Characterization of novel truncated apolipoprotein A-I in human high-density lipoprotein generated by sequential treatment with myeloperoxidase and chymase.

High-density lipoprotein (HDL) cholesterol is a well-known biomarker, which has been associated with reduction in the risk of cardiovascular diseases (CVD). However, some HDL anti-atherosclerotic functions may be impaired without altered HDL-cholesterol (HDL-C) level via its dysfunctional proteins or other physiological reactions in vivo. We previously showed that activated mast cell-derived chymase could modestly cleave apolipoprotein A-I (apoA-I) in HDL3, and further easily cleave lipid-free apoA-I. In contrast, myeloperoxidase (MPO) secreted by macrophages, the main cell type in atherosclerotic plaques, could oxidize HDL proteins, which might modify their tertiary structures, increasing their susceptibility to other enzymes. Here we focused on the co-modification and impact of chymase and MPO, usually secreted during inflammation from cells with possible co-existence in atheromas, on HDL. Only after sequential treatment with MPO and then chymase, two novel truncated apoA-I fragments were generated from HDL. One fragment was 16.5 kDa, and the cleavage site by chymase after MPO modification was the C-terminal of Tyr100 in apoA-I, cross-validated by three different mass spectrometry methods. This novel apoA-I fragment can be trapped in HDL particles to avoid kidney glomerular filtration and has a specific site for antibody generation for ELISA tests. As such, its quantification can be useful in predicting patients with CVD having normal HDL-C levels.

Variation in presepsin and thrombomodulin levels for predicting COVID-19 mortality.

Coronavirus disease (COVID-19) has caused extensive mortality globally; therefore, biomarkers predicting the severity and prognosis of COVID-19 are essential. This study aimed to evaluate the application of presepsin (P-SEP) and thrombomodulin (TM), which are biomarkers of sepsis and endothelial dysfunction, respectively, in the prognosis of COVID-19. Serum P-SEP and TM levels from COVID-19 patients (n = 183) were measured. Disease severity was classified as mild, moderate I, moderate II, or severe based on hemoglobin oxygen saturation and the history of intensive care unit transfer or use of ventilation at admission. Patients in the severe group were further divided into survivors and non-survivors. P-SEP and TM levels were significantly higher in the severe group than those in the mild group, even after adjusting for creatinine values. In addition, TM levels were significantly higher in non-survivors than in survivors. Changes in the P-SEP levels at two time points with an interval of 4.1 ± 2.2 days were significantly different between the survivors and non-survivors. In conclusion, TM and continuous P-SEP measurements may be useful for predicting mortality in patients with COVID-19. Moreover, our data indicate that P-SEP and TM values after creatinine adjustment could be independent predictive markers, apart from renal function.

Decreased rate of blood donors with high ABO antibody titers in Japan and the underlying factors: Comparisons between 2010 and 2021.

BACKGROUND AND OBJECTIVES: Previously (2007), it was reported that ABO antibody titers in Japanese blood donors had decreased significantly compared to 20 years before. Here we evaluated whether further decrease of antibody titers had occurred in recent years, and the potential factors associated with changes in antibody titers. MATERIALS AND METHODS: Serum/plasma from random blood donors in 2010 and 2021 (2010: 3369, 2021: 5796 donors) was classified into low, middle, and high ABO antibody titers according to the reactivity of diluted serum/plasma (2.5-fold and 20-fold) by an automated microplate system. The rates of low/high titer in the two periods were compared. Logistic regression and age-gender-BMI subgroup analyses were conducted to identify the factors that contributed to changes in antibody titers. RESULTS: Compared to 2010, the rate of donors with high ABO antibody titers was　decreased in 2021 for both anti-A and anti-B (anti-A, 2010: 23.8%, 2021: 19.3%; anti-B, 2010: 23.8%, 2021: 16.4%). In logistic regression analysis, age was found to significantly affect both anti-A and anti-B antibody titers (anti-A, adjusted odds ratio 0.36, 95% CI 0.31-0.41; anti-B, 0.42, 0.37-0.47), and BMI (0.82, 0.73-0.92) and other time-related factors (0.79, 0.71-0.88) significantly affect anti-B antibody titers. Subgroup analysis revealed decreased rate of high anti-B titers in the higher age group in 2021. CONCLUSION: The rate of high ABO antibody titers, especially high anti-B titers, was significantly decreased in 2021, and our results suggested an association with aging and obesity of blood donors as well as other time-related factors.

Improvement in bilirubin influence on cholesterol efflux capacity evaluation using the immobilized liposome-bound gel beads method.

INTRODUCTION: High-density lipoprotein (HDL) has a cholesterol efflux capacity (CEC) that protects against atherosclerosis. Recently, we developed an assay for CEC evaluation, named the immobilized liposome-bound gel beads (ILG) method, which is a highly accurate, simple, and safe method for CEC evaluation because it uses liposomes and BODIPY-labeled cholesterol instead of cultured cells and radioactive substances, respectively. Although the ILG method can be implemented in clinical settings, our previous study revealed that bilirubin causes a positive error in the CEC value. Therefore, in the present study, we attempted to improve the influence of bilirubin levels on the ILG method. METHODS: To investigate why bilirubin caused a positive error in CEC values when using the ILG method, 3D fluorescence spectra of BODIPY-labeled cholesterol and bilirubin were measured. To avoid the fluorescence emitted by bilirubin, CEC was measured using the ILG method with shifting of excitation wavelength for BODIPY-labeled cholesterol quantification. In addition, we used bilirubin oxidase to oxidize bilirubin during the incubation time of the ILG method to weaken bilirubin fluorescence. RESULTS: We found that bilirubin emitted fluorescence at the measurement setting of the ILG method. By shifting the excitation wavelength, the positive error caused by bilirubin was improved by approximately 70%. Furthermore, by utilizing bilirubin oxidase, the false-high values of CEC were improved by approximately 80%. CONCLUSIONS: Bilirubin interferes with CEC assay using BODIPY-cholesterol, but we successfully improved the influence of bilirubin on CEC evaluation using the ILG method. These improvements will promote the clinical application of the ILG method.

Development and validation of novel automatable assay for cholesterol efflux capacity.

During the past decade, evaluation of high-density lipoprotein (HDL) functionality has been well studied for predicting cardiovascular disease (CVD) risk. Cholesterol efflux capacity (CEC) is the strongest candidate as the biomarker out of various HDL antiatherosclerotic functions. However, CEC has not yet been introduced clinically because of several technical issues, including the use of radioactive materials and differentiated cells in the assay. Previously, our laboratory developed a radioisotope- and cell-free CEC assay called the immobilized liposome-bound gel beads (ILGs) method to replace the conventional method. However, the separation process of the supernatant was not suitable for installation in an automatic analyzer. The present study aims to develop a new method that is easier to operate. We assumed that the use of magnetic beads instead of gel beads would enable the skip of the centrifugal process. First, similar to the ILG method, porous magnetic beads were treated with liposomes containing fluorescently labeled cholesterol. Fluorescence was observed inside the magnetic beads, and almost the same amount of liposomes as in the ILG method was immobilized successfully. These immobilized liposome-bound magnetic beads (ILMs) were available for CEC assay when HDL and apolipoprotein B-100-depleted serum (BDS) were used as cholesterol acceptors. The ILM method showed sufficient basic performance and a good correlation with the ILG method. Furthermore, when the CEC of 15 serum samples from healthy subjects was measured, a good correlation between HDL-cholesterol level and the ILG method was confirmed. Thus, it was confirmed that the ILM method was successfully developed and could be automated.

Development of internal standard for lipoprotein subclass analysis using dual detection gel-permeation high-performance liquid chromatography system.

The LipoSEARCH® System is an innovative lipoprotein class analysis method based on gel-permeation high-performance liquid chromatography (HPLC). This system uses a gel permeation column to separate the major lipoprotein subclasses (chylomicron, very low-density lipoprotein, low-density lipoprotein, and high-density lipoprotein) in serum according to particle size and splits them into two pathways to measure total cholesterol (TC; esterified + unesterified cholesterol) and triglyceride (TG) concentrations simultaneously to obtain chromatograms for each. These chromatograms were analyzed based on the results of the calibration serum by fitting Gaussian curves to profile the 20 lipoprotein subclasses defined in detail. An important assumption of this HPLC system is its simultaneous detection of two pathways to guarantee the accuracy of each analysis. Therefore, in the present study, we investigated the development of an internal standard that can guarantee the simultaneous detection of this system by adding a pigment to the serum. We focused on quinone pigments with absorption at 550 nm, which is the wavelength used for the enzymatic assay of TC and TG concentrations in the system. As a result, we succeeded in producing overlapping pigment peaks that appeared after the analytical chromatograms in two pathways. It is also suggested that the pigment solution as an internal standard is stable in freezing storage and has little effect on the analysis. The developed internal standard is expected to contribute to the accuracy assurance of lipoprotein analysis by this dual-detection HPLC system.

Novel cholesterol efflux assay using immobilized liposome-bound gel beads: Confirmation and improvement for application in clinical laboratory.

OBJECTIVES: Cholesterol efflux capacity (CEC), an atheroprotective function of high-density lipoprotein, is expected to be a potential biomarker for cardiovascular disease. However, CEC has not been widely introduced for application in clinical laboratories because of the complexity of the conventional CEC assay using cells and radioactive materials. Previously, we developed a novel CEC assay using immobilized liposome-bound gel beads (ILG), which solves these issues. We aimed to confirm the validation and further improve the ILG method for application in the clinical setting. METHODS: Cholesterol efflux capacity values by the ILG method assayed for shorter incubation time (4 h) were compared to those assayed for 16 h (our previous ILG method). To investigate a reference material that can correct the variation between ILG manufacturing lots, bovine serum albumin, human gamma-globulins, and globulin complexes were evaluated. CEC values were also estimated in plasmas obtained with different anticoagulants, serum treated with freeze-thaw cycles, and serum mixed with several interference substances. RESULTS: The CEC of 4- and 16-h incubation times were well correlated. Globulin complexes may be used as a reference material. Plasma can be used as the specimen. The serum and stored temperature of the specimen did not largely affect CEC. Hemoglobin and chyle did not have an effect on CEC, whereas high-bilirubin serum showed elevated CEC. The effect of bilirubin was nearly canceled by subtracting basal fluorescence intensity. CONCLUSIONS: Present ILG method further fulfills some requirements for application in clinical laboratory. Using this reliable simple method, evaluation for clinical significance of CEC is expected.

Effect of myeloperoxidase oxidation and N-homocysteinylation of high-density lipoprotein on endothelial repair function.

Endothelial cell (EC) migration is essential for healing vascular injuries. Previous studies suggest that high-density lipoprotein (HDL) and apolipoprotein A-I (apoA-I), the major protein constituent of HDL, have endothelial healing functions. In cardiovascular disease, HDL is modified by myeloperoxidase (MPO) and N-homocysteine, resulting in apoA-I/apoA-II heterodimer and N-homocysteinylated (N-Hcy) apoA-I formation. This study investigated whether these modifications attenuate HDL-mediated endothelial healing. Wound healing assays were performed to analyze the effect of MPO-oxidized HDL and N-Hcy HDL in vitro. HDL obtained from patients with varying troponin I levels were also examined. MPO-oxidized HDL reduces EC migration compared to normal HDL in vitro, and N-Hcy HDL showed a decreasing trend toward EC migration. EC migration after treatment with HDL from patients was decreased compared to HDL isolated from healthy controls. Increased apoA-I/apoA-II heterodimer and N-Hcy apoA-I levels were also detected in HDL from patients. Wound healing cell migration was significantly negatively correlated with the ratio of apoA-I/apoA-II heterodimer to total apoA-II and N-Hcy apoA-I to total apoA-I. MPO-oxidized HDL containing apoA-I/apoA-II heterodimers had a weaker endothelial healing function than did normal HDL. These results indicate that MPO-oxidized HDL and N-Hcy HDL play a key role in the pathogenesis of cardiovascular disease.

Apolipoprotein C-II and C-III preferably transfer to both high-density lipoprotein (HDL)2 and the larger HDL3 from very low-density lipoprotein (VLDL).

Triglyceride hydrolysis by lipoprotein lipase (LPL), regulated by apolipoproteins C-II (apoC-II) and C-III (apoC-III), is essential for maintaining normal lipid homeostasis. During triglyceride lipolysis, the apoCs are known to be transferred from very low-density lipoprotein (VLDL) to high-density lipoprotein (HDL), but the detailed mechanisms of this transfer remain unclear. In this study, we investigated the extent of the apoC transfers and their distribution in HDL subfractions, HDL2 and HDL3. Each HDL subfraction was incubated with VLDL or biotin-labeled VLDL, and apolipoproteins and lipids in the re-isolated HDL were quantified using western blotting and high-performance liquid chromatography (HPLC). In consequence, incubation with VLDL showed the increase of net amount of apoC-II and apoC-III in the HDL. HPLC analysis revealed that the biotin-labeled apolipoproteins, including apoCs and apolipoprotein E, were preferably transferred to the larger HDL3. No effect of cholesteryl ester transfer protein inhibitor on the apoC transfers was observed. Quantification of apoCs levels in HDL2 and HDL3 from healthy subjects (n = 8) showed large individual differences between apoC-II and apoC-III levels. These results suggest that both apoC-II and apoC-III transfer disproportionately from VLDL to HDL2 and the larger HDL3, and these transfers might be involved in individual triglyceride metabolism.

Marked Changes in Serum Amyloid A Distribution and High-Density Lipoprotein Structure during Acute Inflammation.

High-density lipoprotein- (HDL-) cholesterol measurements are generally used in the diagnosis of cardiovascular diseases. However, HDL is a complicated heterogeneous lipoprotein, and furthermore, it can be converted into dysfunctional forms during pathological conditions including inflammation. Therefore, qualitative analysis of pathophysiologically diversified HDL forms is important. A recent study demonstrated that serum amyloid A (SAA) can remodel HDL and induce atherosclerosis not only over long periods of time, such as during chronic inflammation, but also over shorter periods. However, few studies have investigated rapid HDL remodeling. In this study, we analyzed HDL samples from patients undergoing orthopedic surgery inducing acute inflammation. We enrolled 13 otherwise healthy patients who underwent orthopedic surgery. Plasma samples were obtained on preoperative day and postoperative days (POD) 1-7. SAA, apolipoprotein A-I (apoA-I), and apolipoprotein A-II (apoA-II) levels in the isolated HDL were determined. HDL particle size, surface charge, and SAA and apoA-I distributions were also analyzed. In every patient, plasma SAA levels peaked on POD3. Consistently, the HDL apoA-I : apoA-II ratio markedly decreased at this timepoint. Native-polyacrylamide gel electrophoresis and high-performance liquid chromatography revealed the loss of small HDL particles during acute inflammation. Furthermore, HDL had a decreased negative surface charge on POD3 compared to the other timepoints. All changes observed were SAA-dependent. SAA-dependent rapid changes in HDL size and surface charge were observed after orthopedic surgery. These changes might affect the atheroprotective functions of HDL, and its analysis can be available for the qualitative HDL assessment.

Glycation of HDL Polymerizes Apolipoprotein M and Attenuates Its Capacity to Bind to Sphingosine 1-Phosphate.

AIM: Recently, it has been established that most of the pleiotropic effects of high-density lipoprotein (HDL) are attributed to sphingosine 1-phosphate (S1P), which rides on HDL via apolipoprotein M (ApoM). In subjects with diabetes mellitus, both the pleiotropic effects of HDL and its role in reverse cholesterol transport are reported to be impaired. To elucidate the mechanisms underlying the impaired pleiotropic effects of HDL in subjects with diabetes, from the aspects of S1P and ApoM. METHODS: The incubation of HDL in a high-glucose condition resulted in the dimerization of ApoM. Moreover, the treatment of HDL with methylglyoxal resulted in the modulation of the ApoM structure, as suggested by the results of western blot analysis, isoelectric focusing electrophoresis, and two-dimensional gel electrophoresis, which was reversed by treatment with anti-glycation reagents. RESULTS: The glycation of HDL resulted in impaired binding of the glycated HDL to S1P, and the S1P on glycated HDL degraded faster. In the case of human subjects, on the other hand, although both the serum ApoM levels and the ApoM content in HDL were lower in subjects with diabetes, we did not observe the polymerization of ApoM. CONCLUSIONS: Modulation of the quantity and quality of ApoM might explain, at least in part, the impaired functions of HDL in subjects with diabetes mellitus. ApoM might be a useful target for laboratory testing and/or the treatment of diabetes mellitus.

Higher serum levels of autotaxin and phosphatidylserine-specific phospholipase A1 in patients with lupus nephritis.

BACKGROUND: Recent studies revealed that lysophospholipids (LPLs) and related molecules, such as autotaxin (ATX) and phosphatidylserine-specific phospholipase A1 (PS-PLA1 ), are candidates for novel biomarkers in melanoma, glaucoma and diabetic nephropathy. However, it is not clear whether serum levels of ATX/ PS-PLA1 would be associated with pathological and clinical findings of lupus nephritis (LN). METHODS: In this retrospective cohort study, serum samples were collected from 39 patients with LN and 37 patients with other glomerular diseases. The serum levels of ATX and PS-PLA1 were evaluated for an association with renal pathology and clinical phenotypes of LN. RESULTS: The serum levels of ATX and PS-PLA1 were higher in the patients with LN as compared to those with other glomerular diseases. Among the classes of LN, the patients with class IV showed the trend of lower serum levels of ATX. Moreover, the patients with lower levels of ATX exhibited higher scores of activity index (AI) and chronicity index (CI). The level of ATX tended to be negatively correlated with AI and CI. These results might be explained by the effect of treatment, because the serum levels of ATX and PS-PLA1 were inversely correlated with the daily amount of oral prednisolone. Moreover, they did not reflect the level of proteinuria or kidney survival in LN patients. CONCLUSION: Although the serum levels of ATX and PS-PLA1 were affected by the treatment, these levels were higher in the patients with LN. The potential clinical benefits of these markers need to be clarified in further studies.

Cholesterol transport between red blood cells and lipoproteins contributes to cholesterol metabolism in blood.

Lipoproteins play a key role in transport of cholesterol to and from tissues. Recent studies have also demonstrated that red blood cells (RBCs), which carry large quantities of free cholesterol in their membrane, play an important role in reverse cholesterol transport. However, the exact role of RBCs in systemic cholesterol metabolism is poorly understood. RBCs were incubated with autologous plasma or isolated lipoproteins resulting in a significant net amount of cholesterol moved from RBCs to HDL, while cholesterol from LDL moved in the opposite direction. Furthermore, the bi-directional cholesterol transport between RBCs and plasma lipoproteins was saturable and temperature-, energy-, and time-dependent, consistent with an active process. We did not find LDLR, ABCG1, or scavenger receptor class B type 1 in RBCs but found a substantial amount of ABCA1 mRNA and protein. However, specific cholesterol efflux from RBCs to isolated apoA-I was negligible, and ABCA1 silencing with siRNA or inhibition with vanadate and Probucol did not inhibit the efflux to apoA-I, HDL, or plasma. Cholesterol efflux from and cholesterol uptake by RBCs from Abca1+/+ and Abca1-/- mice were similar, arguing against the role of ABCA1 in cholesterol flux between RBCs and lipoproteins. Bioinformatics analysis identified ABCA7, ABCG5, lipoprotein lipase, and mitochondrial translocator protein as possible candidates that may mediate the cholesterol flux. Together, these results suggest that RBCs actively participate in cholesterol transport in the blood, but the role of cholesterol transporters in RBCs remains uncertain.

Cholesterol Efflux-Independent Modification of Lipid Rafts by AIBP (Apolipoprotein A-I Binding Protein).

OBJECTIVE: AIBP (apolipoprotein A-I binding protein) is an effective and selective regulator of lipid rafts modulating many metabolic pathways originating from the rafts, including inflammation. The mechanism of action was suggested to involve stimulation by AIBP of cholesterol efflux, depleting rafts of cholesterol, which is essential for lipid raft integrity. Here we describe a different mechanism contributing to the regulation of lipid rafts by AIBP. Approach and Results: We demonstrate that modulation of rafts by AIBP may not exclusively depend on the rate of cholesterol efflux or presence of the key regulator of the efflux, ABCA1 (ATP-binding cassette transporter A-I). AIBP interacted with phosphatidylinositol 3-phosphate, which was associated with increased abundance and activation of Cdc42 and rearrangement of the actin cytoskeleton. Cytoskeleton rearrangement was accompanied with reduction of the abundance of lipid rafts, without significant changes in the lipid composition of the rafts. The interaction of AIBP with phosphatidylinositol 3-phosphate was blocked by AIBP substrate, NADPH (nicotinamide adenine dinucleotide phosphate), and both NADPH and silencing of Cdc42 interfered with the ability of AIBP to regulate lipid rafts and cholesterol efflux. CONCLUSIONS: Our findings indicate that an underlying mechanism of regulation of lipid rafts by AIBP involves PIP-dependent rearrangement of the cytoskeleton.

Comparison of a novel cholesterol efflux assay using immobilized liposome-bound gel beads with the conventional method.

Cholesterol efflux capacity (CEC) is an atheroprotective function of high-density lipoprotein (HDL). CEC is currently measured using artificially prepared foam cells composed of cultured macrophage and 3H-cholesterol. However, this conventional method is not suitable for clinical laboratory use due to poor repeatability, complexity, and low safety. Recently, we reported a novel CEC assay, called the immobilized liposome-bound gel beads (ILG) method. The ILG method is an alternative to foam cells, comprising gel beads and 4,4-diflioro-4-bora-3a,4a-s-indacene labeled cholesterol (BODIPY-cholesterol) instead of macrophage and 3H-cholesterol, respectively. The ILG method has shown adequate basic properties and strong correlation with the conventional method. Here, we aimed to compare this new ILG method with the conventional method in-depth. When apoB-depleted serum was used as the cholesterol acceptor (CA), the ILG method had far better reproducibility than the conventional method. The CEC of major HDL subclasses HDL2 and HDL3 had similar results in both the ILG and conventional method. However, the ILG method did not reflect the CEC of apolipoprotein (apo) A-I and a minor HDL subclass which uses ATP-binding cassette transporter A1 on foam cells. Superior reproducibility of the ILG method, which is a limitation of the conventional method, and similar CEC results for major HDL subclasses in the ILG and conventional methods, provide further evidence that the ILG method is promising for measuring CEC clinically. However, some HDL subclasses or apo might have poor CEC correlation between these methods. Further research is therefore needed to confirm the clinical significance of estimating CEC by the ILG method.

Exosomes containing HIV protein Nef reorganize lipid rafts potentiating inflammatory response in bystander cells.

HIV infection has a profound effect on "bystander" cells causing metabolic co-morbidities. This may be mediated by exosomes secreted by HIV-infected cells and containing viral factors. Here we show that exosomes containing HIV-1 protein Nef (exNef) are rapidly taken up by macrophages releasing Nef into the cell interior. This caused down-regulation of ABCA1, reduction of cholesterol efflux and sharp elevation of the abundance of lipid rafts through reduced activation of small GTPase Cdc42 and decreased actin polymerization. Changes in rafts led to re-localization of TLR4 and TREM-1 to rafts, phosphorylation of ERK1/2, activation of NLRP3 inflammasome, and increased secretion of pro-inflammatory cytokines. The effects of exNef on lipid rafts and on inflammation were reversed by overexpression of a constitutively active mutant of Cdc42. Similar effects were observed in macrophages treated with exosomes produced by HIV-infected cells or isolated from plasma of HIV-infected subjects, but not with exosomes from cells and subjects infected with ΔNef-HIV or uninfected subjects. Mice injected with exNef exhibited monocytosis, reduced ABCA1 in macrophages, increased raft abundance in monocytes and augmented inflammation. Thus, Nef-containing exosomes potentiated pro-inflammatory response by inducing changes in cholesterol metabolism and reorganizing lipid rafts. These mechanisms may contribute to HIV-associated metabolic co-morbidities.

Red blood cells participate in reverse cholesterol transport by mediating cholesterol efflux of high-density lipoprotein and apolipoprotein A-I from THP-1 macrophages.

High-density lipoprotein (HDL) plays a main role in reverse cholesterol transport (RCT), one of the most important functions for preventing atherosclerosis. Recent reports have shown that red blood cells (RBCs) can be associated with RCT, an interaction facilitated by albumin. However, the RCT function of RBCs has not been thoroughly elucidated. In this study, the RCT function of RBCs was assessed using cholesterol efflux capacity (CEC) assays, in which [3H]-labeled cholesterol-loaded human acute monocytic leukemia (THP-1) macrophages were incubated with RBCs as a cholesterol acceptor in the presence or absence of HDL or its main component protein apolipoprotein A-I (apoA-I). The CEC of RBCs was found to be dose dependent, enabling uptake of cholesterol from THP-1 macrophages through apoA-I and HDL, and directly from apoA-I and HDL in medium without the presence THP-1 macrophages. Moreover, RBCs could exchange cholesterol with HDL in a bidirectional manner but could only exchange cholesterol with apoA-I in a single direction. Although albumin promoted the movement of cholesterol, synergistic effects were not observed for both apoA-I and HDL, in contrast to previous findings. These results strongly suggested that RBCs may play important roles in RCT by mediating cholesterol efflux as temporary cholesterol storage.

Usefulness of apolipoprotein B-depleted serum in cholesterol efflux capacity assays using immobilized liposome-bound gel beads.

Cholesterol efflux capacity (CEC) in atherosclerotic lesions is the main anti-atherosclerotic function of high-density lipoprotein (HDL). In recent studies, apolipoprotein (apo) B-depleted serum (BDS) obtained with the polyethylene glycol (PEG) precipitation method is used as a cholesterol acceptor (CA) substitution for HDL isolated by ultracentrifugation. However, the suitability of BDS as a CA is controversial. In the present study, CEC obtained from BDS (BDS-CEC) was evaluated based on a parameter, defined as whole-CEC, which was calculated by multiplying CEC obtained using fixed amounts of HDL by cholesterol concentration to HDL-cholesterol (HDL-C) levels in the serum. Significant correlation (r = 0.633) was observed between both CECs. To eliminate systematic errors from possible contamination with serum proteins and low-density lipoprotein (LDL) or very-LDL (VLDL) in BDS-CEC, the deviation of each CEC-BDS from the regression equation was compared with serum protein, LDL, and triglyceride (TG) levels. No correlation was observed between the deviation and the levels of each of these serum components, indicating that the deviations do not derive from systematic error. Further, to evaluate the effects of serum protein on the results, we measured BDS-CEC of reconstituted serum samples prepared using combinations of five levels of serum proteins with five levels of HDL-C. No significant change in BDS-CEC was observed in any combination. These results indicate that BDS-CEC reflects not only the function of HDL but also its concentration in serum.

Serum amyloid A does not affect high-density lipoprotein cholesterol measurement by a homogeneous assay.

BACKGROUND: Serum amyloid A (SAA), which is one of the acute phase proteins, alters the structure of HDL by associating with it during circulation. We focused on whether SAA influences the values of HDL-cholesterol (HDL-C) measurements when using a homogeneous assay. METHODS: HDLs were isolated by ultracentrifugation from serum samples of 248 patients that were stratified into three groups based on their serum SAA concentrations (low: SAA ≤ 8 µg/mL; middle: 8 < SAA ≤ 100 µg/mL; and high: SAA > 100 µg/mL). HDL-C concentrations of the serum samples measured by the homogeneous assay were compared with the total cholesterol concentrations of HDL fractions isolated by ultracentrifugation. RESULTS: HDLs obtained from patients with low SAA concentrations were separated into their general particle sizes and classified as HDL2 and HDL3 by native-gel electrophoresis. On the other hand, HDLs obtained from patients with high SAA concentrations occasionally showed distributions different from the typical sizes of HDL2 and HDL3, such as extremely small or large particles. Nevertheless, HDL-C concentrations measured using the homogeneous assay were strongly correlated with those measured using the ultracentrifugation method, regardless of the SAA concentrations. However, the ratios of HDL-C concentrations obtained by the homogeneous assay to those obtained by the ultracentrifugation method for patients with high SAA concentrations were significantly lower than those of patients with low SAA concentrations. CONCLUSIONS: A large amount of SAA attached to HDL altered the HDL particle size but did not essentially affect HDL-C measurement by homogeneous assay.

Characterization of the cholesterol efflux of apolipoprotein E-containing high-density lipoprotein in THP-1 cells.

High-density lipoprotein (HDL), also known as antiatherogenic lipoprotein, consists of heterogeneous particles in terms of size, density and composition, suggesting differences among HDL subclasses in characteristics and functions. We investigated the role of apolipoprotein E (apoE)-containing HDL, a minor HDL subclass, in the cholesterol efflux capacity (CEC) of HDL, which is its predominant atheroprotective function. The CEC of apoE-containing HDL was similar to that of apoE-deficient HDL, but the former exhibited a greater rate increase (1.48-fold) compared to that of the latter (1.10-fold) by the stimulation of THP-1 macrophages with the Liver X Receptor (LXR) agonist. No difference in CEC was observed without the LXR agonist between apoA-I, the main apolipoprotein in HDL, and apoE, whereas the increase in CEC in response to treatment with the LXR agonist was greater for apoA-I (4.25-fold) than for apoE (2.22-fold). Furthermore, the increase in the CEC of apoE-containing HDL induced by the LXR agonist was significantly reduced by treatment with glyburide, an inhibitor of ATP-binding cassette transporter A1 (ABCA1). These results suggest that apoE-containing HDL, unlike apoE-deficient HDL, is involved in cholesterol efflux via ABCA1.