Inhibition of LPL Expression in Human Monocyte–Derived Macrophages Is Dependent on LDL Oxidation State

Abstract

Abstract-The regulation of macrophage lipoprotein lipase (LPL) secretion and mRNA expression by atherogenic lipoproteins is of critical relevance to foam cell formation. LPL is present in arterial lesions and constitutes a bridging ligand between lipoproteins, proteoglycans, and cell receptors, thus favoring macrophage lipoprotein uptake and lipid accumulation. We investigated the effects of native and of oxidized lipoproteins on the expression of LPL in an in vitro human monocyte-macrophage system. Exposure of mature macrophages (day 12) to highly copper-oxidized human low density lipoprotein (LDL) (100 g protein per milliliter) led to marked reduction in the expression of LPL activity (Ϫ62%, PϽ0.01) and mRNA level (Ϫ47%, PϽ0.05); native LDL, acetylated LDL, and LDL oxidized for Ͻ6 hours were without effect. The reduction in LPL activity became significant at a threshold of 6 hours of LDL oxidation (Ϫ31%, PϽ0.05). Among the biologically active sterols formed during LDL oxidation, only 7␤-hydroxycholesterol (5 g/mL) induced a minor reduction in macrophage LPL activity, whereas 25-hydroxycholesterol was without effect. By contrast, lysophosphatidylcholine, whose LDL content increased in parallel with the degree of oxidation, induced significant reductions in LPL activity and mRNA levels at concentrations of 2 to 20 mol/L (Ϫ34% to Ϫ53%, PϽ0.01).Our results demonstrate that highly oxidized LDL (Ͼ6-hour oxidation) exerts negative feedback on LPL secretion in human monocytes-macrophages via a reduction in mRNA levels. By contrast, native LDL and mildly oxidized LDL (Ͻ6-hour oxidation) did not exert a feedback effect on LPL expression. We speculate that the content of lysophosphatidylcholine and, to a lesser degree, of 7␤-hydroxycholesterol in oxidized LDLs is responsible for the downregulation of LPL activity and mRNA abundance in human monocyte-derived macrophages and may therefore modulate LPL-mediated pathways of lipoprotein uptake during conversion of macrophages to foam cells. 2-5The mechanism of the conversion of macrophages to foam cells containing large amounts of cholesterol, cholesteryl esters, and TGs is presently the subject of extensive investigation. 6 The uptake of lipoproteins by such cells involves several receptors of distinct specificity, including scavenger receptors, 7 CD36, 8 and Fc receptors 9 for oxLDL; the LDL receptor; the LDL receptor-related protein receptor 10,11 ; the VLDL receptor for TG-rich lipoproteins 12 ; and last, membrane-binding proteins for uptake of certain TG-rich lipoproteins. 13 These cellular uptake mechanisms are rendered complex by the contribution of additional factors such as heparan sulfate proteoglycans of the matrix and cell plasma membranes and equally by LPL, 14 which may serve to "anchor" lipoproteins to the cell surface.LPL is a key enzyme of lipoprotein metabolism that hydrolyzes the TG component of chylomicrons and VLDL. 15 Moreover, LPL can act as a ligand to create a "bridge" between TG-rich lipoproteins and several receptors of the LDL family.16 LPL may al...

Inhibition of Lipopolysaccharide-Stimulated Chronic Obstructive Pulmonary Disease Macrophage Inflammatory Gene Expression by Dexamethasone and the p38 Mitogen-Activated Protein Kinase Inhibitor N-cyano-N′-(2-{[8-(2,6-difluorophenyl)-4-(4-fluoro-2-methylphenyl)-7-oxo-7,8-dihydropyrido[2,3-d] pyrimidin-2-yl]amino}ethyl)guanidine (SB706504)

“…These cultures have been shown previously to consist of entirely CD68ϩ macrophages (Stengel et al, 1998), and we obtained similar data in MDMs from three subjects using flow cytometry (data not shown). Media were removed from MDMs before the addition of drugs (1 ml/well), 100 nM dexamethasone (Sigma-Aldrich, St. Louis, MO), and/or 3 M SB706504 (gift from GlaxoSmithKline) previously stored in DMSO.…”

“…Peripheral blood mononuclear cells were isolated from whole blood as described previously (Stengel et al, 1998;Kent et al, 2008). These cultures have been shown previously to consist of entirely CD68ϩ macrophages (Stengel et al, 1998), and we obtained similar data in MDMs from three subjects using flow cytometry (data not shown).…”

Adipocyte-Derived Plasma Protein, Adiponectin, Suppresses Lipid Accumulation and Class A Scavenger Receptor Expression in Human Monocyte-Derived Macrophages

“…In addition, adiponectin decreased the steady-state mRNA levels of class A MSR and LPL mRNA without altering those of apoE. Because class A MSR, LPL, and apoE reportedly increase during monocyte-to-macrophage differentiation, 29,31,32 adiponectin may partially modulate macrophage differentiation and affect lipid accumulation in the macrophages. The promoter assay of class A MSR revealed that adiponectin suppressed the class A MSR expression at the transcriptional level, although further studies are needed to clarify the precise transcriptional regulatory mechanism by which adiponectin decreases the class A MSR expression.…”

mTOR Inhibition Induces Endothelial Progenitor Cell Death

“…After weeks in culture, these cells grow to confluence, as described (17). The effects of immunosuppressants were also assessed in these late outgrowth EPC, human aortic endothelial cells (HAEC, Clonetics, Cambrex Bioscience) and in macrophages generated from the same initial population of PBMC (18).…”