The conformation of apolipoprotein A-I in high-density lipoproteins is influenced by core lipid composition and particle size: a surface plasmon resonance study

Plasma high-density lipoproteins (HDL) are a heterogeneous group of particles that vary in size as well as lipid and apoprotein composition. The effect of HDL core lipid composition and particle size on apolipoprotein (apo) A-I structure was studied using surface plasmon resonance (SPR) analysis of the binding of epitope-defined monoclonal antibodies. The association and dissociation rate constants of 12 unique apo A-I-specific monoclonal antibodies for isolated plasma HDL were calculated. In addition, the association rate constants of the antibodies were determined for homogeneous preparations of spherical reconstituted HDL (rHDL) that contained apo A-I as the sole apolipoprotein and differed either in their size or in their core lipid composition. This analysis showed that lipoprotein size affected the conformation of domains dispersed throughout the apo A-I molecule, but the conformation of the central domain between residues 121 and 165 was most consistently modified. In contrast, replacement of core cholesteryl esters with triglyceride in small HDL modified almost the entire molecule, with only two key N-terminal domains of apo A-I being unaffected. This finding suggested that the central and C-terminal domains of apo A-I are in direct contact with rHDL core lipids. This immunochemical analysis has provided valuable insight into how core lipid composition and particle size affect the structure of specific domains of apo A-I on HDL.     

Modulation of substrate selectivity in plasma lipid transfer protein reaction over structural variation of lipid particle

The modulation of substrate selectivity of human plasma LTP reaction is the subject of the present investigation. The moderate selectivity by a factor of 5 to 6 was observed in the LTP-catalyzed transfer of cholesteryl ester over triacylglycerol between plasma lipoproteins. On the other hand, the transfer of cholesteryl ester by LTP was highly selective over the negligible transfer of triacylglycerol, by a factor of 60 to 500, between the microemulsions with LDL size, regardless of the activators such as human and pig apolipoprotein (apo) A-I, human apo C-III and apo E that bound to the surface of the emulsion in equilibrium. The presence of free cholesterol in these microemulsions reduced slightly the rate of cholesteryl ester transfer but had no effect on triacylglycerol transfer. Other surface-active reagents such as cholic acid, Triton X-100 and Tween-20, did not have an effect on the triacylglycerol transfer either. Triacylglycerol transfer by LTP became measurable between such lipid particles as prepared by co-sonication of lipid with pig apo A-I and isolated as the mixed-microemulsions in the density of LDL and HDL. In these conditions, the substrate selectivity for cholesteryl ester over triacylglycerol was a factor of 6 to 16 mimicking the ratio in plasma lipoproteins. The conformation of pig apo A-I estimated by circular dichroism showed that its apparent helical content was further more induced when apo A-I was integrated into the mixed-microemulsion by co-sonication than the lipid-bound apo A-I in equilibrium. Apo A-I, thus integrated into lipid particles, was highly resistant to the denaturation by guanidine hydrochloride while the lipid-bound apo A-I in equilibrium was denatured as readily as the lipid-free protein. Thus, triacylglycerol transfer by LTP was induced by structural modulation of substrate-carrying lipid particles such as higher integration of apolipoproteins.     

Apolipoprotein A-II inhibits high density lipoprotein remodeling and lipid-poor apolipoprotein A-I formation

The high density lipoproteins (HDL) in human plasma are classified on the basis of apolipoprotein composition into those containing apolipoprotein (apo) A-I but not apoA-II, (A-I)HDL, and those containing both apoA-I and apoA-II, (A-I/A-II)HDL. Cholesteryl ester transfer protein (CETP) transfers core lipids between HDL and other lipoproteins. It also remodels (A-I)HDL into large and small particles in a process that generates lipid-poor, pre-beta-migrating apoA-I. Lipid-poor apoA-I is the initial acceptor of cellular cholesterol and phospholipids in reverse cholesterol transport. The aim of this study is to determine whether lipid-poor apoA-I is also formed when (A-I/A-II)rHDL are remodeled by CETP. Spherical reconstituted HDL that were identical in size had comparable lipid/apolipoprotein ratios and either contained apoA-I only, (A-I)rHDL, or (A-I/A-II)rHDL were incubated for 0-24 h with CETP and Intralipid(R). At 6 h, the apoA-I content of the (A-I)rHDL had decreased by 25% and there was a concomitant formation of lipid-poor apoA-I. By 24 h, all of the (A-I)rHDL were remodeled into large and small particles. CETP remodeled approximately 32% (A-I/A-II)rHDL into small but not large particles. Lipid-poor apoA-I did not dissociate from the (A-I/A-II)rHDL. The reasons for these differences were investigated. The binding of monoclonal antibodies to three epitopes in the C-terminal domain of apoA-I was decreased in (A-I/A-II)rHDL compared with (A-I)rHDL. When the (A-I/A-II)rHDL were incubated with Gdn-HCl at pH 8.0, the apoA-I unfolded by 15% compared with 100% for the apoA-I in (A-I)rHDL. When these incubations were repeated at pH 4.0 and 2.0, the apoA-I in the (A-I)rHDL and the (A-I/A-II)rHDL unfolded completely. These results are consistent with salt bridges between apoA-II and the C-terminal domain of apoA-I, enhancing the stability of apoA-I in (A-I/A-II)rHDL and possibly contributing to the reduced remodeling and absence of lipid poor apoA-I in the (A-I/A-II)rHDL incubations.     

Effect in vitro of apolipoprotein A-V on the structure and functions of recombinant high density lipoprotein

The neighboring position of apolipoprotein A-I (apoA-I) and apolipoprotein A-V (apoA-V) gene and the modulation of apoA-V on the concentrations, size and maturation of high density lipoprotein (HDL) may indicate a special relationship between apoA-V and HDL. To assess the effects of apoA-V on HDL structure and related functions in vitro, a series of recombinant HDL (rHDL) were synthesized in vitro with various mass ratios of recombinant apoA-I: apoA-V. An increase in apoA-V in rHDL resulted in enhanced lipid-binding ability, increased phospholipid content and larger particle size. Furthermore, the lipid-free and lipid-bound apoA-V in rHDL showed antioxidant capacity against low density lipoprotein (LDL) in vitro. In THP-1 derived macrophages, apoA-V of rHDL was shown to have no influence on the uptake of oxidized LDL (oxLDL) and intracellular lipid accumulation. Thus, the addition of apoA-V to rHDL resulted in changes in several rHDL properties, including increased lipid-binding ability, phospholipid content, particle size and antioxidant capacity. These alterations may explain the modulation of apoA-V on HDL in vivo and the beneficial functions of apoA-V on atherosclerosis.     

Formation of spherical, reconstituted high density lipoproteins containing both apolipoproteins A-I and A-II is mediated by lecithin:cholesterol acyltransferase

Previous studies have provided detailed information on the formation of spherical high density lipoproteins (HDL) containing apolipoprotein (apo) A-I but no apoA-II (A-I HDL) by an lecithin:cholesterol acyltransferase (LCAT)-mediated process. In this study we have investigated the formation of spherical HDL containing both apoA-I and apoA-II (A-I/A-II HDL). Incubations were carried out containing discoidal A-I reconstituted HDL (rHDL), discoidal A-II rHDL, and low density lipoproteins in the absence or presence of LCAT. After the incubation, the rHDL were reisolated and subjected to immunoaffinity chromatography to determine whether A-I/A-II rHDL were formed. In the absence of LCAT, the majority of the rHDL remained as either A-I rHDL or A-II rHDL, with only a small amount of A-I/A-II rHDL present. By contrast, when LCAT was present, a substantial proportion of the reisolated rHDL were A-I/A-II rHDL. The identity of the particles was confirmed using apoA-I rocket electrophoresis. The formation of the A-I/A-II rHDL was influenced by the relative concentrations of the precursor discoidal A-I and A-II rHDL. The A-I/A-II rHDL included several populations of HDL-sized particles; the predominant population having a Stokes' diameter of 9.9 nm. The particles were spherical in shape and had an electrophoretic mobility slightly slower than that of the alpha-migrating HDL in human plasma. The apoA-I:apoA-II molar ratio of the A-I/A-II rHDL was 0.7:1. Their major lipid constituents were phospholipids, unesterified cholesterol, and cholesteryl esters. The results presented are consistent with LCAT promoting fusion of the A-I rHDL and A-II rHDL to form spherical A-I/A-II rHDL. We suggest that this process may be an important source of A-I/A-II HDL in human plasma.     

The use of chemical cross-linking and mass spectrometry to elucidate the tertiary conformation of lipid-bound apolipoprotein A-I

PURPOSE OF REVIEW: The purpose of this review is to highlight recent advances in mass spectrometry and its use for identifying the lipid-bound conformation of apolipoprotein A-I. Given the current interest in understanding the structure of HDL apolipoprotein A-I, this approach seems ideal in assessing its dual role as mediator of lipid efflux and modulator of cellular inflammation. RECENT FINDINGS: A large number of different technical approaches have been employed over the past 25 years in attempts to solve the lipid-bound conformation of apolipoprotein A-I. Since the X-ray crystal structure of lipid-free Delta43 apolipoprotein A-I was reported in 1997, a 'double belt' model describing lipid-bound apolipoprotein A-I conformation for recombinant HDL has prevailed. Recent studies have focused on determining the exact helix-helix registry and salt-bridging partners found on a two apolipoprotein A-I molecule disc as well as on spherical HDL particles. Investigations are all aimed at defining the conformation of lipid-bound apolipoprotein A-I which may provide an explanation for how specific domains of apolipoprotein A-I interact with important HDL-modifying proteins that ultimately determine the apolipoprotein's fate in circulation. SUMMARY: Recent advances in mass spectrometric sequencing of cross-linked peptides provide an excellent tool to help define protein tertiary structure. This approach has provided refined structural information on apolipoprotein A-I folding which had eluded all previous approaches.     

Oxidized-phospholipids in reconstituted high density lipoprotein particles affect structure and function of recombinant paraoxonase 1

Paraoxonase 1 (PON1) is an HDL-associated enzyme and exhibits anti-inflammatory, anti-diabetic, and anti-atherogenic properties. Association of PON1 to HDL particles increases the stability and activity of PON1 and is important for the normal functioning of the enzyme. HDL particles are made up of lipid and protein constituents and apolipoprotein A-I (apoA-I) is a principal protein constituent of HDL that facilitates various biological activities of HDL. In many disease conditions the oxidized phospholipid (Ox-PL) content of HDL is found to be increased and an inverse correlation between the activity of PON1 and oxidation of the HDL is observed. However, the molecular details of the inhibitory action of the Ox-PL-containing HDL on the function of PON1 are not clear yet. In this study we have assembled reconstituted HDL (rHDL) particles with and without Ox-PL and compared their effect on the structure and function of ¹³C-labeled recombinant PON1 (¹³C-rPON1) by employing attenuated total reflectance Fourier transformed infrared (ATR-FTIR) spectroscopy and enzymatic assay. Our results show that the presence of the Ox-PL in the rHDL particles alters the structure of rPON1 and decreases its lactonase activity.     

Remodeling of apolipoprotein E-containing spherical reconstituted high density lipoproteins by phospholipid transfer protein

Phospholipid transfer protein (PLTP) transfers phospholipids between HDL and other lipoproteins in plasma. It also remodels spherical, apolipoprotein A-I (apoA-I)-containing HDL into large and small particles in a process involving the dissociation of lipid-free/lipid-poor apoA-I. ApoE is another apolipoprotein that is mostly associated with large, spherical HDL that do not contain apoA-I. Three isoforms of apoE have been identified in human plasma: apoE2, apoE3, and apoE4. This study investigates the remodeling of spherical apoE-containing HDL by PLTP and the ability of PLTP to transfer phospholipids between apoE-containing HDL and phospholipid vesicles. Spherical reconstituted high density lipoproteins (rHDL) containing apoA-I [(A-I)rHDL], apoE2 [(E2)rHDL], apoE3 [(E3)rHDL], or apoE4 [(E4)rHDL] as the sole apolipoprotein were prepared by incubating discoidal rHDL with low density lipoproteins and lecithin:cholesterol acyltransferase. PLTP remodeled the spherical, apoE-containing rHDL into large and small particles without the dissociation of apoE. The PLTP-mediated remodeling of apoE-containing rHDL was more extensive than that of (A-I)rHDL. PLTP transferred phospholipids from small unilamellar vesicles to apoE-containing rHDL in an isoform-dependent manner, but at a rate slower than that for spherical (A-I)rHDL. It is concluded that apoE enhances the capacity of PLTP to remodel HDL but reduces the ability of HDL to participate in PLTP-mediated phospholipid transfers.     

A key point mutation (V156E) affects the structure and functions of human apolipoprotein A-I

A naturally occurring point mutant of human apolipoprotein A-I (apoA-I), V156E, which is associated with extremely low plasma apoA-I and high density lipoprotein (HDL) levels, and coronary artery disease (Huang, W., Sasaki, J., Matsunaga, A., Nanimatsu, H., Moriyama, K., Han, H. Kugi, M., Koga, T., Yamaguchi, K., and Arakawa, K. (1998) Arterioscler. Throm. Vasc. Biol. 18, 389-396), was produced in an Escherichia coli expression system. The purified recombinant proapoA-I V156E mutant was examined in its structural and functional properties, both, in the lipid-free and lipid-bound states. In the lipid-free form the mutant protein exhibited small changes in conformation, but was more stable, and quite resistant to self-association, compared with control apoA-I. The V156E mutant was able to interact with phospholipid (PL) at high PL:protein ratios (95:1, mol/mol), but was inefficient in forming reconstituted HDL (rHDL) complexes at lower PL:protein ratios (40:1). In the lipid-bound, rHDL state, the mutant protein was somewhat more alpha-helical and formed a larger complex (110 A) than control apoA-I (97 A). Furthermore, the rHDL particles containing the V156E mutant did not rearrange to smaller particles in the presence of low density lipoproteins, and had minimal reactivity with lecithin-cholesterol acyltransferase (LCAT), compared with rHDL particles made with control apoA-I. These results suggest a key role for Val-156, or the adjacent central region of apoA-I in the modulation of apoA-I conformation, stability, and self-association in solution, and in the formation of small HDL, the conformational adaptability of apoA-I leading to structural rearrangements of HDL, and the activation of LCAT.     

A proteolytic method for distinguishing between lipid-free and lipid-bound apolipoprotein A-I

Recent studies indicate that certain lipid-poor forms of apolipoprotein (apo)A-I may be particularly important in promoting cholesterol release from overburdened cells in the periphery. However, a detailed understanding of the physiological relevance of these species has been hampered by the difficulty in measuring them. As part of a search for a rapid assay for these forms of apoA-I, we have observed that the protease enteropeptidase can specifically cleave human lipid-free apoA-I but not its lipid-bound form. Enteropeptidase cleaved lipid-free apoA-I at a single site at amino acid 188, resulting in an N-terminal fragment of 22 kDa. However, apoA-I was not susceptible to enteropeptidase when present in reconstituted high-density lipoprotein (rHDL) particles as small as 6 nm in diameter or in human HDL(3) particles, even at extremely high enzyme-to-protein ratios and extended reaction times. We capitalized on this observation to develop an assay for the measurement of lipid-poor apoA-I in in vitro systems. Densitometry was used to generate a standard curve from sodium dodecyl sulfate polyacrylamide gels to determine the amounts of the N-terminal proteolytic fragment in unknown samples treated with enteropeptidase. This system could accurately quantify apoA-I that had been displaced from rHDL particles and human HDL(3) with purified apoA-II. On the basis of the results, a system of nomenclature is proposed for "lipid-free," "lipid-poor," and "lipid bound" apoA-I.The reported method distinguishes forms of apoA-I by a conformational parameter without previous separation of the species. This simple and inexpensive method will be useful for understanding the characteristics of plasma HDL that are favorable for the dissociation of apoA-I.     

Structure of apolipoprotein A-I in three homogeneous, reconstituted high density lipoprotein particles

To elucidate further the conformation of human apolipoprotein A-I (apoA-I) in lipid-bound states and its effect on the reaction with lecithin cholesterol acyltransferase (LCAT), we prepared reconstituted HDL (rHDL) particles from a reaction mixture containing dipalmitoylphosphatidylcholine/cholesterol/apoA-I in the molar ratios of 150:7.5:1. The particles were separated by gel filtration into three classes of highly homogeneous and reproducible discs with diameters of 97, 136, and 186 A, containing 2, 3, and 4 molecules of apoA-I/disc, respectively, and increasing proportions of phospholipid and cholesterol. These three classes of particles were then investigated by a variety of fluorescence techniques, to probe the average environment and mobility of the tryptophan (Trp) residues in the structure of apoA-I. We found small, gradual changes in the fluorescence parameters with changes in the size of the rHDL, consistent with a shift of Trp residues to a more hydrophobic and more rigid environment, as well as an increased resistance of apoA-I to denaturation by guanidine hydrochloride in the larger particles. In contrast, circular dichroism measurements and binding studies with seven monoclonal antibodies indicated a similar alpha-helical structure (73%) for apoA-I in all the particles, and similar exposure of apoA-I epitopes in the COOH-terminal two-thirds of the apolipoprotein. Thus the structure of apoA-I is comparable for the three classes of particles and is consistent with the presence of eight alpha-helical segments per apoA-I in contact with the lipid. In addition, we obtained the apparent kinetic parameters for the reaction of the rHDL particles with lecithin cholesterol acyltransferase. The apparent Km values were similar but the apparent Vmax decreased almost 8-fold, going from the 97- to the 186-A particles; therefore, the decreasing reactivity for the larger particles can be attributed mainly to differences in the catalytic rate constant. The rate limiting step is probably affected by local structural differences in the apoA-I, or by the interfacial properties of the lipid.     

Cholesterol mobilization by free and lipid-bound apoAI(Milano) and apoAI(Milano)-apoAII heterodimers

Despite very low plasma levels of HDL, carriers of the apolipoprotein AI Arg173 --> Cys mutation apoAI(Milano) (AIM) have no apparent increase in risk for atherosclerotic vascular disease. HDL apolipoprotein species in AIM carriers include apoAI-AII heterodimers, previously found to confer the enhanced ability of tyrosyl radical-oxidized HDL to mobilize cholesterol for removal from cultured cells. To determine whether enhanced mobilization of cholesterol by apoprotein species in AIM explains a cardioprotective action of this mutation, we examined the ability of lipid-free and lipid-bound AIM and AIM-AII heterodimers to deplete cholesterol from cultured cells. Free AIM and AIM-AII heterodimers showed a decreased capacity to act as acceptors of cholesterol from cholesterol-loaded human fibroblasts compared with native apoAI but similar capacities to deplete fibroblasts of the pool of cholesterol available for esterification by acyl-CoA:cholesterol acyltransferase (ACAT). Discoidal reconstituted HDL (rHDL) containing apoAI depleted both of these cholesterol pools more readily than AIM-containing rHDL when compared at equivalent rHDL protein levels, but similar abilities of these rHDL to deplete cell cholesterol were seen when compared at equivalent phospholipid levels. Spherical rHDL generated using the whole lipid fraction of HDL and apoAI or AIM showed similar capacities to deplete total and ACAT-accessible cell cholesterol when compared at similar protein levels, but an increased capacity of AIM-containing particles was seen when compared at equivalent phospholipid levels. Unlike the apoAI-AII heterodimer in tyrosylated HDL, AIM-AII heterodimer-containing spherical rHDL showed no increased capacity to deplete either of these pools of cholesterol. These results suggest a similar or better capacity of native apoAI in lipid-free or lipid-bound form in discoidal rHDL to enhance the mobilization of cellular cholesterol when compared to AIM in its free or lipid-bound forms. Any increase in depletion of cellular cholesterol by lipid-bound AIM in spherical rHDL appears related to altered phospholipid-binding rather than intrinsic cholesterol-mobilizing characteristics of this protein compared to native apoAI. The lack of major differences in these studies in cholesterol mobilization by native apoAI and AIM, or by apoAIM-AII heterodimers, suggests that any protection against atherosclerosis conferred by this mutation is likely related to other beneficial vascular effects of AIM.     

Regulation of reconstituted high density lipoprotein structure and remodeling by apolipoprotein E

Apolipoprotein E (apoE) enters the plasma as a component of discoidal HDL and is subsequently incorporated into spherical HDL, most of which contain apoE as the sole apolipoprotein. This study investigates the regulation, origins, and structure of spherical, apoE-containing HDLs and their remodeling by cholesteryl ester transfer protein (CETP). When the ability of discoidal reconstituted high density lipoprotein (rHDL) containing apoE2 [(E2)rHDL], apoE3 [(E3)rHDL], or apoE4 [(E4)rHDL] as the sole apolipoprotein to act as substrates for LCAT were compared with that of discoidal rHDL containing apoA-I [(A-I)rHDL], the rate of cholesterol esterification was (A-I)rHDL > (E2)rHDL approximately (E3)rHDL > (E4)rHDL. LCAT also had a higher affinity for discoidal (A-I)rHDL than for the apoE-containing rHDL. When the discoidal rHDLs were incubated with LCAT and LDL, the resulting spherical (E2)rHDL, (E3)rHDL, and (E4)rHDL were larger than, and structurally distinct from, spherical (A-I)rHDL. Incubation of the apoE-containing spherical rHDL with CETP and Intralipid(R) generated large fusion products without the dissociation of apoE, whereas the spherical (A-I)rHDLs were remodeled into small particles with the formation of lipid-poor apoA-I. In conclusion, i) apoE activates LCAT less efficiently than apoA-I; ii) apoE-containing spherical rHDLs are structurally distinct from spherical (A-I)rHDL; and iii) the CETP-mediated remodeling of apoE-containing spherical rHDL differs from that of spherical (A-I)rHDL.     

The charge and structural stability of apolipoprotein A-I in discoidal and spherical recombinant high density lipoprotein particles.

The details of how high density lipoprotein (HDL) microstructure affects the conformation and net charge of apolipoprotein (apo) A-I in various classes of HDL particles have been investigated in homogeneous recombinant HDL (rHDL) particles containing apoA-I, palmitoyl-oleoyl phosphatidylcholine (POPC) and cholesteryl oleate. Isothermal denaturation with guanidine HCl was used to monitor alpha-helix structural stability, whereas electrokinetic analyses and circular dichroism were used to determine particle charge and apoA-I secondary structure, respectively. Electrokinetic analyses show that at pH 8.6 apoA-I has a net negative charge on discoidal (POPC.apoA-I) particles (-5.2 electronic units/mol of apoA-I) which is significantly greater than that of apoA-I either free in solution or on spherical (POPC.cholesteryl oleate.apoA-I) rHDL (approximately -3.5 electronic units). Raising the POPC content (32-128 mol/ml of apoA-I) of discoidal particles 1) increases the particle major diameter from 9.3 to 12.1 nm, 2) increases the alpha-helix content from 62 to 77%, and 3) stabilizes the helical segments by increasing the free energy of unfolding (delta GD degree) from 1.4 to 3.0 kcal/mol of apoA-I. Raising the POPC content (28-58 mol/mol of apoA-I) of spherical particles 1) increases the particle diameter from 7.4 to 12.6 nm, 2) increases the percent alpha-helix from 62 to 69%, and 3) has no significant effect on delta GD degree (2.2 kcal/mol of apoA-I). This study shows that different HDL subspecies maintain particular apoA-I conformations that confer unique charge and structural characteristics on the particles. It is likely that the charge and conformation of apoA-I are critical molecular properties that modulate the metabolism of HDL particles and influence their role in cholesterol transport.     

Oxidized phospholipid content destabilizes the structure of reconstituted high density lipoprotein particles and changes their function

High density lipoprotein (HDL) particles are made up of lipid and protein constituents and apolipoprotein A-I (apoA-I) is a principal protein component that facilitates various biological activities of HDL particles. Increase in Ox-PL content of HDL particles makes them 'dysfunctional' and such modified HDL particles not only lose their athero-protective properties but also acquire pro-atherogenic and pro-inflammatory functions. The details of Ox-PL-induced alteration in the molecular properties of HDL particles are not clear. Paraoxonase 1 (PON1) is an HDL-associated enzyme that possesses anti-inflammatory and anti-atherogenic properties; and many of the athero-protective functions of HDL are attributed to the associated PON1. In this study we have characterized the physicochemical properties of reconstituted HDL (rHDL) particles containing varying amounts of Ox-PL and have compared their PON1 stimulation capacity. Our results show that increased Ox-PL content (a) modifies the physicochemical properties of the lipid domain of the rHDL particles, (b) decreases the stability and alters the conformation as well as orientation of apoA-I molecules on the rHDL particles, and (c) decreases the PON1 stimulation capacity of the rHDL particles. Our data indicate that the presence of Ox-PLs destabilizes the structure of the HDL particles and modifies their function.     

Immune-regulation of the apolipoprotein A-I/C-III/A-IV gene cluster in experimental inflammation

Apolipoprotein A-IV is a member of the apo A-I/C-III/A-IV gene cluster. In order to investigate its hypothetical coordinated regulation, an acute phase was induced in pigs by turpentine oil injection. The hepatic expression of the gene cluster as well as the plasma levels of apolipoproteins were monitored at different time periods. Furthermore, the involvement of the inflammatory mediators' interleukins 1 and 6 and tumor necrosis factor in the regulation of this gene cluster was tested in cultured pig hepatocytes, incubated with those mediators and apo A-I/C-III/A-IV gene cluster expression at the mRNA level was measured. In response to turpentine oil-induced inflammation, a decreased hepatic apo A-IV mRNA expression was observed (independent of apo A-I and apo C-III mRNA) not correlating with the plasma protein levels. The distribution of plasma apo A-IV experienced a shift from HDL to larger particles. In contrast, the changes in apo A-I and apo C-III mRNA were reflected in their corresponding plasma levels. Addition of cytokines to cultured pig hepatocytes also decreased apo A-IV and apo A-I mRNA levels. All these results show that the down-regulation of apolipoprotein A-I and A-IV messages in the liver may be mediated by interleukin 6 and TNF-alpha. The well-known HDL decrease found in many different acute-phase responses also appears in the pig due to the decreased expression of apolipoprotein A-I and the enlargement of the apolipoprotein A-IV-containing HDL.     

Characterization of reconstituted high-density lipoprotein particles formed by lipid interactions with human serum amyloid A

The acute-phase human protein serum amyloid A (SAA) is enriched in high-density lipoprotein (HDL) in patients with inflammatory diseases. Compared with normal HDL containing apolipoprotein A-I, which is the principal protein component, characteristics of acute-phase HDL containing SAA remain largely undefined. In the present study, we examined the physicochemical properties of reconstituted HDL (rHDL) particles formed by lipid interactions with SAA. Fluorescence and circular dichroism measurements revealed that although SAA was unstructured at physiological temperature, α-helix formation was induced upon binding to phospholipid vesicles. SAA also formed rHDL particles by solubilizing phospholipid vesicles through mechanisms that are common to other exchangeable apolipoproteins. Dynamic light scattering and nondenaturing gradient gel electrophoresis analyses of rHDL after gel filtration revealed particle sizes of approximately 10 nm, and a discoidal shape was verified by transmission electron microscopy. Thermal denaturation experiments indicated that SAA molecules in rHDL retained α-helical conformations at 37 °C, but were almost completely denatured around 60 °C. Furthermore, trypsin digestion experiments showed that lipid binding rendered SAA molecules resistant to protein degradation. In humans, three major SAA1 isoforms (SAA1.1, 1.3, and 1.5) are known. Although these isoforms have different amino acids at residues 52 and 57, no major differences in physicochemical properties between rHDL particles resulting from lipid interactions with SAA isoforms have been found. The present data provide useful insights into the effects of SAA enrichment on the physicochemical properties of HDL.     

Evidence that phospholipids play a key role in pre-beta apoA-I formation and high-density lipoprotein remodeling

The initial plasma acceptor of unesterified cholesterol and phospholipids from peripheral cells has been identified as pre-beta migrating, lipid-free, or lipid-poor apolipoprotein (apo) A-I (pre-beta apoA-I). Pre-beta apoA-I is formed when plasma factors, such as cholesteryl ester transfer protein (CETP), remodel high-density lipoproteins (HDL). The aim of this study is to determine how phospholipids influence pre-beta apoA-I formation during the CETP-mediated remodeling of HDL. Reconstituted HDL (rHDL) containing either 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC), 1-palmitoyl-2-linoleoyl phosphatidylcholine (PLPC), 1-palmitoyl-2-arachidonyl phosphatidylcholine (PAPC), or 1-palmitoyl-2-docosahexanoyl phosphatidylcholine (PDPC) as the only phospholipid were prepared. The rHDL were comparable in size and core lipid/protein molar ratio and contained only cholesteryl esters in their core and apoA-I as the sole apolipoprotein. The (POPC)rHDL, (PLPC)rHDL, (PAPC)rHDL, and (PDPC)rHDL were respectively incubated for 0-24 h with CETP and microemulsions containing triolein and either POPC, PLPC, PAPC, or PDPC. The rate at which the rHDL were depleted of core lipids and remodeled to small particles varied widely with (POPC)rHDL < (PLPC)rHDL < (PDPC)rHDL approximately (PAPC)rHDL. Pre-beta apoA-I was not formed in the (POPC)rHDL incubations. Pre-beta apoA-I was apparent by 24 h in the (PLPC)rHDL incubations and by 12 h in the (PAPC)rHDL and (PDPC)rHDL incubations. The enhanced formation of pre-beta apoA-I in the (PAPC)rHDL and (PDPC)rHDL incubations reflected the increased core lipid depletion of the particles combined with the destabilization and progressive exclusion of apoA-I from the particle surface. In conclusion, these results show that phospholipids play a key role in the CETP-mediated remodeling of rHDL and pre-beta apoA-I formation.     

Mechanism of dissociation of human apolipoprotein A-I from complexes with dimyristoylphosphatidylcholine as studied by guanidine hydrochloride denaturation

The reversibility of the binding of human apolipoprotein A-I (apo A-I) to phospholipid has been monitored through the influence of guanidine hydrochloride (Gdn-HCl) on the isothermal denaturation and renaturation of apo A-1/dimyristoylphosphatidylcholine (DMPC) complexes at 24 degree C. Denaturation was studied by incubating discoidal 1:100 and vesicular 1:500 mol/mol apo A-I/DMPC complexes with up to 7 M Gdn-HCl for up to 72 h. Unfolding of apo A-I molecules was observed from circular dichroism spectra while the distribution of protein between free and lipid-associated states was monitored by density gradient ultracentrifugation. The ability of apo A-I to combine with DMPC in the presence of Gdn-HCl at 24 degrees C was also investigated by similar procedures. In both the denaturation and renaturation of 1:100 and 1:500 complexes, the final values of the molar ellipticity and the ratio of free to bound apo A-I at various concentrations of Gdn-HCl are dependent on the initial state of the lipid and protein; apo A-I is more resistant to denaturation when Gdn-HCl is added to existing complexes than to a mixture of apo A-I and DMPC. There is an intermediate state in the denaturation pathway of apo A-I/DMPC complexes which is not present in the renaturation; the intermediate comprises partially unfold apo A-I molecules still associated with the complex by some of their apolar residues. Complete unfolding of the alpha helix and subsequent desorption of the apo A-I molecules from the lipid/water interface involve cooperative exposure of these apolar residues to the aqueous phase. The energy barrier associated with this desorption step makes the binding of apo A-I to DMPC a thermodynamically irreversible process. Consequently, binding constants of apo A-I and PC cannot be calculated simply from equilibrium thermodynamic treatments of the partitioning of protein between free and bound states. Apo A-I molecules do not exchange freely between the lipid-free and lipid-bound states, and extra work is required to drive protein molecules off the surface. The required increased in surface pressure can be achieved by a net mass transfer of protein to the surface; in vivo, increases in the surface pressure of lipoproteins by lipolysis can cause protein desorption.