These findings suggest that abnormal HDL capacity to mediate cholesterol efflux is a key driver of excess CVD in patients on chronic hemodialysis and may explain why statins have limited effect to decrease CV events. The findings also suggest cellular cholesterol transporters as potential therapeutic targets to decrease CV risk in this population.

Macrophages incubated with mildly oxidized low density lipoprotein (OxLDL), aggregated low density lipoprotein (AggLDL), or cholesteryl ester-rich lipid dispersions (DISPs) accumulate cholesterol in lysosomes followed by an inhibition of lysosomal cholesteryl ester (CE) hydrolysis.

Peroxidation of plasma lipoproteins has been implicated in the endothelial cell activation and monocyte adhesion that initiate atherosclerosis, but the exact mechanisms underlying this activation remain unclear. Lipid peroxidation generates lipid aldehydes, including the ␥-ketoaldehydes (␥KA), also termed isoketals or isolevuglandins, that readily modify the amine headgroup of phosphatidylethanolamine (PE). We hypothesized that aldehyde modification of PE could mediate some of the proinflammatory effects of lipid peroxidation. We found that PE modified by ␥KA (␥KA-PE) induced THP-1 monocyte adhesion to human umbilical cord endothelial cells. ␥KA-PE also induced expression of adhesion molecules and increased MCP-1 and IL-8 mRNA in human umbilical cord endothelial cells. To determine the structural requirements for ␥KA-PE activity, we tested several related compounds. PE modified by 4-oxo-pentanal induced THP-1 adhesion, but N-glutaroyl-PE and C 18:0 N-acyl-PE did not, suggesting that an N-pyrrole moiety was essential for cellular activity. As the N-pyrrole headgroup might distort the membrane, we tested the effect of the pyrrolePEs on membrane parameters. ␥KA-PE and 4-oxo-pentanal significantly reduced the temperature for the liquid crystalline to hexagonal phase transition in artificial bilayers, suggesting that these pyrrole-PE markedly altered membrane curvature. Additionally, fluorescently labeled ␥KA-PE rapidly internalized to the endoplasmic reticulum (ER); ␥KA-PE induced C/EBP homologous protein CHOP and BiP expression and p38 MAPK activity, and inhibitors of ER stress reduced ␥KA-PE-induced C/EBP homologous protein CHOP and BiP expression as well as EC activation, consistent with ␥KA-PE inducing ER stress responses that have been previously linked to inflammatory chemokine expression. Thus, ␥KA-PE is a potential mediator of the inflammation induced by lipid peroxidation.Lipid peroxidation has been implicated in a host of pathological processes, including inflammation and atherosclerosis, but there is still much that is unknown about the mechanisms linking lipid peroxidation to the activation of inflammatory responses. Lipid peroxidation produces a plethora of lipid aldehydes ( Fig. 1), including malondialdehyde, acrolein, and 4-hydroxynonenal (HNE).3 It also produces a large family of ␥-ketoaldehydes (␥KA), regioisomers that have been given the trivial name of isoketals or isolevuglandins. Exposure of vascular cells to various aldehydes results in endothelial dysfunction, secretion of cytokines, and recruitment of monocytes (1-7), all of which are key steps in the initiation of the chronic inflammatory conditions leading to atherosclerosis.Despite these potentially important inflammatory effects, the mechanisms whereby lipid aldehydes induce their effects on vascular cells are still poorly understood. Significant effort has focused on the effects of protein and DNA modification by lipid aldehydes. However, recent studies have shown that lipid aldehydes, including malondialdehyde, acrolein, HNE, and ␥KA, a...

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Human macrophages incubated for prolonged periods with mildly oxidized LDL (oxLDL) or cholesteryl ester-rich lipid dispersions (DISP) accumulate free and esterified cholesterol within large, swollen lysosomes similar to those in foam cells of atherosclerosis. The cholesteryl ester (CE) accumulation is, in part, the result of inhibition of lysosomal hydrolysis due to increased lysosomal pH mediated by excessive lysosomal free cholesterol (FC). To determine if the inhibition of hydrolysis was long lived and further define the extent of the lysosomal defect, we incubated THP-1 macrophages with oxLDL or DISP to produce lysosome sterol engorgement and then chased with acetylated LDL (acLDL). Unlike oxLDL or DISP, CE from acLDL normally is hydrolyzed rapidly. Three days of incubation with oxLDL or DISP produced an excess of CE in lipid-engorged lysosomes, indicative of inhibition. After prolonged oxLDL or DISP pretreatment, subsequent hydrolysis of acLDL CE was inhibited. Coincident with the inhibition, the lipid-engorged lysosomes failed to maintain an acidic pH during both the initial pretreatment and subsequent acLDL incubation. This indicates that the alterations in lysosomes were general, long lived, and affected subsequent lipoprotein metabolism. This same phenomenon, occurring within atherosclerotic foam cells, could significantly affect lesion progression.

Mice deficient in scavenger receptor class B type I (SR-BI) and apolipoprotein E (apoE) [double knockout (DKO) mice] develop dyslipidemia, accelerated atherosclerosis, and myocardial infarction, and die prematurely. We examined effects of apoE and SR-BI deficiency on macrophage cholesterol homeostasis. DKO macrophages had increased total cholesterol (TC) stores (220-380 mg/mg protein) compared with apoE 2/2 cells (40 mg/mg), showed significant lysosomal lipid engorgement, and increased their TC by 34% after exposure to HDL. DKO macrophages from apoE 2/2 mice reconstituted with DKO bone marrow showed less cholesterol accumulation (89 mg/mg), suggesting that the dyslipidemia of DKO mice explains part of the cellular cholesterol defect. However, analyses of DKO and apoE 2/2 macrophages from transplanted apoE 2/2 mice revealed a role for macrophage SR-BI, inasmuch as the TC in DKO macrophages increased by 10% in the presence of HDL, whereas apoE 2/2 macrophage TC decreased by 33%. After incubation with HDL, the free cholesterol (FC) increased by 29% in DKO macrophages, and decreased by 8% in apoE 2/2 cells, and only DKO cells had FC in large peri-nuclear pools. Similar trends were observed with apoA-I as an acceptor. Thus, the abnormal cholesterol homeostasis of DKO macrophages is due to the plasma lipid environment of DKO mice and to altered trafficking of macrophage cholesterol. Both factors are likely to contribute to the accelerated atherosclerosis in DKO mice.-

Free radical-induced oxidation products of polyunsaturated fatty acids esterified to phospholipids have been implicated in a number of human diseases including atherosclerosis and neurodegenerative diseases. Some of these phospholipid oxidation products have potent biological activities and likely contribute to human pathophysiological conditions. Oxidation products have also been used as markers of oxidative stress in vivo. Identification and quantification of phospholipid oxidation products are often performed by analyzing the oxidized free fatty acid moieties after hydrolysis from the phospholipids head groups by gas chromatography-mass spectrometry (GC-MS) or liquid chromatography-mass spectrometry (LC-MS). We now describe the definitive identification of intact oxidized products of glycerophospholipids including glycerophosphatidylcholine (GPC), glycerophosphatidylethanolamine (GPE), and glycerophosphatidylserine (GPS) in vitro and in vivo using iontrap MS. For these analyses, the negative ions of the oxidation products of phospholipids are fragmented to MS n and unequivocal structural characterization is obtained based on collision-induced dissociation (CID) of the sn-2 carboxylate ion. This technique overcomes the need to hydrolyze fatty acids from phospholipids in the analysis. The method has been used to identify a number of oxidation products of glycerophospholipids including hydroxyeicosatetraenoates (HETEs) and isoprostanes (IsoPs) esterified to different classes of glycerophospholipids in vitro and in vivo. These studies thus provide a new approach to identify the intact oxidation products of glycerolphospholipids.

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