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.

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.

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.

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.