Plasma Membrane Lipids: An Important Binding Site for All Lipoprotein Classes.

Cholesterol is one of the main constituents of plasma membranes; thus, its supply is of utmost importance. This review covers the known mechanisms of cholesterol transfer from circulating lipoprotein particles to the plasma membrane, and vice versa. To achieve homeostasis, the human body utilizes cellular de novo synthesis and extracellular transport particles for supply of cholesterol and other lipids via the blood stream. These lipoprotein particles can be classified according to their density: chylomicrons, very low, low, and high-density lipoprotein (VLDL, LDL, and HDL, respectively). They deliver and receive their lipid loads, most importantly cholesterol, to and from cells by several redundant routes. Defects in one of these pathways (e.g., due to mutations in receptors) usually are not immediately fatal. Several redundant pathways, at least temporarily, compensate for the loss of one or more of them, but the defects trigger systemic diseases, such as atherosclerosis later on. Recently, intracellular membrane-membrane contact sites were shown to be involved in intracellular cholesterol transfer and the plasma membrane itself has been proposed to act as a binding site for lipoprotein-mediated cargo unloading.

Cholesterol transfer at the plasma membrane.

Cholesterol homeostasis is of central importance for life. Therefore, cells have developed a divergent set of pathways to meet their cholesterol needs. In this review, we focus on the direct transfer of cholesterol from lipoprotein particles to the cell membrane. More molecular details on the transfer of lipoprotein-derived lipids were gained by recent studies using phospholipid bilayers. While amphiphilic lipids are transferred right after contact of the lipoprotein particle with the membrane, the transfer of core lipids is restricted. Amphiphilic lipid transfer gains special importance in genetic diseases impairing lipoprotein metabolism like familial hypercholesterolemia. Taken together, these data indicate that there is a constant exchange of amphiphilic lipids between lipoprotein particles and the cell membrane.

Serum and Lipoprotein Particle miRNA Profile in Uremia Patients.

microRNAs (miRNAs) are post-transcriptional regulators of messenger RNA (mRNA), and transported through the whole organism by-but not limited to-lipoprotein particles. Here, we address the miRNA profile in serum and lipoprotein particles of healthy individuals in comparison with patients with uremia. Moreover, we quantitatively determined the cellular lipoprotein-particle-uptake dependence on the density of lipoprotein particle receptors and present a method for enhancement of the transfer efficiency. We observed a significant increase of the cellular miRNA level using reconstituted high-density lipoprotein (HDL) particles artificially loaded with miRNA, whereas incubation with native HDL particles yielded no measurable effect. Thus, we conclude that no relevant effect of lipoprotein-particle-mediated miRNA-transfer exists under in vivo conditions though the miRNA profile of lipoprotein particles can be used as a diagnostic marker.

Effect of chronic kidney disease on macrophage cholesterol efflux.

AIMS: Patients with chronic kidney disease (CKD) have a high risk to develop atherosclerosis. The capacity of high-density lipoproteins (HDL) or serum to accept cholesterol from macrophages and the capacity of macrophages to export excess cholesterol are critical for the atheroprotective role of reverse cholesterol transport. HDL cholesterol acceptor capacity was reported to be decreased in middle aged hemodialysis patients, but the role of confounding factors remains unclear. MAIN METHODS: We measured the cholesterol acceptor capacity (CAC) of HDL or serum in 12 pediatric and 17 young adult patients with CKD stages 3-5, 14 young adult hemodialysis patients and 15 adult renal transplant recipients without associated diseases and matched controls using THP-1 macrophages. Moreover we studied the cholesterol export capacity (CEC) of patients' monocyte-derived macrophages (HMDMs) to control serum or HDL. KEY FINDINGS: In adults with CKD stages 3-5 serum CAC was slightly increased, whereas CEC of HMDMs was unaltered in both, adult and pediatric patients. In hemodialysis patients, however, serum CAC was markedly reduced to 85±11% of control (p<0.001), presumably due to low serum apolipoprotein A-I. Interestingly, CEC of HMDMs from dialysis patients was increased. In transplant patients no alterations were found. SIGNIFICANCE: CKD without hemodialysis does not reduce cholesterol export from macrophages. Hemodialysis patients might benefit from therapies aiming to restore serum CAC by increasing apolipoprotein A-I. The enhanced export of cholesterol by HMDMs from dialysis patients may represent an adaptive response.

Profound Changes in Sex Hormone Levels during Cross-Sex Hormone Therapy of Transsexuals do not Alter Serum Cholesterol Acceptor Capacity.

INTRODUCTION: Men and postmenopausal women exhibit a higher risk for atherosclerosis than premenopausal women. These differences were often attributed to sex steroids, but the role of estrogen and testosterone in atherosclerosis are more complex than anticipated. Cross-sex hormone therapy of transsexuals is an interesting model, which has been used to study hormonal effects on serum lipid profile, insulin resistance, and body composition. However, studies on macrophage cholesterol efflux, the first step in reverse cholesterol transport, are not available. AIM: The aim of this study was to evaluate the effect of cross-sex hormone therapy in transsexuals on the capacity of serum to accept cholesterol from macrophages. METHODS: Cholesterol acceptor capacity (CAC) of serum from transsexuals before and after at least 6 months of hormone treatment was measured using macrophage tissue culture models. MAIN OUTCOME MEASURES: CAC of serum using the human acute monocytic leukemia cell line (THP-1 cells). RESULTS: Unexpectedly, the CAC of serum from male to female (MtF) transsexuals was not increased, but decreased after hormone therapy. Serum from female to male (FtM) transsexuals showed no changes in CAC. CONCLUSIONS: Despite drastic changes in hormone status, no increase in CAC was detected in MtF patients, and no alteration in CAC was seen in FtM patients. These data further challenge the traditional view that estrogen and testosterone exert beneficial and detrimental effects, respectively, on lipoprotein metabolism and ultimately atherosclerosis.

Combined light and electron microscopy using diaminobenzidine photooxidation to monitor trafficking of lipids derived from lipoprotein particles.

Diaminobenzidine (DAB) photooxidation is a method for conversion of fluorescent signals into electron-dense precipitates that are visible in the electron microscope. Recently, we have applied this method to analyze organelles involved in holo-high density lipoprotein (HDL) particle uptake at the ultrastructural level. In the present work we extended the spectrum of molecules visualized via photooxidation to monitor the uptake of HDL-derived lipids in HepG2 cells. By the combined light-electron microscopic method and with the aid of the DAB photooxidation technique, it became possible for the first time to visualize different intracellular pathways of lipoprotein particle-derived lipids and analyze the compartments involved at the ultrastructural level. HDL-Alexa 568 was used to visualize holo-HDL particle uptake. Reconstituted HDL particles containing the fluorescent cholesterol analogues Bodipy-cholesterol, Bodipy-cholesteryl oleate, or cholesteryl Bodipy-ester were used to visualize uptake of the HDL-associated sterol. In Bodipy-cholesteryl oleate and cholesteryl Bodipy-ester, the cholesterol moiety or the fatty acid moiety is fluorescently labeled, respectively; in contrast, Bodipy-cholesterol is an analogue of free cholesterol. The cellular compartments involved in their intracellular routes after uptake were analyzed in the fluorescence and electron microscope after DAB photooxidation. Bodipy-cholesterol was found to be localized in tubular endosomes and multivesicular bodies (MVBs), in the trans-Golgi network, and in stacked Golgi cisternae. In contrast, HepG2 cells incubated with HDL containing Bodipy-cholesteryl oleate or cholesteryl Bodipyester gave an uptake pattern comparable to that of holo-HDL particles, with MVBs being involved. Bodipy-cholesteryl oleate was also found in lysosomes. These results indicate that HDL-derived cholesterol and cholesteryl ester are transported by different intracellular pathways in HepG2 cells. Thus, the DAB photooxidation method enables the analysis of intracellular transport of lipoprotein particle-derived lipids at the light and at the ultrastructural level.

Effect of obesity on plasma clusterin, [corrected] a proposed modulator of leptin action.

Clusterin, a protein constituent of HDL, was recently shown to bind plasma leptin in vitro and has been proposed to modulate leptin activity. To gain insight into a possible role for plasma clusterin in human obesity, we measured plasma clusterin, leptin, soluble leptin receptor (sObR), and lipoproteins in 70 obese adolescents (12.4 ± 1.6 y; BMI-SD score (SDS-BMI) 2.35 ± 0.47) before and after 3 wk of weight reduction in a dietary camp and in 44 normal weight controls. Binding of plasma leptin to HDL or clusterin was studied using ultracentrifugation and immunoaffinity chromatography. During weight reduction, clusterin decreased from 14.6 ± 4.1 to 10.3 ± 2.9 mg/dL, p < 0.001) in obese adolescents, whereas sObR increased. However, baseline plasma clusterin in obese adolescents did not differ from controls. Clusterin did not correlate with SDS-BMI, weight loss, leptin, or lipoproteins. Only ∼ 1% of plasma leptin was associated with clusterin/apoA-I complexes or with HDL. Our results do not support a role for plasma clusterin as an important leptin-binding protein or modulator of leptin action. The decrease of plasma clusterin during weight reduction may be an effect of the hypocaloric diet rather than being directly linked to weight loss.

Scavenger receptor, Class B, Type I provides an alternative means for beta-VLDL uptake independent of the LDL receptor in tissue culture.

Recent evidence suggests that scavenger receptor, class B, type I (SR-BI) plays a physiological role in VLDL metabolism. SR-BI was reported to mediate beta-VLDL uptake; however, cellular details of this process are not well characterized. In the present study we show that SR-BI delivers cholesterol derived from beta-VLDL to LDL receptor negative SR-BI over-expressing Chinese Hamster Ovarian cells (ldlA7-SRBI). Cell association of beta-VLDL was approximately 3 times higher after SR-BI over-expression, which was competed by beta-VLDL, but only to a lesser extent by HDL and LDL. Almost all of the associated beta-VLDL was located intracellularly, and therefore could not be released by a 50-fold excess of unlabeled beta-VLDL. beta-VLDL was degraded at a rate of 6 ng beta-VLDL/mg cell protein and hour. In contrast to ldlA7 cells, beta-VLDL association was competed by LDL in cells with a functional LDL receptor like CHO and HepG2 cells, indicating a strong impact of the LDL receptor in beta-VLDL uptake. beta-VLDL degradation was similar to ldlA7-SRBI cells. When beta-VLDL uptake was followed using fluorescence microscopy, beta-VLDL showed a different uptake pattern in SR-BI over-expressing cells, ldlA7-SRBI, compared to LDL receptor containing cells, CHO and HepG2.

Characterization of endocytic compartments after holo-high density lipoprotein particle uptake in HepG2 cells.

Holo-high density lipoprotein (HDL) particle uptake, besides selective lipid uptake, constitutes an alternative pathway to regulate cellular cholesterol homeostasis. In the current study, the cellular path of holo-HDL particles was investigated in human liver carcinoma cells (HepG2) using combined light and electron microscopical methods. The apolipoprotein moiety of HDL was visualized with different markers: horseradish peroxidase, colloidal gold and the fluorochrome Alexa(568), used in fluorescence microscopy and after photooxidation correlatively at the ultrastructural level. Time course experiments showed a rapid uptake of holo-HDL particles, an accumulation in endosomal compartments, with a plateau after 1-2 h of continuous uptake, and a clearance 1-2 h upon replacement by unlabeled HDL. Correlative microscopy, using HDL-Alexa(568)-driven diaminobenzidine (DAB) photooxidation, identified the fluorescent organelles as DAB-positive multivesicular bodies (MVBs) in the electron microscope; their luminal contents but not the internal vesicles were stained. Labeled MVBs increased in numbers and changed shapes along with the duration of uptake, from polymorphic organelles with multiple surface domains and differently shaped protrusions dominating at early times of uptake to compact bodies with mainly tubular appendices and densely packed vesicles after later times. Differently shaped and labeled surface domains and appendices, as revealed by three dimensional reconstructions, as well as images of homotypic fusions indicate the dynamics of the HDL-positive MVBs. Double staining visualized by confocal microscopy, along with the electron microscopic data, shows that holo-HDL particles after temporal storage in MVBs are only to a minor degree transported to lysosomes, which suggests that different mechanisms are involved in cellular HDL clearance, including resecretion.