Evidence for a complex relationship between apoA-V and apoC-III in patients with severe hypertriglyceridemia

The relevance of apolipoprotein A-V (apoA-V) for human lipid homeostasis is underscored by genetic association studies and the identification of truncation-causing mutations in the APOA5 gene as a cause of type V hyperlipidemia, compatible with an LPL-activating role of apoA-V. An inverse correlation between plasma apoA-V and triglyceride (TG) levels has been surmised from animal data. Recent studies in human subjects using (semi)quantitative immunoassays, however, do not provide unambiguous support for such a relationship. Here, we used a novel, validated ELISA to measure plasma apoA-V levels in patients (n = 28) with hypertriglyceridemia (HTG; 1.8-78.7 mmol TG/l) and normolipidemic controls (n = 42). Unexpectedly, plasma apoA-V levels were markedly increased in the HTG subjects compared with controls (1,987 vs. 258 ng/ml; P < 0.001). In the HTG group, apoA-V and TG were positively correlated (r = +0.44, P = 0.02). In addition, we noted an increased level of the LPL-inhibitory protein apoC-III in the HTG group (45.8 vs. 10.6 mg/dl in controls; P < 0.001). The correlation between apoA-V and TG levels in the HTG group disappeared (partial r = +0.09, P = 0.65) when controlling for apoC-III levels. In contrast, apoC-III and TG remained positively correlated in this group when controlling for apoA-V (partial r = +0.43, P = 0.025). Our findings suggest that in HTG patients, increased TG levels are accompanied by high plasma levels of apoA-V and apoC-III, apolipoproteins with opposite modes of action. This study provides evidence for a complex interaction between apoA-V and apoC-III in patients with severe HTG.     

Altered apolipoprotein A-V expression during the acute phase response is independent of plasma triglyceride levels in mice and humans

Plasma triglyceride (TG) levels are altered during the acute phase response (APR). Plasma levels of the recently discovered apolipoprotein A-V (apoA-V) are inversely associated with plasma TG. The aim of this study was to investigate the change of apoA-V plasma levels and hepatic apoA-V expression during the APR in relation to plasma TG. During human APR plasma apoA-V was decreased as were plasma TG (each P<0.01). Also early in the course of the murine APR plasma apoA-V levels and hepatic apoA-V expression were decreased and changed in the same direction as plasma TG. Treatment of HepG2 cells with TNF-alpha and IL-1beta decreased apoA-V mRNA levels early by 42% and 55%, respectively (each P<0.001). However, in promoter/reporter assays the human apoA-V promoter was unresponsive to proinflammatory cytokines. Instead, we demonstrate that a significant decrease in apoA-V mRNA stability in response to treatment with TNF-alpha and IL-1beta is the underlying basis of decreased apoA-V expression during the APR (P<0.05). These data demonstrate that (i) apoA-V expression decreases early during the APR due to changes in mRNA stability, and (ii) during the APR apoA-V is not inversely related to plasma TG levels in mice and humans, thereby identifying a relevant pathophysiological setting, in which the previously reported close inverse association between these parameters does not hold true.     

Apolipoprotein A-V dependent modulation of plasma triacylglycerol: a puzzlement

The discovery of apolipoprotein A-V (apoA-V) in 2001 has raised a number of intriguing questions about its role in lipid transport and triglyceride (TG) homeostasis. Genome wide association studies (GWAS) have consistently identified APOA5 as a contributor to plasma TG levels. Single nucleotide polymorphisms (SNP) within the APOA5 gene locus have been shown to correlate with elevated plasma TG. Furthermore, transgenic and knockout mouse models support the view that apoA-V plays a critical role in maintenance of plasma TG levels. The present review describes recent concepts pertaining to apoA-V SNP analysis and their association with elevated plasma TG. The interaction of apoA-V with glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1 (GPIHBP1) is discussed relative to its postulated role in TG-rich lipoprotein catabolism. The potential role of intracellular apoA-V in regulation of TG homeostasis, as a function of its ability to associate with cytosolic lipid droplets, is reviewed. While some answers are emerging, numerous mysteries remain with regard to this low abundance, yet potent, modulator of TG homeostasis. Given the strong correlation between elevated plasma TG and heart disease, there is great scientific and public interest in deciphering the numerous biological riddles presented by apoA-V. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.     

Apolipoprotein A-V-heparin interactions: implications for plasma lipoprotein metabolism

Transgenic and gene disruption experiments in mice have revealed that apolipoprotein (apo) A-V is a potent regulator of plasma triglyceride (TG) levels. To investigate the molecular basis of apoA-V function, the ability of isolated recombinant apoA-V to modulate lipoprotein lipase (LPL) activity was examined in vitro. With three distinct lipid substrate particles, including very low-density lipoprotein (VLDL), a TG/phospholipid emulsion, or dimyristoylphosphatidylcholine liposomes, apoA-V had little effect on LPL activity. In the absence or presence apolipoprotein C-II, apoA-V marginally inhibited LPL activity. On the other hand, apoA-V-dimyristoylphosphatidylcholine disc particles bound to heparin-Sepharose and were specifically eluted upon application of a linear gradient of NaCl. The interaction of apoA-V with sulfated glycosaminoglycans was further studied by surface plasmon resonance spectroscopy. ApoA-V showed strong binding to heparin-coated chips, and binding was competed by free heparin. ApoA-V enrichment enhanced binding of apoC-II-deficient chylomicrons and VLDL to heparin-coated chips. When LPL was first bound to the heparin-coated chip, apoA-V-enriched chylomicrons showed binding. Finally, human pre- and post-heparin plasma samples were subjected to immunoblot analysis with anti-apoA-V IgG. No differences in the amount of apoA-V present were detected. Taken together, the results show that apoA-V lipid complexes bind heparin and, when present on TG-rich lipoprotein particles, may promote their association with cell surface heparan sulfate proteoglycans. Through such interactions, apoA-V may indirectly affect LPL activity, possibly explaining its inverse correlation with plasma TG levels.     

The C terminus of apolipoprotein A-V modulates lipid-binding activity

Human apolipoprotein A-V (apoA-V) is a potent modulator of plasma triacylglycerol (TG) levels. To probe different regions of this 343-amino-acid protein, four single Trp apoA-V variants were prepared. The variant with a Trp at position 325, distal to the tetraproline sequence at residues 293-296, displayed an 11-nm blue shift in wavelength of maximum fluorescence emission upon lipid association. To evaluate the structural and functional role of this C-terminal segment, a truncated apoA-V comprising amino acids 1-292 was generated. Far UV circular dichroism spectra of full-length apoA-V and apoA-V-(1-292) were similar, with approximately 50% alpha-helix content. In guanidine HCl denaturation experiments, both full-length and truncated apoA-V yielded biphasic profiles consistent with the presence of two structural domains. The denaturation profile of the lower stability component (but not the higher stability component) was affected by truncation. Truncated apoA-V displayed an attenuated ability to solubilize l-alpha-dimyristoylphosphatidylcholine phospholipid vesicles compared with full-length apoA-V, whereas a peptide corresponding to the deleted C-terminal segment displayed markedly enhanced kinetics. The data support the concept that the C-terminal region is not required for apoA-V to adopt a folded protein structure, yet functions to modulate apoA-V lipid-binding activity; therefore, this concept may be relevant to the mechanism whereby apoA-V influences plasma TG levels.     

Effect of apolipoprotein A-V on plasma triglyceride, lipoprotein size, and composition in genetically engineered mice

Transgenic (Tg) mice that overexpress the human apolipoprotein A-V gene (APOA5) yet lack an endogenous mouse apoa5 gene (APOA5 Tg mice) were generated. Subsequently, the effect of human apoA-V expression on plasma triglyceride (TG) concentration and lipoprotein and apolipoprotein distribution was determined and compared with that in mice deficient in apoA-V (apoa5(-/-) mice). NMR analysis of plasma lipoproteins revealed that APOA5 Tg mice had a very low VLDL concentration (26.4 +/- 7.7 nmol/dl), whereas VLDL in apoa5(-/-) mice was 18- fold higher (467 +/- 152 nmol/dl). SDS-PAGE analysis of the d < 1.063 g/ml plasma fraction revealed that the apoB-100/apoB-48 ratio was 14-fold higher in APOA5 Tg versus apoa5(-/-) mice and that the apoE/total apoB ratio was 7-fold greater in APOA5 Tg versus apoa5(-/-) mice. It is anticipated that a reduction in apoB-100/apoB-48 ratio as well as that for apoE/apoB would impair the uptake of VLDL and remnants in apoa5(-/-) mice, thereby contributing to increased plasma TG levels. The concentration of apoA-V in APOA5 Tg mice was 12.5 +/- 2.9 microg/ml, which is approximately 50- to 100-fold higher than that reported for normolipidemic humans. ApoA-V was predominantly associated with HDL but was rapidly and efficiently redistributed to apoA- V-deficient VLDL upon incubation. Consistent with findings reported for human subjects, apoA-V concentration was positively correlated with TG levels in normolipidemic APOA5 Tg mice. It is conceivable that, in a situation in which apoA-V is chronically overexpressed, complex interactions among factors regulating TG homeostasis may result in a positive correlation of apoA-V with TG concentrations.     

Apolipoprotein A-V, triglycerides and risk of coronary artery disease: the prospective Epic-Norfolk Population Study

In mouse models, apolipoprotein A-V (apoA-V) exhibits triglyceride (TG)-lowering effects. We investigated the apoA-V/TG relationship and the association of apoA-V with coronary artery disease (CAD) risk by determining serum apoA-V levels and genotypes in a nested case-control (n = 1,034/2,031) study. Both univariate and multivariate apoA-V levels showed no association with future CAD (P = 0.4 and 0.5, respectively). Unexpectedly, there was a significant positive correlation between serum apoA-V and TG in men and women (r = 0.36 and 0.28, respectively, P < 0.001 each) but a negative correlation between apoA-V and LPL mass (r = -0.14 and -0.12 for men and women respectively, P < 0.001 each). The frequency of the c.56C>G polymorphism did not differ between cases and controls despite significant positive association of c.56G with both apoA-V and TG levels. For -1131T>C, the minor allele was significantly associated with lower apoA-V yet higher TG levels and was overrepresented in cases (P = 0.047). The association of -1131T>C with CAD risk, however, was independent of apoA-V levels and likely acts through linkage disequilibrium with APOC3 variants. The positive correlation of apoA-V levels with TG levels, negative correlation with LPL levels, and lack of association with CAD risk highlight the need for further human studies to clarify the role of apoA-V.     

Apolipoprotein A-V association with intracellular lipid droplets

Apolipoprotein A-V (apoA-V) plays a key role in the regulation of triglyceride (TG) metabolism. Given the very low concentration of apoA-V in plasma, we hypothesized that apoA-V may influence plasma TG levels by affecting the assembly and/or secretion of apoB-containing lipoproteins. When apoA-V was overexpressed in cultured Hep3B cells, neither the amount of apoB secreted nor the density distribution of apoB-containing lipoproteins was affected. Fluorescence microscopy and cell lysate immunoprecipitation studies revealed that apoA-V is not associated with apoB intracellularly, yet immunoprecipitation of apoA-V from the cell culture medium resulted in coprecipitation of apoB. These data suggest that the apoA-V association with apoB-containing lipoproteins is a postsecretory event. Confocal fluorescence microscopy revealed the presence of apoA-V in distinct cellular structures. Based on Nile Red staining, we identified these structures to be intracellular lipid droplets. These data suggest that apoA-V has a unique association with cellular lipids and, therefore, may be involved in the storage or mobilization of intracellular lipids.     

Influence of apoA-V gene variants on postprandial triglyceride metabolism: impact of gender

Although apolipoprotein A-V (apoA-V) polymorphisms have been consistently associated with fasting triglyceride (TG) levels, their impact on postprandial lipemia remains relatively unknown. In this study, we investigate the impact of two common apoA-V polymorphisms (-1131 T>C and S19W) and apoA-V haplotypes on fasting and postprandial lipid metabolism in adults in the United Kingdom (n = 259). Compared with the wild-type TT, apoA-V -1131 TC heterozygotes had 15% (P = 0.057) and 21% (P = 0.002) higher fasting TG and postprandial TG area under the curve (AUC), respectively. Significant (P = 0.038) and nearly significant (P = 0.057) gender x genotype interactions were observed for fasting TG and TG AUC, with a greater impact of genotype in males. Lower HDL-cholesterol was associated with the rare TC genotype (P = 0.047). Significant linkage disequilibrium was found between the apoA-V -1131 T>C and the apoC-III 3238 C>G variants, with univariate analysis indicating an impact of this apoC-III single nucleotide polymorphism (SNP) on TG AUC (P = 0.015). However, in linear regression analysis, a significant independent association with TG AUC (P = 0.007) was only evident for the apoA-V -1131 T>C SNP, indicating a greater relative importance of the apoA-V genotype.     

Biogenesis of apolipoprotein A-V and its impact on VLDL triglyceride secretion

Apolipoprotein A-V (apoA-V) is a potent regulator of intravascular triglyceride (TG) metabolism, yet its plasma concentration is very low compared with that of other apolipoproteins. To examine the basis for its low plasma concentration, the secretion efficiency of apoA-V was measured in stably transfected McA-RH7777 rat hepatoma cells. Pulse-chase experiments revealed that only ～20% of newly synthesized apoA-V is secreted into culture medium within 3 h postsynthesis and that ～65% undergoes presecretory turnover; similar results were obtained with transfected nonhepatic Chinese hamster ovary cells. ApoA-V secreted by McA-RH7777 cells was not associated with cell surface heparin-competable binding sites. When stably transfected McA-RH7777 cells were treated with oleic acid, the resulting increase in TG synthesis caused a reduction in apoA-V secretion, a reciprocal increase in cell-associated apoA-V, and movement of apoA-V onto cytosolic lipid droplets. In a stably transfected doxycycline-inducible McA-RH7777 cell line, apoA-V expression inhibited TG secretion by ～50%, increased cellular TG, and reduced Z-average VLDL(1) particle diameter from 81 to 67 nm; however, no impact on apoB secretion was observed. These data demonstrate that apoA-V inefficiently traffics within the secretory pathway, that its intracellular itinerary can be regulated by changes in cellular TG accumulation, and that apoA-V synthesis can modulate VLDL TG mobilization and secretion.     

Structure and interfacial properties of human apolipoprotein A-V

Apolipoprotein A-V (apoA-V), the newest member of the plasma apolipoprotein family, was recently discovered by comparison of the mouse and human genomes. Studies in rodents and population surveys of human apoA-V polymorphisms have noted a strong effect of apoA-V on plasma triglyceride levels. Toward the elucidation of the biologic function of apoA-V, we used spectroscopic and surface chemistry techniques to probe its structure and interfacial activity. Computer-assisted sequence analysis of apoA-V predicts that it is very hydrophobic, contains a significant amount of alpha-helical secondary structure, and probably is composed of discrete structural regions with varying degrees of lipid affinity. Fluorescence spectroscopy of recombinant human apoA-V provided evidence of tertiary folding, and light scattering studies indicated that apoA-V transforms dimyristoylphosphatidylcholine vesicles into discoidal complexes with an efficiency similar to that of apoA-I. Surface chemistry techniques revealed that apoA-V displays high affinity, low elasticity, and slow binding kinetics at hydrophobic interfaces, properties we propose may retard triglyceride-rich particle assembly. Metabolic labeling and immunofluorescence studies of COS-1 cells transfected with human apoA-V demonstrated that apoA-V is poorly secreted, remains associated with the endoplasmic reticulum, and does not traffic to the Golgi. Given that overexpression of the apoA-V gene lowers plasma triglycerides in mice, these data together suggest that apoA-V may function intracellularly to modulate hepatic VLDL synthesis and/or secretion.     

A sandwich enzyme-linked immunosorbent assay for human plasma apolipoprotein A-V concentration

Apolipoprotein A-V (apoA-V) is a recently discovered apolipoprotein that appears to have a role in plasma triglyceride (TG) transport. We have developed an ELISA for apoA-V using monoclonal antibodies that has a lower limit of detection of 0.3 ng/ml and linearity up to 20 ng/ml. The ELISA was then used to quantify plasma apoA-V in 196 healthy subjects and 106 patients with insulin-resistant diabetes mellitus. In the healthy subjects, total apoA-V concentration was 179.2 +/- 74.8 ng/ml, and it was greater in females than in males (P < 0.005). It was correlated positively with the plasma HDL cholesterol (r = 0.32, P < 0.0001), apoA-I (r = 0.27, P = 0.0001), and apoE (r = 0.18, P = 0.011) concentrations and negatively with plasma TG concentration (r = -0.22, P = 0.021). In relation to single nucleotide polymorphism 3 (-1131C/T) of the apoA-V gene, apoA-V concentration was higher in the T/T type than in the C/C type (P < 0.01). Plasma TG concentration was lower in the T/T type than in the C/C or C/T type (P < 0.05). ApoA-V concentration was lower in the diabetic patients (69.4 +/- 44.3 ng/ml; P < 0.01) than in the healthy controls.     

ApoE2-associated hypertriglyceridemia is ameliorated by increased levels of apoA-V but unaffected by apoC-III deficiency

Apolipoprotein E2 (apoE2)-associated hyperlipidemia is characterized by a disturbed clearance of apoE2-enriched VLDL remnants. Because excess apoE2 inhibits LPL-mediated triglyceride (TG) hydrolysis in vitro, we investigated whether direct or indirect stimulation of LPL activity in vivo reduces the apoE2-associated hypertriglyceridemia. Here, we studied the role of LPL and two potent modifiers, the LPL inhibitor apoC-III and the LPL activator apoA-V, in APOE2-knockin (APOE2) mice. Injection of heparin in APOE2 mice reduced plasma TG by 53% and plasma total cholesterol (TC) by 18%. Adenovirus-mediated overexpression of LPL reduced plasma TG by 85% and TC by 40%. Both experiments indicate that the TG in apoE2-enriched particles is a suitable substrate for LPL. Indirect activation of LPL activity via deletion of Apoc3 in APOE2 mice did not affect plasma TG levels, whereas overexpression of Apoa5 in APOE2 mice did reduce plasma TG by 81% and plasma TC by 41%. In conclusion, the hypertriglyceridemia in APOE2 mice can be ameliorated by the direct activation of LPL activity. Indirect activation of LPL via overexpression of apoA-V does, whereas deletion of apoC-III does not, affect the plasma TGs in APOE2 mice. These data indicate that changes in apoA-V levels have a dominant effect over changes in apoC-III levels in the improvement of APOE2-associated hypertriglyceridemia.     

Apolipoprotein A-V associates with intrahepatic lipid droplets and influences triglyceride accumulation

Apolipoprotein A-V (apoA-V), secreted solely by the liver, is a low abundance protein that strongly influences plasma triglyceride (TG) levels. In vitro, in transfected hepatoma cell lines apoA-V is largely retained within the cell in association with cytosolic lipid droplets (LD). To evaluate if this is true in vivo, in the present study the amount of apoA-V in the plasma compartment versus liver tissue was determined in APOA5 transgenic (Tg) mice. The majority of total apoA-V (∼ 80%) was in the plasma compartment. Injection of APOA5 Tg mice with heparin increased plasma apoA-V protein levels by ∼ 25% indicating the existence of a heparin-releasable pool. Intrahepatic apoA-V was associated with LD isolated from livers of wild type (WT) and APOA5 Tg mice. Furthermore, livers from APOA5 Tg mice contained significantly higher amounts of TG than livers from WT or apoa5 knockout mice suggesting that apoA-V influences intrahepatic TG levels.     

Structure-function studies of human apolipoprotein A-V: a regulator of plasma lipid homeostasis

To investigate structure and function relations of a new member of the exchangeable apolipoprotein family that modulates plasma lipid levels, recombinant human apolipoprotein (apo) A-V was produced in Escherichia coli and isolated by a combination of nickel chelation affinity chromatography and reversed-phase HPLC. Antibodies directed against apoA-V were generated and employed in immunoblotting experiments. Anti-apoA-V IgG gave a strong response against recombinant apoA-V from E. coli and human apoA-V expressed in transgenic mice, but did not recognize human apoA-I or apoA-IV. In neutral-pH buffers, at concentrations of >0.1 mg/mL, isolated lipid-free apoA-V is poorly soluble. By contrast, apoA-V is soluble in 50 mM sodium citrate (pH 3.0). Far-UV circular dichroism analysis and spectral deconvolution reveal that apoA-V possesses 32% alpha-helix, 33% beta-sheet, 16% beta-turn, and 18% random coil secondary structure conformers. Temperature-induced denaturation studies gave rise to a transition midpoint of 47.1 degrees C. Upon being cooled to ambient temperature from 85 degrees C, apoA-V failed to recover all of the negative ellipticity present in unheated apoA-V. ApoA-V interacts with bilayer vesicles of dimyristoylphosphatidylcholine to form discoidal complexes with diameters in the range of 15-20 nm. However, apoA-V was a poor activator of lecithin:cholesterol acyltransferase where the activity was 8.5 +/- 1.8% of that of apoA-I. Furthermore, apoA-V failed to support enhanced efflux of cholesterol from cAMP-treated J774 macrophages, although low levels of efflux were obtained from unstimulated cells. Taken together, the results demonstrate recombinant apoA-V possesses unique structural and functional characteristics, in keeping with its proposed role in the modulation of plasma lipid levels.     

Effects of apoA-V on HDL and VLDL metabolism in APOC3 transgenic mice

Apolipoprotein A-V (apoA-V) and apoC-III are exchangeable constituents of VLDL and HDL. ApoA-V counteracts the effect of apoC-III on triglyceride (TG) metabolism with poorly defined mechanisms. To better understand the effects of apoA-V on TG and cholesterol metabolism, we delivered apoA-V cDNA into livers of hypertriglyceridemic APOC3 transgenic mice by adenovirus-mediated gene transfer. In response to hepatic apoA-V production, plasma TG levels were reduced significantly as a result of enhanced VLDL catabolism without alternations in VLDL production. This effect was associated with reduced apoC-III content in VLDL. Increased apoA-V production also resulted in decreased apoC-III and increased apoA-I content in HDL. Furthermore, apoA-V-enriched HDL was associated with enhanced LCAT activity and increased cholesterol efflux. This effect, along with apoE enrichment in HDL, contributed to HDL core expansion and alpha-HDL formation, accounting for significant increases in both the number and size of HDL particles. As a result, apoA-V-treated APOC3 transgenic mice exhibited decreased VLDL-cholesterol and increased HDL-cholesterol levels. ApoA-V-mediated reduction of apoC-III content in VLDL represents an important mechanism by which apoA-V acts to ameliorate hypertriglyceridemia in adult APOC3 transgenic mice. In addition, increased apoA-V levels accounted for cholesterol redistribution from VLDL to larger HDL particles. These data suggest that in addition to its TG-lowering effect, apoA-V plays a significant role in modulating HDL maturation and cholesterol metabolism.     

Synthesis of recombinant high density lipoprotein with apolipoprotein A-I and apolipoprotein A-V

It has been shown that apolipoprotein A-V (apoA-V) over-expression significantly lowers plasma triglyceride levels and decreases atherosclerotic lesion development. To assess the feasibility of recombinant high density lipoprotein (rHDL) reconstituted with apoA-V and apolipoprotein A-I (apoA-I) as a therapeutic agent for hyperlipidemic disorder and atherosclerosis, a series of rHDL were synthesized in vitro with various mass ratios of recombinant apoA-I and apoA-V. It is interesting to find that apoA-V of rHDL had no effect on lipoprotein lipase (LPL) activation in vitro and very low density lipoprotein (VLDL) clearance in HepG2 cells and in vivo. By contrast, LPL activation and VLDL clearance were inhibited by the addition of apoA-V to rHDL. Furthermore, the apoA-V of rHDL could not redistribute from rHDL to VLDL after incubation at 37°C for 30 min. These findings suggest that an increase of apoA-V in rHDL could not play a role in VLDL clearance in vitro and in vivo, which could, at least in part, attribute to the lost redistribution of apoA-V from rHDL to VLDL and LPL binding ability of apoA-V in rHDL. The therapeutic application of rHDL reconstituted with apoA-V and apoA-I might need the construction of rHDL from which apoA-V could freely redistribute to VLDL.     

Apolipoprotein A-V N-terminal Domain Lipid Interaction Properties in Vitro Explain the Hypertriglyceridemic Phenotype Associated with Natural Truncation Mutants

The N-terminal 146 residues of apolipoprotein (apo) A-V adopt a helix bundle conformation in the absence of lipid. Because similarly sized truncation mutants in human subjects correlate with severe hypertriglyceridemia, the lipid binding properties of apoA-V(1–146) were studied. Upon incubation with phospholipid in vitro, apoA-V(1–146) forms reconstituted high density lipoproteins 15–17 nm in diameter. Far UV circular dichroism spectroscopy analyses of lipid-bound apoA-V(1–146) yielded an α-helix secondary structure content of 60%. Fourier transformed infrared spectroscopy analysis revealed that apoA-V(1–146) α-helix segments align perpendicular with respect to particle phospholipid fatty acyl chains. Fluorescence spectroscopy of single Trp variant apoA-V(1–146) indicates that lipid interaction is accompanied by a conformational change. The data are consistent with a model wherein apoA-V(1–146) α-helices circumscribe the perimeter of a disk-shaped bilayer. The ability of apoA-V(1–146) to solubilize dimyristoylphosphatidylcholine vesicles at a rate faster than full-length apoA-V suggests that N- and C-terminal interactions in the full-length protein modulate its lipid binding properties. Preferential association of apoA-V(1–146) with murine plasma HDL, but not with VLDL, suggests that particle size is a determinant of its lipoprotein binding specificity. It may be concluded that defective lipoprotein binding of truncated apoA-V contributes to the hypertriglyceridemia phenotype associated with truncation mutations in human subjects.     

An apolipoprotein A-V gene SNP is associated with marked hypertriglyceridemia among Asian-American patients

Apolipoprotein A-V (apoA-V) is an important regulator of plasma levels of triglyceride (TG) in mice. In humans, APOA5 genetic variation is associated with TG in several populations. In this study, we determined the effects of the p.185Gly>Cys (c.553G>T; rs2075291) polymorphism on plasma TG levels in subjects of Chinese ancestry living in the United States and in a group of non-Chinese Asian ancestry. The frequency of the less common cysteine allele was 4-fold higher (15.1% vs. 3.7%) in Chinese high-TG subjects compared with a low-TG group (Chi-square = 20.2; P < 0.0001), corresponding with a 4.45 times higher risk of hypertriglyceridemia (95% confidence interval, 2.18-9.07; P < 0.001). These results were replicated in the non-Chinese Asians. Heterozygosity was associated, in the high-TG group, with a doubling of TG (P < 0.001), mainly VLDL TG (P = 0.014). All eleven TT homozygotes had severe hypertriglyceridemia, with mean TG of 2,292 +/- 447 mg/dl. Compared with controls, carriers of the T allele had lower postheparin lipoprotein lipase activity but not hepatic lipase activity. In Asian populations, this common polymorphism can lead to profound adverse effects on lipoprotein profiles, with homozygosity accounting for a significant number of cases of severe hypertriglyceridemia. This specific apoA-V variant has a pronounced effect on TG metabolism, the mechanism of which remains to be elucidated.     

High postprandial triglyceridemia in patients with type 2 diabetes and microalbuminuria

Microalbuminuria (MA) is an independent risk factor for atherosclerosis in patients with type 2 diabetes mellitus (T2DM). Postprandial lipemia is also associated with excess cardiovascular risk. However, the association between MA and postprandial lipemia in diabetes has not been investigated. A total of 64 patients with T2DM, 30 with and 34 without MA, were examined. Plasma total triglycerides (TGs), triglycerides contained in chylomicrons (CM-TG), and TGs in CM-deficient plasma were measured at baseline and every 2 h for 6 h after a mixed meal. Postheparin LPL and HL activities were also determined. Plasma levels of apolipoprotein A-V (apoA-V), apoC-II, and apoC-III were measured in the fasting state and 2 h postprandially. Patients with MA had higher postprandial total TG levels than those without MA (P < 0.001); this increase been attributed mainly to CM-TG. LPL activity and fasting concentrations of the measured apolipoproteins were not different between the studied groups, whereas HL activity was higher in the patients with MA. ApoC-II and apoC-III levels did not change postprandially in either study group, whereas apoA-V increased more in the patients with MA. These data demonstrate for the first time that MA is characterized by increased postprandial lipemia in patients with T2DM and may explain in part the excess cardiovascular risk in these patients.