Acute effects of oxidized low density lipoprotein on metabolic responses in macrophages

Application of multianalyte microphysiometry to characterize macrophage metabolic responses to oxidized LDL and effects of an apoA-1 mimetic

Although the interaction of macrophages with oxidized low density liopoprotein (oxLDL) is critical to the pathogenesis of atherosclerosis, relatively little is known about their metabolic response to oxLDL. Our development of the multianalyte microphysiometer (MAMP) allows for simultaneous measurement of extracellular metabolic substrates and products in real-time. Here, we use the MAMP to study changes in the metabolic rates of RAW-264.7 cells undergoing respiratory burst in response to oxLDL. These studies indicate that short duration exposure of macrophages to oxLDL results in time-dependent increases in glucose and oxygen consumption and in lactate production and extracellular acidification rate. Since apolipoprotein A-I (apoA-I) and apoA-I mimetics prevent experimental atherosclerosis, we hypothesized that the metabolic response of the macrophage during respiratory burst can be modulated by apoA-I mimetics. We tested this hypothesis by examining the effects of the apoA-I peptide mimetic, L-4F, alone and complexed with 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) on the macrophage metabolic response to oxLDL. L-4F and the DMPC/L-4F complexes attenuated the macrophage respiratory burst in response to oxLDL. The MAMP provides a novel approach for studying macrophage ligand-receptor interactions and cellular metabolism and our results provide new insights into the metabolic effects of oxLDL and mechanism of action of apoA-I mimetics.

Highly upregulated expression of CD36 and MSR1 in circulating monocytes of patients with acute coronary syndromes

Acute Coronary Syndromes (ACS) are a group of disorders caused by the significant reduction of circulation in coronary arteries. The most common reason of the dysfunction is a blood clot formed in place of plaque rupture. The role of scavenger receptors in development and progression of atherosclerosis has been confirmed in many animal experiments, however the knowledge about contribution of the receptors in the development of ACS symptoms in humans still remains insufficient. The aim of this work was to define the expression of two scavenger receptors: CD36 and MSR1 in monocytes of patients with ACS after the onset of symptoms and after the 6 months of treatment. The analysis of CD36 and MSR1 expression was carried out with the use of real-time PCR and flow cytometry. Analyses of lipid and glucose concentration in blood and the level of inflammatory markers in plasma were performed additionally for all ACS patients. All data obtained during the research were analyzed using statistical tests, such as Mann Whitney test, Wilcoxon test, or correlation. In all patients with symptoms of ACS the amount of CD36 and MSR1 mRNA in circulating monocytes, as well as the density of both receptors on the cells surface was significantly higher. Re-analysis of subjects after 6 months of treatment, showed a significant decrease in the CD36 and MSR1 expression in all patients who received atorvastatin. The results of presented studies demonstrate that both investigated receptors are involved in the development and/or progression of ACS.

Spontaneous remodeling of HDL particles at acidic pH enhances their capacity to induce cholesterol efflux from human macrophage foam cells

HDL particles may enter atherosclerotic lesions having an acidic intimal fluid. Therefore, we investigated whether acidic pH would affect their structural and functional properties. For this purpose, HDL(2) and HDL(3) subfractions were incubated for various periods of time at different pH values ranging from 5.5 to 7.5, after which their protein and lipid compositions, size, structure, and cholesterol efflux capacity were analyzed. Incubation of either subfraction at acidic pH induced unfolding of apolipoproteins, which was followed by release of lipid-poor apoA-I and ensuing fusion of the HDL particles. The acidic pH-modified HDL particles exhibited an enhanced ability to promote cholesterol efflux from cholesterol-laden primary human macrophages. Importantly, treatment of the acidic pH-modified HDL with the mast cell-derived protease chymase completely depleted the newly generated lipid-poor apoA-I, and prevented the acidic pH-dependent increase in cholesterol efflux. The above-found pH-dependent structural and functional changes were stronger in HDL(3) than in HDL(2). Spontaneous acidic pH-induced remodeling of mature spherical HDL particles increases HDL-induced cholesterol efflux from macrophage foam cells, and therefore may have atheroprotective effects.

Clinical significance of enzymatic lysophosphatidylcholine (LPC) assay data in patients with sepsis

Lysophosphatidylcholine (LPC) has been suggested to serve as a useful prognostic marker for sepsis. However, existing LPC assays are complicated, time-consuming, and of limited application in real clinical situations. Thus, we investigated the serum LPC levels in sepsis patients using an enzymatic assay and analyzed the correlations between the serum LPC concentration and clinical characteristics. We prospectively collected blood samples from suspected sepsis patients, commencing on day 1 of sepsis. We analyzed all samples using an enzymatic assay. Additionally, we analyzed the serum LPC concentrations in a control group of 21 healthy blood donors. A total of 105 patients who fulfilled the sepsis criteria were included. The mean serum LPC concentration was 43.49 ± 33.09 μmol/L in sepsis patients, which was much lower than that of 21 healthy controls (234.68 ± 30.33 μmol/L, p<0.001). Bacteremic sepsis was associated with a lower serum LPC concentration than non-bacteremic sepsis (34.8 ± 26.85 vs. 49.05 ± 35.63 μmol/L, p<0.05). No difference in serum LPC concentration was evident between survivors and non-survivors. The serum LPC concentration tended to decrease with the severity of sepsis. The day 1 serum LPC concentration was decreased in patients with sepsis, especially when bacteremia was present. However, the serum LPC level did not correlate with disease severity and did not predict mortality from sepsis.

Acidic Extracellular Environments Strongly Impair ABCA1-Mediated Cholesterol Efflux From Human Macrophage Foam Cells

Objective— In the deep microenvironments of advanced human atherosclerotic lesions, the intimal fluid becomes acidic. We examined the effect of an acidic extracellular pH on cholesterol removal (efflux) from primary human macrophages.

Methods and Results— When cholesterol efflux from acetyl-low-density lipoprotein-loaded macrophages to various cholesterol acceptors was evaluated at pH 7.5, 6.5, or 5.5, the lower the pH the more was cholesterol efflux reduced. The reduction of efflux to lipid-free apolipoprotein A-I was stronger than to high-density lipoprotein 2 or to plasma. Cholesterol efflux to every acceptor was severely compromised also at neutral pH when the macrophages had been loaded with cholesterol at acidic pH, or when both loading and efflux were carried out at acidic pH. Compatible with these observations, the typical upregulation of ABCA1 and ABCG1 mRNA levels in macrophages loaded with cholesterol at neutral pH was rapidly attenuated in acidic medium. The secondary structure of apolipoprotein A-I did not changed over the pH range studied, supporting the notion that the inhibitory effect of acidic pH on cholesterol efflux rather impaired the ability of the foam cells to facilitate ABCA1-mediated cholesterol release. Secretion of apolipoprotein E from the foam cells was fully inhibited when the pH was 5.5, which further reduced cholesterol efflux.

Conclusion— An acidic pH reduces cholesterol efflux via different pathways and particularly impairs the function of the ABCA1 transporter. The pH-sensitive function of human macrophage foam cells in releasing cholesterol may accelerate lipid accumulation in deep areas of advanced atherosclerotic plaques where the intimal fluid is acidic.

Effects of adrenergic agents on rat peritoneal macrophages activated in vitro by acetylated low-density lipoprotein

Macrophages interact with modified lipoproteins and alter their functional status. In this study, the effects of the adrenergic agents adrenaline, isoproterenol, and dobutamine on macrophages activated with acetylated low-density lipoprotein were tested. The aim of this investigation was to determine whether adrenergic agents influence biologically significant functions of these cells in an in vitro model of macrophage-lipoprotein acute interaction. Rat peritoneal macrophages were incubated with acetylated low-density lipoprotein for 16 h, with or without adrenergic agents. Hydrogen peroxide and nitric oxide production and acid phosphatase activities in the supernatant and cell lysate were assayed. Adrenaline and isoproterenol inhibited the production of hydrogen peroxide, stimulated the production of nitric oxide, and increased the extracellular activity of acid phosphatase in the lipoprotein-activated cells. Dobutamine increased the extracellular, but decreased the intracellular acid phosphatase activity. Adrenaline and dobutamine also produced changes in the cell size and nuclear morphology of the macrophages. Macrophages are activated in vitro by acetylated low-density lipoprotein, and their functions and morphology are modified under the influence of adrenergic agents. Certain effects could be attributed to changes in cyclic AMP levels.

A glance at G-protein-coupled receptors for lipid mediators: a growing receptor family with remarkably diverse ligands

A plethora of lipid-like molecules known to act as intracellular second messengers are now recognized to signal cells through plasma membrane 7 transmembrane G-protein-coupled receptors (GPCRs). This has been the result of a decade-long genetic hunt for novel sequences encoding 7 transmembrane receptor proteins and the efforts to pair novel sequences with biologically active substances of (partly) unknown molecular mechanism of action. Identification of novel GPCR ligand pairs represents the first step to shed more light into the mode of action of novel cellular signaling molecules in human health and disease and might represent a fruitful source for the development of new drugs, judged on the successful history of GPCR as drug targets. Since 2000, more than 16 reports became available on lipid mediators--as diverse as lysophospholipids, arachidonic acid metabolites, short-, medium-, and long-chain fatty acids as well as steroid-like molecules--exerting their effects as extracellular mediators via rhodopsin-like family GPCRs. These reports have opened new avenues for research in human lipid receptor physiology and pharmacology. Here, the current knowledge on the recently deorphanized lipid receptors, including their isolation, expression pattern, function, and possible physiological or pathological roles will be reviewed.

The pivotal role of scavenger receptor CD36 and phagocyte-derived oxidants in oxidized low density lipoprotein-induced adhesion to endothelial cells

Adhesion of phagocytes to endothelial cells constitutes a crucial step in atherogenesis. Oxidized low density lipoproteins (LDL) are supposed to facilitate the adhesion process. We investigated the molecular mechanisms by which mildly and extensively hypochlorite-oxidized LDL force adhesion of murine macrophages and human neutrophils to human umbilical venous endothelial cells. After 1h of co-incubation of macrophages, endothelial cells, and lipoproteins adhesion significantly increased to 160+/-13% (S.E.M., n=5) in the presence of mildly oxidized lipoprotein, and 210+/-11% (S.E.M., n=5) in the presence of extensively oxidized lipoprotein. Similar results were obtained with neutrophils. CD36 antibody (FA6-152) significantly reduced adhesion to 102+/-7% (S.E.M., n=5) using mildly oxidized low density lipoprotein and to 179+/-16% (S.E.M., n=5) using extensively oxidized low density lipoprotein. Native high density lipoprotein and to a lesser extent methionine-oxidized high density lipoprotein significantly counteracted the effects of low density lipoprotein. Prior incubation of endothelial cells with modified lipoproteins followed by their removal and subsequent incubation with macrophages or neutrophils resulted in only minor changes of adhesion. This suggests that the direct contact of low density lipoprotein with phagocytes followed by activation of a respiratory burst with release of reactive oxygen species facilitates the adhesion process. Accordingly, the addition of antioxidants (superoxide dismutase and catalase) to the co-incubation medium was followed by a significant decrease in phagocyte adhesion. It is concluded that oxidized low density lipoprotein-induced respiratory burst activation of phagocytes with subsequent release of oxidants constitutes a crucial step in promoting the adhesion of phagocytes to endothelial cells.

Therapeutic effects of lysophosphatidylcholine in experimental sepsis

Sepsis represents a major cause of death in intensive care units. Here we show that administration of lysophosphatidylcholine (LPC), an endogenous lysophospholipid, protected mice against lethality after cecal ligation and puncture (CLP) or intraperitoneal injection of Escherichia coli. In vivo treatment with LPC markedly enhanced clearance of intraperitoneal bacteria and blocked CLP-induced deactivation of neutrophils. In vitro, LPC increased bactericidal activity of neutrophils, but not macrophages, by enhancing H(2)O(2) production in neutrophils that ingested E. coli. Incubation with an antibody to the LPC receptor, G2A, inhibited LPC-induced protection from CLP lethality and inhibited the effects of LPC in neutrophils. G2A-specific antibody also blocked the inhibitory effects of LPC on certain actions of lipopolysaccharides (LPS), including lethality and the release of tumor necrosis factor-alpha (TNF-alpha) from neutrophils. These results suggest that LPC can effectively prevent and treat sepsis and microbial infections.

pH Heterogeneity of human and rabbit atherosclerotic plaques; a new insight into detection of vulnerable plaque

BACKGROUND

Atherosclerotic plaques are heterogeneous with respect to inflammation, calcification, vascularity, oxygen, and temperature. We hypothesized that they also vary in pH and measured pH in living human carotid endarterectomized atherosclerotic plaques (CEA), Watanabe heritable hyperlipidemic (WHHL) rabbit aortas and human umbilical arteries (HUA).

METHODS AND RESULTS

We measured pH of CEA of 48 patients, nine WHHL rabbit aortas and 11 HUA specimens (as controls) using a glass type microelectrode mounted on a micromanipulator in a 37 degrees C incubator. We also used single emission and also dual emission fluorescence ratio imaging microscopy employing pH-sensitive probes to confirm pH heterogeneity. Mean pH measured at 415 points of CEA was 7.55+/-0.32; at 275 points of WHHL rabbit aortas it was 7.40+/-0.43; and in 233 points of HUA it was 7.24+/-0.1. In CEA, pH of yellow (lipid-rich) areas was significantly lower than pH in calcified areas (7.15+/-0.01 vs. 7.73+/-0.01, P<0.0001). The coefficients of variation (heterogeneity) of pH in CEA, WHHL rabbit aortas, and HUA were 0.038+/-0.010, 0.039+/-0.007, and 0.009+/-0.003, respectively (P=0.0001). Fluorescence microscopic imaging confirmed pH heterogeneity in both humans and rabbits but not in HUA. In a variance components analysis 82% of the heterogeneity was due to the within-plaque variation and 2% was attributable to between-plaque variation.

CONCLUSIONS

Our findings support the hypothesis of pH heterogeneity in plaques, and suggest a possible role for detecting low pH in the detection of plaque vulnerability. The source of pH heterogeneity particularly acidic pH, its impact on the stability of plaques and its potential clinical utility in locating vulnerable plaques remain to be evaluated.

Continuous measurement of glucose utilization in heart myoblasts

Understanding quantitative aspects of cell energy metabolism and how it is influenced by environment is central to biology, medicine, and biotechnology. Most methods used for measuring metabolic fluxes associated with energy metabolism require considerable personnel effort or high maintenance instrumentation. The microphysiometer is a commercially available instrument that measures acid extrusion rates, which are commonly used for drug screening. With the addition of oxygen sensors, the instrument can also be used to measure cell oxygen consumption rates and thereby calculate glycolytic fluxes. In the work described here, oxygen consumption and acid extrusion rates were used to measure glucose utilization by the H9c2 rat heart myoblast cell line and these results are compared with fluxes measured with a radiometric assay. Both assays were used to investigate changes in H9c2 energy metabolism due to cell stimulation with carbachol and insulin. The results demonstrate the utility of the microphysiometer method for measuring both transient and sustained changes in partitioning of glucose utilization between glycolysis and oxidation in live cells.

A product of growth arrest-specific gene 6 modulates scavenger receptor expression in human vascular smooth muscle cells

Although Gas6 is identified as a growth factor for vascular smooth muscle cells (VSMCs), its roles in these cells have not been clearly elucidated. To examine the role of Gas6 in atherosclerosis, we examined the effects of Gas6 on scavenger receptor family expression in VSMCs. Scavenger receptor class A, one of the scavenger receptor family members, was upregulated in VSMCs by Gas6. Furthermore, the atherogenic lipoprotein, oxidized LDL, induced Gas6 production in these cells. These results indicate that Gas6 plays an important role in foam cell formation in human VSMCs.

Prostaglandin f2alpha treatment in vivo, but not in vitro, stimulates protein kinase C-activated superoxide production by nonsteroidogenic cells of the rat corpus luteum

Luteal regression is associated with the generation of reactive oxygen species (ROS). To determine the nature of the ROS generator, cells isolated from luteinized rat ovaries were examined for ROS production using luminol-amplified chemiluminescence (LCL). Cells cultured for 2-48 h exhibited minimal LCL, but there was a significant (30- to 50-fold), rapid (maximum at 3-5 min), and dose-dependent increase in LCL in response to phorbol ester (phorbol 12-myristate 13-acetate; TPA; ED50 = 0.03 microM) and diacylglycerol (1,2-dioctanoyl-glycerol; ED50 = 30 microM). The TPA-induced response was cell number dependent and was virtually abolished by superoxide dismutase, freezing, or heating (95 degrees C for 5 min). Zymosan, known to induce a phagocytic response in leukocytes, stimulated a superoxide (O2-.) response with a slow onset (maximum at 40 to 60 min) and a maximum about one third of that observed for TPA. The response to TPA and zymosan was inhibited by the NADPH/NADH-oxidase inhibitor, diphenylene iodonium (ID50 = 5 microM for TPA), but not by the mitochondrial inhibitors, potassium cyanide, rotenone, or sodium azide. Fractionation of cells by centrifugal elutriation showed that TPA-stimulated O2-. production coeluted with the nonsteroidogenic cells and that little, if any, O2-. generation coeluted with the steroidogenic cells. Cells isolated 1, 2, and 4 h after in vivo treatment with a luteolytic dose of prostaglandin F2alpha (PGF2alpha) showed a significant increase in TPA-stimulated O2-. production at 2 h, whereas luteal cells or corpora lutea incubated directly with 1 microM PGF2alpha did not show any increase in response. Corpora lutea isolated from naturally regressed ovaries (18 days after ovulation) showed a significantly elevated level of TPA-stimulated O2-. production. In conclusion, there is a superoxide generator in luteinized ovaries that is activated through a protein kinase C pathway, localized in nonsteroidogenic cells, transiently increased during PGF2alpha-induced luteolysis in vivo, and elevated during natural luteal regression.