Characterization of apoA-I-containing lipoprotein subpopulations secreted by HepG2 cells

Influence of apolipoprotein A-I and apolipoprotein A-II availability on nascent HDL heterogeneity

It is important to understand HDL heterogeneity because various subspecies possess different functionalities. To understand the origins of HDL heterogeneity arising from the existence of particles containing only apoA-I (LpA-I) and particles containing both apoA-I and apoA-II (LpA-I+A-II), we compared the abilities of both proteins to promote ABCA1-mediated efflux of cholesterol from HepG2 cells and form nascent HDL particles. When added separately, exogenous apoA-I and apoA-II were equally effective in promoting cholesterol efflux, although the resultant LpA-I and LpA-II particles had different sizes. When apoA-I and apoA-II were mixed together at initial molar ratios ranging from 1:1 to 16:1 to generate nascent LpA-I+A-II HDL particles, the particle size distribution altered, and the two proteins were incorporated into the nascent HDL in proportion to their initial ratio. Both proteins formed nascent HDL particles with equal efficiency, and the relative amounts of apoA-I and apoA-II incorporation were driven by mass action. The ratio of lipid-free apoA-I and apoA-II available at the surface of ABCA1-expressing cells is a major factor in determining the contents of these proteins in nascent HDL. Manipulation of this ratio provides a means of altering the relative distribution of LpA-I and LpA-I+A-II HDL particles.

ABCA1 and biogenesis of HDL

Mammalian somatic cells do not catabolize cholesterol and therefore export it for sterol homeostasis at cell and whole body levels. This mechanism may reduce intracellularly accumulated excess cholesterol, and thereby would contribute to the prevention or cure of the initial stage of atherosclerotic vascular lesion. High-density lipoprotein (HDL) plays a central role in this reaction by removing cholesterol from cells and transporting it to the liver, the major cholesterol catabolic site. Two independent mechanisms have been identified for cellular cholesterol release. The first is non-specific diffusion-mediated cholesterol "efflux" from the cell surface, in which cholesterol is trapped by various extracellular acceptors including lipoproteins. Extracellular cholesterol esterification of HDL provides a driving force for the net removal of cell cholesterol by this pathway, and some cellular factors may enhance this reaction. The other mechanism is an apolipoprotein-mediated process to generate new HDL particles by removing cellular phospholipid and cholesterol. This reaction is mediated by a membrane protein ATP-binding cassette transporter A1 (ABCA1), and lipid-free or lipid-poor helical apolipoproteins recruit cellular phospholipid and cholesterol to assemble HDL particles. The reaction is composed of two elements: the assembly of HDL particles with phospholipid by apolipoprotein, and cholesterol enrichment in HDL. ABCA1 is essential for the former step and the latter requires further intracellular events. ABCA1 is a rate-limiting factor of HDL assembly and is regulated by transcriptional and post-transcriptional factors. Post-transcriptional regulation of ABCA1 involves modulation of its calpain-mediated degradation.

Assembly of High-Density Lipoprotein

Mammalian somatic cells do not catabolize cholesterol and need to export it for its homeostasis at the levels of cells and whole bodies. This reaction may reduce intracellularly accumulated cholesterol in excess and would contribute to prevention or regression of the initial stage of atherosclerosis. High-density lipoprotein (HDL) is thought to play a main role in this reaction, and 2 independent mechanisms are proposed for this reaction. First, cholesterol is exchanged in a nonspecific physicochemical manner between cell surface and extracellular lipoproteins, and cholesterol esterification on HDL provides a driving force for net removal of cell cholesterol. Second, apolipoproteins directly interact with cells and generate HDL by removing cellular phospholipid and cholesterol. This reaction is a major source of plasma HDL and is mediated by a membrane protein, ABCA1. Lipid-free or lipid-poor helical apolipoproteins primarily recruit cellular phospholipid to assemble HDL particles, and cholesterol enrichment in these particles is regulated independently. ABCA1 is a rate-limiting factor of the HDL assembly and is regulated by transcriptional factors and posttranscriptional factors. Posttranscriptional regulation of ABCA1 includes modulation of its calpain-mediated degradation.

Assembly of high density lipoprotein by the ABCA1/apolipoprotein pathway

PURPOSE OF REVIEW

Mammalian somatic cells do not catabolize cholesterol and therefore need to export it for sterol homeostasis at the levels of cells and whole bodies. This mechanism may reduce intracellularly accumulated cholesterol in excess, and thereby would contribute to the prevention or cure of the initial stage of atherosclerotic vascular lesions.

RECENT FINDINGS

HDL is thought to play a main role in this reaction on the basis of epidemiological evidence and in-vitro experimental data. Two independent mechanisms have been identified for this reaction. One is non-specific diffusion-mediated cholesterol 'efflux' from the cell surface, and cholesterol is trapped by various extracellular acceptors including lipoproteins. Extracellular cholesterol esterification on HDL provides a driving force for the net removal of cell cholesterol, and some cellular factors may enhance this reaction. The other mechanism is an apolipoprotein-mediated process to generate HDL by removing cellular phospholipid and cholesterol. This reaction is mediated by a membrane protein ABCA1, and lipid-free or lipid-poor helical apolipoproteins recruit cellular phospholipid and cholesterol to assemble HDL particles. The reaction is composed of two elements: the assembly of HDL particles with phospholipid by apolipoprotein, and cholesterol enrichment in HDL. ABCA1 is essential for the former step, and the latter step requires further intracellular events.

SUMMARY

ABCA1 is a rate-limiting factor of HDL assembly and is regulated by transcriptional factors and posttranscriptional factors. Posttranscriptional regulation of ABCA1 involves the modulation of its calpain-mediated degradation.

Antioxidant vitamins and lipid therapy: end of a long romance?

During the past decade, the perception flourished that lipid and antioxidant therapy were 2 independent avenues for cardiovascular protection. However, studies have shown that commonly used antioxidant vitamin regimens do not prevent cardiovascular events. We found that the addition of antioxidant vitamins to simvastatin-niacin therapy substantially blunts the expected rise in the protective high density lipoprotein (HDL)2 cholesterol and lipoprotein(A-I) subfractions of HDL, with apparent adverse effects on the progression of coronary artery disease. To better understand this effect, 12 apolipoproteins, receptors, or enzymes that contribute to reverse cholesterol transport have been examined in terms of their relationship to HDL2 and lipoprotein(A-I) levels and the potential for antioxidant modulation of their gene expression. Three plausible candidate mechanisms are identified: (1) antioxidant stimulation of cholesteryl ester transfer protein expression/activity, (2) antioxidant suppression of macrophage ATP binding cassette transmembrane transporter A1 expression, and/or (3) antioxidant suppression of hepatic or intestinal apolipoprotein A-I synthesis or increase in apolipoprotein A-I catabolism. In summary, antioxidant vitamins E and C and beta-carotene, alone or in combination, do not protect against cardiovascular disease. Their use for this purpose may create a diversion away from proven therapies. Because these vitamins blunt the protective HDL2 cholesterol response to HDL cholesterol-targeted therapy, they are potentially harmful in this setting. We conclude that they should rarely, if ever, be recommended for cardiovascular protection.

The benefits of niacin in atherosclerosis

Niacin favorably alters all major lipid subfractions at pharmacologic doses. Alone or in combination, it promotes regression of coronary artery disease, decreases coronary events, stroke, and total mortality. Major recent progress in niacin is in four areas. Firstly, recent data indicate that it increases high-density lipoprotein (HDL) and lowers triglycerides and low-density lipoprotein (LDL) by mechanisms different from statins, fibrates, and bile-sequestrants, giving rationale for combination therapy to achieve synergistic effects for complete lipid goal achievement. Secondly, new data on an extended-release preparation of niacin given once nightly indicates that it is as effective and has greater tolerability than immediate-release niacin. Thirdly, preliminary data with a single tablet formulation extended-release niacin and an HMG CoA reductase inhibitor (lovastatin) shows it to be safe and very effective, especially for raising HDL. Finally, emerging evidence indicates that niacin can be used effectively and safely in patients with type 2 diabetes mellitus, who often have low HDL levels.

Release of cellular cholesterol: molecular mechanism for cholesterol homeostasis in cells and in the body

Most mammalian somatic cells are unable to catabolize cholesterol and therefore need to export it in order to maintain sterol homeostasis. This mechanism may also function to reduce excessively accumulated cholesterol, which would thereby contribute to prevention or cure of the initial stage of atherosclerotic vascular lesion. High-density lipoprotein (HDL) has been believed to play a main role in this reaction based on epidemiological evidence and in vitro experimental data. At least two independent mechanisms are identified for this reaction. One is non-specific diffusion-mediated cholesterol 'efflux' from cell surface. Cholesterol molecules desorbed from cells can be trapped by various extracellular acceptors including various lipoproteins and albumin, and extracellular cholesterol esterification mainly on HDL may provide a driving force for the net removal of cell cholesterol by maintaining a cholesterol gradient between lipoprotein surface and cell membrane. The other is apolipoprotein-mediated process to generate new HDL by removing cellular phospholipid and cholesterol. The reaction is initiated by the interaction of lipid-free or lipid-poor helical apolipoproteins with cellular surface resulting in assembly of HDL particles with cellular phospholipid and incorporation of cellular cholesterol into the HDL being formed. Thus, HDL has dual functions as an active cholesterol acceptor in the diffusion-mediated pathway and as an apolipoprotein carrier for the HDL assembly reaction. The impairment of the apolipoprotein-mediated reaction was found in Tangier disease and other familial HDL deficiencies to strongly suggest that this is a main mechanism to produce plasma HDL. The causative mutations for this defect was identified in ATP binding cassette transporter protein A1, as a significant step for further understanding of the reaction and cholesterol homeostasis.

Very Small Apolipoprotein A-I-containing Particles from Human Plasma: Isolation and Quantification by High-Performance Size-Exclusion Chromatography

Background: Very small apolipoprotein (apo) A-I-containing lipoprotein (Sm LpA-I) particles with pre-β electrophoretic mobility may play key roles as “nascent” and/or “senescent” HDL; however, methods for their isolation are difficult and often semiquantitative.

Methods: We developed a preparative method for separating Sm LpA-I particles from human plasma by high-performance size-exclusion chromatography (HP-SEC), using two gel permeation columns (Superdex 200 and Superdex 75) in series and measuring apo A-I content in column fractions in 30 subjects with HDL-cholesterol (HDL-C) concentrations of 0.4–3.83 mmol/L.

Results: Three major sizes of apo A-I-containing particles were detected: an ∼15-nm diameter (∼700 kDa) species; a 7.5–12 nm (100–450 kDa) species; and a 5.8–6.3 nm species (40–60 kDa, Sm LpA-I particles), containing 0.2–3%, 80–96%, and 2–15% of plasma total apo A-I, respectively. Two subjects with severe HDL deficiency had increased relative apo A-I content in Sm LpA-I: 25% and 37%, respectively. The percentage of apo A-I in Sm LpA-I correlated positively with fasting plasma triglyceride concentrations (r = 0.581; P <0.0005) and inversely with total apo A-I (r = −0.551; P <0.0013) and HDL-C concentrations (r = −0.532; P <0.0017), although the latter two relationships were largely attributable to extremely hypoalphalipoproteinemic subjects. The percentage of apo A-I in Sm LpA-I correlated with that in pre-β-migrating species by crossed immunoelectrophoresis (r = 0.98; P <0.0001; n = 24) and with that in the d >1.21 kg/L fraction by ultracentrifugation (r = 0.86; P <0.001; n = 20). Sm LpA-I particles, on average, appear to contain two apo A-I and four phospholipid molecules but little or no apo A-II, triglyceride, or cholesterol.

Conclusions: We present a new HP-SEC method for size separation of native HDL particles from plasma, including Sm Lp A-I, which may play important roles in the metabolism of HDL and in its contribution(s) to protection against atherosclerosis. This method provides a basis for further studies of the structure and function of Sm Lp A-I.

Mechanism of action of niacin on lipoprotein metabolism

It is generally accepted that the increased concentrations of apolipoprotein (apo) B containing very low-density lipoproteins (VLDL) and low-density lipoproteins (LDL), and decreased levels of apo AI containing high-density lipoproteins (HDL) are correlated to atherosclerotic cardiovascular disease. Current evidence indicates that the post-translational apo-B degradative processes regulate the hepatic assembly and secretion of VLDL and the subsequent generation of LDL particles. The availability of triglycerides (TG) for the addition to apo B during intracellular processing appears to play a central role in targeting apo B for either intracellular degradation or assembly and secretion as VLDL particles. Based on the availability of TG, the liver secretes either dense TG-poor VLDL2 or large TG-rich VLDL1 particles, and these particles serve as precursors for the formation of more buoyant or small, dense LDL particles by lipid transfer protein- and hepatic lipase-mediated processes. HDLs are a heterogenous class of lipoproteins, and apo AI (the major protein of HDL) participates in reverse cholesterol transport, a process by which excess cholesterol is eliminated. Recent studies indicate that HDL particles containing only apo A-I (LPA-I) are more effective in reverse cholesterol transport and more anti-atherogenic than HDL particles containing both apo A-I and apo A-II (LPA-I + A-II).

Comparison of the effects of triphasic oral contraceptives with desogestrel or levonorgestrel on apolipoprotein A-I-containing high-density lipoprotein particles

Recent observations suggest that the risk of coronary artery disease (CAD) is associated with both the level and composition of the two major populations of apolipoprotein (apo)-defined high-density lipoprotein (HDL) particles: those containing both apo A-I and apo A-II [Lp(AI,AII)] and those containing apo A-I without apo A-II [Lp(AI)]. While sex hormones are known to affect HDL, their influence on these apo-defined HDL particles is not known. We have determined the effects of two triphasic oral contraceptive (OC) formulations on these HDL particles in healthy normolipidemic women aged 21 to 35 years. The formulations contain comparable quantities of ethinyl estradiol (EE) and either desogestrel (DG), a minimally androgenic progestin, or levonorgestrel (LN), a more androgenic progestin. Lipid and lipoprotein levels were measured during the third week of the normal menstrual cycle and the sixth month of OC use. The DG/EE formulation significantly increased total cholesterol (C) 15%, triglyceride (TG) 99%, phospholipid (PL) 17%, apo A-I 28%, apo A-II 34%, apo B 21%, very-low-density lipoprotein cholesterol (VLDL-C) 238%, HDL-C 20%, and HDL3-C 28% (P < .02 to .005, n = 11), but not low-density lipoprotein cholesterol (LDL-C). The LN/EE formulation also increased total C 15%, TG 33%, apo A-I 15%, HDL3-C 21% (P < .05, n = 10), apo B 30% (P < .005), and, additionally, LDL-C 19% (P < .05). Both formulations increased Lp(AI,AII) (DG/EE, 34%, P < .005; LN/EE, 24%, P < .01). These changes reflected comparable increases of small (7.0 to 8.2 nm) and medium (8.2 to 9.2 nm) particles in the LN/EE group and a predominant increase of medium-sized particles in the DG/EE group. Also, in the LN/EE group but not the DG/EE group, there were fewer large (9.2 to 11.2 nm) particles. Lp(AI) increased only in the DG/EE group (25%, P = .075) and was due to the presence of more large particles. The level of Lp(AI) did not change in the LN/EE group, but the lipid/A-I ratio of these particles was lower (P = .012) and there were more small particles. Thus, triphasic OC formulations with progestins of different androgenicity had different effects on VLDL, LDL, and the level and composition of HDL particles with and without apo A-II, possibly reflecting estrogen/progestin/androgen balance. Estrogen dominance increases both Lp(AI,AII) and Lp(AI) and favors large Lp(AI) particles, while progestin/androgen dominance increases only Lp(AI,AII) and favors small particles. Because of the importance of HDL in the arterial wall physiology, OC formulations with different estrogen and progestin content may affect arterial wall health to a different extent.

Acute effects of intravenous infusion of ApoA1/phosphatidylcholine discs on plasma lipoproteins in humans

To investigate the metabolism of nascent HDLs, apoA1/phosphatidylcholine (apoA1/PC) discs were infused IV over 4 hours into 7 healthy men. Plasma total apoA1 and phospholipid (PL) concentrations increased during the infusions. The rise in plasma apoA1 was greatest in small prebeta-migrating particles not present in the infusate. Total HDL unesterified cholesterol (UC) also increased simultaneously. After stopping the infusion, the concentrations of apoA1, PL, HDL UC, and small prebeta HDLs decreased, whereas those of HDL cholesteryl ester (CE) and large alpha-migrating apoA1 containing HDLs increased. ApoB-containing lipoproteins became enriched in CEs. Addition of apoA1/PC discs to whole blood at 37 degrees C in vitro also generated small prebeta HDLs, but did not augment the transfer of UC from erythrocytes to plasma. We conclude that the disc infusions increased the intravascular production of small prebeta HDLs in vivo, and that this was associated with an increase in the efflux and esterification of UC derived from fixed tissues. The extent to which the increase in tissue cholesterol efflux was dependent on that in prebeta HDL production could not be determined. Infusion of discs also reduced the plasma apoB and apoA2 concentrations, and increased plasma triglycerides and apoC3. Thus, nascent HDL secretion may have a significant impact on prebeta HDL production, reverse cholesterol transport and lipoprotein metabolism in humans.

Displacement of apo A-I from HDL by apo A-II or its C-terminal helix promotes the formation of pre-beta1 migrating particles and decreases LCAT activation

The displacement of apolipoprotein (apo) A-I by apo A-II is a major event in the remodeling of high density lipoproteins (HDL). In the present study, we investigated the displacement of apo A-I both from native and reconstituted HDL (rHDL) by either apo A-II or by the C-terminal helical peptide (i.e. residues 53-70). We studied the remodeling process of the original particles, the changes in size and composition and in their lecithin:cholesterol acyltransferase (LCAT) activating properties. Using gel filtration, we show that, at low apo A-II/AI ratios, the initial lipid apolipoprotein complex containing 2 mol apo A-I is remodeled into a mixed complex containing apo A-I and apo A-II, involving the displacement of one apo A-I by apo A-II. Upon addition of a larger amount of apo A-II, the rHDL particles become more heterogeneous and of larger size. Immunoblotting of the particles separated by non denaturing gradient gel electrophoresis shows that most of the apo A-I remains associated with the largest particles. The LCAT activation properties of the remodeled complexes decrease upon addition of either apo A-II or its C-terminal helix. This decrease is more pronounced when rHDL are incubated with the apo A-II C-terminal helix than with native apo A-II, as VmaX decreases from 28 to 16 and 7 nmol cholesteryl ester/ml per h respectively, whereas Km remains unchanged. The displacement of apo A-I observed with rHDL also occurred with native HDL particles as demonstrated by two-dimensional gel electrophoresis, using pyrene-phospholipid labeled HDL. Displacement of apo A-I generates pre-beta1 migrating particles containing apo A-I and phospholipids. We therefore propose that apo A-II has a dual effect on the role of HDL in reverse cholesterol transport: displacement of apo A-I from rHDL results in a negative control of the LCAT activity, while generation of pre-beta1 migrating particles enhances the formation of potential acceptors of cellular cholesterol.

Estradiol stimulates apolipoprotein A-I- but not A-II-containing particle synthesis and secretion by stimulating mRNA transcription rate in Hep G2 cells

Estrogen therapy increases plasma HDL levels, which may reduce cardiovascular risk in postmenopausal women. The mechanism of action of estrogen in influencing various steps in hepatic HDL and apolipoprotein (apo) A-I synthesis and secretion are not fully understood. In this study, we have used the human hepatoblastoma cell line (Hep G2) as an in vitro model system to delineate the effect of estradiol on multiple regulatory steps involved in hepatic HDL metabolism. Incubation of Hep G2 cells with estradiol resulted in the following statistically significant findings: (1) increased accumulation of apoA-I in the medium without affecting uptake/removal of radiolabeled HDL-protein; (2) accelerated incorporation of [3H]leucine into apoA-I; (3) selective increase in [3H]leucine incorporation into lipoprotein (LP) A-I but not LP A-I+A-II HDL particles (HDL particles without and with apoA-II, respectively); (4) increased ability of apoA-I-containing particles to efflux cholesterol from fibroblasts; (5) stimulated steady state apoA-I but not apoA-II mRNA expression; and (6) increased newly transcribed apoA-I mRNA message without effect on apoA-I mRNA half-life. The data indicate that estradiol stimulates newly transcribed hepatic apoA-I mRNA, resulting in a selective increase in LP A-I, a subfraction of HDL that is associated with decreased atherosclerotic cardiovascular disease, especially in premenopausal women.

Gemfibrozil stimulates apolipoprotein A-I synthesis and secretion by stabilization of mRNA transcripts in human hepatoblastoma cell line (Hep G2)

Gemfibrozil is a widely used drug that elevates plasma HDL and lowers triglycerides and LDL. The mechanism of action of this pharmacological agent on HDL metabolism is not established. Since the liver is the major organ involved in HDL production and removal, we assessed the effect of gemfibrozil on the modulation of apoA-I (a major protein of HDL)-containing particles by a human hepatoblastoma cell line (Hep G2). Incubation of Hep G2 cells with gemfibrozil resulted in the following statistically significant findings: (1) increased accumulation of apoA-I in the medium without affecting uptake of radiolabeled HDL-protein or HDL-apoA-I; (2) accelerated incorporation of [3H]leucine and [35S]methionine into apoA-I; (3) equivalent increases in [3H]leucine incorporation into HDL particles without and with apoA-II (LpA-I and LpA-I+A-II, respectively); (4) equal efflux of fibroblast cholesterol by harvested LpA-I and LpA-I+A-II particles; (5) increased steady state apoA-I mRNA without affecting apoA-I transcription; and (6) increased apoA-I mRNA half-life (2.2-fold). These data indicate that gemfibrozil stabilizes apoA-I mRNA transcripts, resulting in increased translation of functional apoA-I-containing particles capable of effluxing cellular cholesterol, thus defining a major mechanism by which gemfibrozil increases HDL.

Characterization of subspecies of apolipoprotein A-I-containing lipoprotein in homozygotes for familial lecithin:cholesterol acyltransferase deficiency

We characterized the two species of lipoproteins containing apolipoprotein A-I (apoA-I), one containing only apoA-I (LpA-I) and the other containing apoA-I and apoA-II (LpA-I/A-II), in four homozygotes for familial lecithin: cholesterol acyltransferase (LCAT) deficiency. Two homozygotes lacked both LCAT mass and activity, whereas the other two had some residual LCAT mass and activity. In these patients, the amount of all apoA-I-containing lipoproteins was one fourth that of normal control subjects, and > 60% was LpA-I. The chemical composition of both LpA-I and LpA-I/A-II is characterized by markedly decreased ratios of neutral to polar lipids compared with those of normals and the sizes of LpA-I and LpA-I/A-II particles are shifted to smaller and larger diameter ranges when compared with those of normal particles. Changes in particle diameter are also reflected in slower electrophoretic mobilities of both LpA-I and LpA-I/A-II particles. All of these abnormalities were more evident in the two homozygotes who lacked LCAT activity. Incubation of LCAT-deficient plasma with LCAT markedly corrected the chemical and physical abnormalities in both LpA-I and LpA-I/A-II particles. These data, taken together, emphasize the importance of LCAT in modifying the chemical composition, size, and shape of LpA-I and LpA-I/A-II particles.

Distribution of lecithin-cholesterol acyltransferase in normolipidemic and dyslipidemic plasma

Plasma lecithin-cholesterol acyltransferase levels and cholesterol esterification rates have been reported to be different between normolipidemic and dyslipidemic subjects. Since apolipoprotein A-I is the presumed primary physiological activator of lecithin-cholesterol acyltransferase, the distribution of the enzyme among A-I-containing lipoprotein particles and A-I-free plasma in normolipidemic and dyslipidemic subjects was examined. A-I-containing lipoprotein particles with and without apolipoprotein A-II were isolated from plasma by immunoaffinity chromatography, and the lecithin-cholesterol acyltransferase mass in these particles and in the A-I-free plasma was quantified by radioimmunoassay. The plasma lecithin-cholesterol acyltransferase concentration was comparable between normolipidemic men (5.9 +/- 1.1 micrograms/ml, n = 15) and women (5.8 +/- 1.1 micrograms/ml, n = 19), with 71 +/- 8% located in particles without apolipoprotein A-II, 17.6 +/- 6% in particles containing A-II, and 12 +/- 6% in the A-I-free plasma. In patients with elevated cholesterol (n = 12), triglyceride (n = 10), and with renal failure (n = 15) plasma levels of the enzyme were significantly higher (6.7 +/- 1.2, 6.9 +/- 1.3, and 6.6 +/- 1.3 micrograms/ml, respectively) (P < 0.05). In all three patient groups, a higher proportion of the enzyme (27 +/- 12%, 33 +/- 12%, and 19 +/- 9%) was not apo A-I associated. This phenomenon was also observed in plasma samples after incubation at 37 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of pravastatin on apolipoprotein-specific high density lipoprotein subpopulations and low density lipoprotein subclass phenotypes in patients with primary hypercholesterolemia

The HMG-CoA reductase inhibitor class of cholesterol-lowering agents reduces very low density lipoproteins (VLDL) and low density lipoproteins (LDL) and slightly increases high density lipoproteins (HDL). However, the effects of these agents on subclasses within the LDL and HDL fractions are not well understood. We have employed an HMG-CoA reductase inhibitor, pravastatin, to determine if LDL subclass phenotypes, as determined by gradient gel electrophoresis, and HDL particles containing both apolipoprotein (apo) A-I and A-II, Lp(AI w AII), and those containing apo A-I but not A-II, Lp(AI w/o AII) are affected by pravastatin (10 mg daily). Twenty-four subjects with LDL-cholesterol (LDL-C) > 160 mg/dl, triglyceride (TG) < 350 mg/dl and no recent myocardial infarction or secondary causes of hypercholesterolemia were enrolled. Compared with an age- and sex-matched normolipidemic reference group (controls), the hypercholesterolemic subjects had reduced levels of Lp(AI w/o AII) and increased levels of Lp(AI w AII) at baseline. In addition, both of their HDL subpopulations had significantly more small (7.0-8.2 nm) particles (P < 0.02 and 0.0001) but significantly fewer large (9.2-11.2 nm) particles (P < 0.002 and 0.0001). Pravastatin induced statistically significant (P < 0.001) reductions in plasma total C (15%), LDL-C (18%), and apo B (16%). While apo A-I and A-II levels increased 5% (P < 0.001) and 6% (P < 0.05), respectively, concentration, composition, and size abnormalities in Lp(AI w AII) and Lp(AI w/o AII) persisted. Lp(a), apo E and cholesteryl ester transfer protein (CETP) levels also did not change. Although changes in LDL subclass phenotypes were observed, all changes involved the intermediate phenotype, and no significant changes in LDL peak particle diameter were seen in either group. Interrelationships between CETP, LDL subclass phenotypes and HDL subpopulations were also seen.

CONCLUSIONS

Although pravastatin decreased plasma apo B and LDL lipid concentrations, no major changes were seen in LDL subclass phenotypes or HDL subpopulations even in the presence of abnormalities associated with arteriosclerosis. Similarly, CETP, which is believed to play a role in HDL and LDL particle size distribution, did not change with pravastatin treatment. Further research is needed to determine the pathophysiological basis of abnormal HDL and LDL subclasses in hypercholesterolemia and explore methods of rectifying the abnormalities.

Lipoprotein heterogeneity in end-stage renal disease

Fifteen patients on chronic maintenance hemodialysis without any additional known cause for dyslipidemia were arbitrarily divided into two groups based on fasting plasma triglyceride levels. The hypertriglyceridemic patients (plasma triglyceride levels above 170 mg/dl, N = 7) also had decreased high density lipoprotein (HDL) cholesterol levels and decreased post-heparin plasma lipoprotein lipase activity compared to the normotriglyceridemic patients (N = 8). All lipoprotein fractions collected by density gradient ultracentrifugation were triglyceride-enriched in the hypertriglyceridemic patients. Both groups of patients had elevated intermediate density lipoprotein levels, heterogeneity in the distribution of low density lipoproteins (LDL) and apoprotein-specific HDL subpopulations, and abnormalities in the size and composition of both LDL and HDL. The described alterations tended to be more marked in hypertriglyceridemic patients and are not detected by the usual laboratory evaluation of lipoproteins. These lipoprotein abnormalities have been shown to be atherogenic in patients without renal disease and are likely to contribute to the high prevalence of premature atherosclerosis in end-stage renal disease.

Interaction between high-density lipoprotein subpopulations in apo B-free and abetalipoproteinemic plasma

Two populations of high-density lipoprotein (HDL) particles exist in human plasma. Both contain apolipoprotein (apo) A-I, but only one contains apo A-II: Lp(AI w AII) and Lp(AI w/o AII). To study the extent of interaction between these particles, apo B-free plasma prepared by the selective removal of apo B-containing lipoproteins (LpB) from the plasma of three normolipidemic (NL) subjects and whole plasma from two patients with abetalipoproteinemia (ABL) were incubated at 37 degrees C for 24 h. Apo B-free plasma samples were used to avoid lipid-exchange between HDL and LpB. Lp(AI w AII) and Lp(AI w/o AII) were isolated from each apo B-free plasma sample before and after incubation and their protein and lipid contents quantified. Before incubation, ABL plasma had reduced levels of Lp(AI w AII) and Lp(AI w/o AII), (40% and 70% of normals, respectively). Compared to the HDL of apo B-free NL plasma, ABL HDL had higher relative contents of free cholesterol, phospholipid and total lipid, and contained more particles with apparent hydrated Stokes diameter in the 9.2-17.0 nm region. These differences were particularly pronounced in particles without apo A-II. Despite their differences, the total cholesterol contents of Lp(AI w AII) increased, while that of Lp(AI w/o AII) decreased in all five plasma samples and the amount of apo A-I in Lp(AI w AII) increased by 6-8 mg/dl in four during the incubation. These compositional changes were accompanied by a relative reduction of particles in the 7.0-8.2 nm Stokes diameter size region and an increase of particles in the 9.2-11.2 nm region. These data are consistent with intravascular modulation between HDL particles with and without apo A-II. The observed increase in apo A-II-associated cholesterol and apo A-I, could involve either the transfer of cholesterol and apo A-I from particles without apo A-II to those with A-II, or the transfer of apo A-II from Lp(AI w AII) to Lp(AI w/o AII). The exact mechanism and direction of the transfer remain to be determined.

Isolation and partial characterization of lipoprotein A-II (LP-A-II) particles of human plasma

High density lipoproteins (HDL) consist of a mixture of chemically and functionally distinct families of particles defined by their characteristic apolipoprotein (Apo) composition. The two major lipoprotein families are lipoprotein A-I (LP-A-I) and lipoprotein A-I:A-II (LP-A-I:A-II). This study describes the isolation of a third minor HDL family of particles referred to as lipoprotein A-II (LP-A-II) because it lacks ApoA-I and contains ApoA-II as its main or sole apolipoprotein constituent. Because ApoA-II is an integral protein constituent of three distinct lipoprotein families (LP-A-I:A-II, LP-A-II: B:C:D:E and LP-A-II), LP-A-II particles were isolated from whole plasma by sequential immunoaffinity chromatography on immunosorbers with antisera to ApoA-II, ApoB and ApoA-I, respectively. In normolipidemic subjects, the concentration of LP-A-II particles, based on ApoA-II content, is 4-18 mg/dl accounting for 5-20% of the total ApoA-II not associated with ApoB-containing lipoproteins. The lipid composition of LP-A-II particles is characterized by low percentage of triglycerides and cholesterol esters and a high percentage of phospholipids in comparison with lipid composition of LP-A-I and LP-A-II: A-II. The major part of LP-A-II particles contain ApoA-II as the sole apolipoprotein constituent; however, small subsets of LP-A-II particles may also contain ApoD and other minor apolipoproteins. The lipid/protein ratio of LP-A-II is higher than those of LP-A-I and LP-A-I:A-II. In homozygous ApoA-I and ApoA-I/ApoC-III deficiencies, LP-A-II particles are the only ApoA-containing high density lipoprotein with levels found to be within the same range (7-13 mg/dl) as those of normolipidemic subjects. However, in contrast to normal LP-A-II, their lipid composition is characterized by higher percentages of triglycerides and cholesterol esters and a lower percentage of phospholipids and their apolipoprotein composition by the presence of ApoC-peptides and ApoE in addition to ApoA-II and ApoD. These results show that LP-A-II particles are a minor HDL family and suggest that, in the absence of ApoA-I-containing lipoproteins, they become an efficient acceptor/donor of ApoC-peptides and ApoE required for a normal metabolism of triglyceride-rich lipoproteins. Their other possible functional roles in lipid transport remain to be established in future experiments.