postprandial states. Diabetic patients had reduced levels of LpA-I particles that were protein enriched and phospholipid depleted. Gradient gel electrophoresis showed that control LpA-I particles had five distinct populations, whereas diabetic particles lacked the largest one. LpA-I isolated from diabetic plasma exhibited a decreased capacity to induce cholesterol efflux from Ob 1771 adipose cells both in fasting (15.1 +/- 10.0% versus 7.5 +/- 2.7%, P < .05) and postprandial (17.7 +/- 11.2% versus 7.7 +/- 3.9%, P < .05) states, whereas only control particles showed significantly higher ability to promote cholesterol efflux after the test meal (P = .02). Lecithin:cholesterol acyltransferase activity measured with an exogenous substrate showed a 54% increase and an 18% decrease postprandially for control subjects and patients, respectively. Thus, the different abnormalities found in the fasting state were further amplified in the postprandial situation. This resulted in LpA-I particles with aberrant size and composition and decreased ability to accomplish their antiatherogenic role in type II diabetic patients.

Lipid composition of HDL subfractions in dog plasma and lymph

We report the lipid composition of dog plasma and peripheral lymph lipoproteins as separated into pre-beta, alpha, and pre-alpha fractions by agarose gel electrophoresis. Plasma lipoproteins with alpha mobility have a composition different from that of plasma lipoproteins with pre-alpha mobility, having 9% versus 11% free cholesterol, 21% versus 17% cholesterol ester, 1% versus 16% triacylglycerol, and 69% versus 56% phospholipid. On the other hand, lymph alpha and pre-alpha lipoproteins have compositions that are quite similar (9% versus 7% free cholesterol, 17% versus 17% cholesterol ester, 2% versus 4% triacylglycerol, and 71% versus 71% phospholipid). The lipid compositions of plasma and lymph alpha lipoproteins are quite similar (9% versus 9% free cholesterol, 21% versus 17% cholesterol ester, 1% versus 2% triacylglycerol, and 70% versus 72% phospholipid). The lipid compositions of plasma and lymph pre-alpha lipoproteins are different (11% versus 7% free cholesterol, 17% versus 17% cholesterol ester, 16% versus 4% triacylglycerol, and 56% versus 71% phospholipid). Peripheral lymph lipoproteins with pre-beta mobility contained 15% cholesterol, 13% cholesterol ester, 10% triacylglycerol, and 61% phospholipid. Compared with plasma, peripheral lymph lipoproteins are free cholesterol-enriched in all fractions. Calculated stoichiometric ratios of lipid to apoA-I per particle, alpha lipoproteins have two molecules of apoA-I per particle, and pre-alpha lipoproteins have four molecules of apoA-I per particle.

Cholesterol efflux, cholesterol esterification, and cholesteryl ester transfer by LpA-I and LpA-I/A-II in native plasma

HDLs encompass structurally heterogeneous particles that fulfill specific functions in reverse cholesterol transport. Two-dimensional nondenaturing polyacrylamide gradient gel electrophoresis (2D-PAGGE) of normal plasma and subsequent immunoblotting with anti-apolipoprotein (apo) A-I antibodies differentiates an abundant particle with electrophoretic alpha-mobility and less abundant particles with electrophoretic pre-beta-mobility (pre beta 1-LpA-I, pre beta 2-LpA-I, pre beta 3-LpA-I). Immunodetection with anti-apoA-II antibodies identifies a single particle with alpha-mobility. To differentiate alpha-migrating HDL without apo A-II (alpha-LpA-I) from those with apoA-II (alpha-LpA-I/A-II), we combined 2D-PAGGE with immunoadsorption of apoA-II. Incubation of plasma with [3H]cholesterol-labeled fibroblasts in combination with immunosubtracting 2D-PAGGE allowed us to analyze the role of alpha-LpA-I and alpha-LpA-I/A-II in the uptake and esterification of cell-derived cholesterol in native plasma. Depending on the duration of incubations with cells, alpha-LpA-I took up two to four times more [3H]cholesterol than alpha-LpA-I/A-II. Irrespective of the duration of incubation, two to three times more [3H]cholesteryl esters accumulated in alpha-LpA-I than in alpha-LpA-I/A-II. Subsequent incubations in the presence of an inhibitor of lecithin:cholesterol acyltransferase led to preferential accumulation of [3H]cholesteryl esters in alpha-LpA-I/A-II. In conclusion, our data indicate that alpha-LpA-I is more effective than alpha-LpA-I/A-II in both uptake and esterification of cell-derived cholesterol. Moreover, alpha-LpA-I/A-II appears to accumulate cholesteryl esters, at least partially, from alpha-LpA-I.

Modulation of cholesterol efflux from Fu5AH hepatoma cells by the apolipoprotein content of high density lipoprotein particles. Particles containing various proportions of apolipoproteins A-I and A-II

The influence of apolipoproteins (apo) A-I and A-II on the ability of high density lipoproteins (HDL) to remove cholesterol from cultured Fu5AH rat hepatoma cells was studied independently on alterations in the overall structure and lipid composition of the lipoprotein particles. To this end, apoA-I was progressively replaced by apoA-II in ultracentrifugally isolated HDL3 without inducing changes in other remaining lipoprotein components. As apoA-II was progressively substituted for apoA-I in HDL3 (A-II:A-I+A-II percentage mass: 29.5, 47.6, 71.5, 97.4, and 98.9%), the rate of cholesterol efflux from Fu5AH hepatoma gradually and significantly decreased after 2 or 4 h of incubation at 37 degrees C (cholesterol efflux: 30.4 +/- 0.8, 24.1 +/- 1.0, 19.8 +/- 1.2, 15.7 +/- 1.4, and 13.4 +/- 1.3%/2h, respectively; 38.4 +/- 1.5, 29.2 +/- 0.9, 27.0 +/- 0.2, 20.4 +/- 0.4, and 17.5 +/- 1.0%/4h, respectively) (p < 0.01 with all A-II-enriched HDL3 fractions as compared with non-enriched homologues). In agreement with data obtained with total HDL3, increasing the A-II:A-I+A-II percentage mass in HDL3 particles containing initially only apoA-I (HDL3-A-I) progressively reduced cellular cholesterol efflux. After 2 h of incubation, cholesterol efflux correlated negatively with A-II:A-I+A-II percentage mass (r = -0.86; p < 0.0001; n = 20), but not with either free cholesterol:phospholipid ratio, A-I+A-II:total lipid ratio or mean size of HDL3. As determined by using Spearman rank correlation analysis, the A-II:A-I+A-II% mass ratio correlated negatively with the apparent maximal efflux (Vmax(efflux)) (rho = -0.68; p < 0.05, n = 10), but not with the HDL3 concentration required to obtain 50% of maximal efflux (Km(efflux)) (rho = -0.08; not significant, n = 10). It was concluded that the apoA-I and apoA-II content of HDL3 is one determinant of its ability to promote cholesterol efflux from Fu5AH rat hepatoma cells.

Apolipoprotein A-I-containing particles and reverse cholesterol transport: evidence for connection between cholesterol efflux and atherosclerosis risk

It is now clearly established that apo A-I-containing lipoproteins exist as two major families, those containing apo A-I and apo A-II (LpA-I:A-II) and those containing apo A-I but free of apo A-II (LpA-I). Metabolic studies utilizing radiolabeled lipoprotein particles suggested that there is a kinetic difference between LpA-I and LpA-I:A-II family and support the concept that there may be important functional differences between the lipoprotein particles present within HDL. Of considerable significance was the finding that proteins stimulating reverse cholesterol transport (lecithin:cholesterol acyltransferase (LCAT), cholesteryl ester transfer protein (CETP)) are mainly present in LpA-I and not in LpA-I:A-II family. Cholesterol efflux mediated by A-I-containing particles has been studied in different cells. Long term exposure to LpA-I family promoted cholesterol efflux whereas less efflux was observed in the presence of LpA-I:A-II family. The fact that LpA-I:A-II family can inhibit the LpA-I promoted cholesterol efflux strongly supports the role of apo A-II as an antagonist in the production of cholesterol efflux. These results which emphasize that LpA-I and LpA-I:A-II families behave as distinct entities have been confirmed in other studies showing that they have different clinical significance. The results in mice transgenic for apo A-I indicate that overexpression of apo A-I induces more cholesterol efflux and protects C57BL/6 mice from atherosclerosis. Increased expression of apo A-II in mice appears to decrease cholesterol efflux and to promote rather than retard aortic fatty streak development.(ABSTRACT TRUNCATED AT 250 WORDS)