Constitutive Receptor-independent Low Density Lipoprotein Uptake and Cholesterol Accumulation by Macrophages Differentiated from Human Monocytes with Macrophage-Colony-stimulating Factor (M-CSF)

Low concentrations of oxidized low density lipoprotein (OxLDL) are cytoprotective for phagocytes, although the underlying mechanisms remain unclear. We investigated signaling pathways used by OxLDL to attenuate apoptosis in monocytic cells. OxLDL at 25-50 mg/ml inhibited staurosporine-induced apoptosis in THP-1 cells and mouse peritoneal macrophages, and it was cytoprotective in human primary monocytes upon serum withdrawal. Attenuated cell demise was reversed by blocking extracellular signal-regulated kinase (ERK) signaling. Translocation of cytochrome c to the cytosol was attenuated by OxLDL, which again demanded ERK signaling. Analysis of Bcl-2 family proteins revealed phosphorylation of Bad at serine 112 as well as ERK-dependent inhibition of Mcl-1 degradation. Although the formation of reactive oxygen species (ROS) is an established signal generated by OxLDL, ROS scavengers did not interfere with cell protection by OxLDL. Thus, activation of the ERK signaling pathway by OxLDL is important to protect phagocytes from apoptosis.-Namgaladze, D., A. Kollas, and B. Brü ne. Oxidized LDL attenuates apoptosis in monocytic cells by activating ERK signaling. J. Lipid Res. 2008. 49: 58-65.

Section: Discussionmentioning

confidence: 82%

Oxidized LDL attenuates apoptosis in monocytic cells by activating ERK signaling

Namgaladze

Kollas

Brüne

2008

“…Under this condition, z70% of LDL cholesterol internalization could be inhibited by the addition of cold LDL, indicating its dependence on LDLr (Fig. 4A), whereas the fraction of LDL cholesterol uptake, which could not be inhibited by cold LDL, may be mediated by macropinocytosis (13)(14)(15) (Fig. 4B), indicating that the majority of LDL cholesterol uptake efflux (B, F), cellular ACAT-generated cholesteryl ester content (C, G), and mb-CD extraction at 4 or 37jC (D) were measured.…”

Section: Specificity Of Ldl and Acldl Cholesterol Internalizationmentioning

confidence: 99%

“…The cell can also export excess cholesterol to appropriate extracellular acceptors by transfer mechanisms through cholesterol gradients that involve mostly HDL, mediated via scavenger receptor class B type I (SR-BI) (4) and the ABCG1/G4 transporter (5,6) or the apolipoprotein A-I (apoA-I)-mediated pathway that operates through ABCA1 (7)(8)(9)(10). In addition to the canonical receptor pathway for regulated LDL uptake by the LDL receptor (LDLr) (11,12), the macrophage can take up LDL via receptor-independent pathways, such as macropinocytosis, which, in a dosedependent manner, can lead to macrophage foam cell formation under certain conditions in vitro (13)(14)(15).…”

mentioning

confidence: 99%

Different cellular traffic of LDL-cholesterol and acetylated LDL-cholesterol leads to distinct reverse cholesterol transport pathways

Wang

Kiss

Franklin

et al. 2007

Endocytosis of LDL and modified LDL represents regulated and unregulated cholesterol delivery to macrophages. To elucidate the mechanisms of cellular cholesterol transport and egress under both conditions, various primary macrophages were labeled and loaded with cholesterol or cholesteryl ester from LDL or acetylated low density lipoprotein (AcLDL), and the cellular cholesterol traffic pathways were examined. Confocal microscopy using fluorescently labeled 3,3 ¶-dioctyldecyloxacarbocyanine perchlorate-labeled LDL and 1,1 ¶-dioctyldecyl-3,3,3 ¶,3 ¶-tetramethylindodicarbocyanine perchlorate-labeled AcLDL demonstrated their discrete traffic pathways and accumulation in distinct endosomes. ABCA1-mediated cholesterol efflux to apolipoprotein A-I (apoA-I) was much greater for AcLDLloaded macrophages compared with LDL. Treatment with the liver X receptor ligand 22-OH increased efflux to apoA-I in AcLDL-loaded but not LDL-loaded cells. In contrast, at a level equivalent to AcLDL, LDL-derived cholesterol was preferentially effluxed to HDL, in keeping with increased ABCG1. In vivo studies of reverse cholesterol transport (RCT) from cholesterol-labeled macrophages injected intraperitoneally demonstrated that LDL-derived cholesterol was more efficiently transported to the liver and secreted into bile than AcLDL-derived cholesterol. This indicates a greater efficiency of HDL than lipid-poor apoA-I in interstitial fluid in controlling in vivo RCT.These assays, taken together, emphasize the importance of mediators of diffusional cholesterol efflux in RCT.

“…Heretofore, researchers believed that macrophages could internalize only modified lipoproteins, typically through a variety of scavenger receptors (3,4). In recent studies, we have demonstrated an alternative mechanism for macrophage foam cell formation that does not depend on LDL modification or macrophage receptors (5)(6)(7). In this mechanism of foam cell formation, macrophages take up LDL through receptor-independent, fluid-phase pinocytosis.…”

mentioning

confidence: 99%

“…In this mechanism of foam cell formation, macrophages take up LDL through receptor-independent, fluid-phase pinocytosis. Receptor-independent fluid-phase uptake of 125 I-LDL does not show saturation of uptake with increasing 125 I-LDL concentrations and cannot be inhibited by excess amounts of unlabeled LDL (5,7). Pinocytosis of fluid and solute such as LDL contained in the fluid can occur either through uptake of fluid by micropinocytosis within small vesicles (micropinosomes) less than 0.1 mm in diameter, or by macropinocytosis within large vacuoles (macropinosomes) usually greater than 1 mm (8,9).…”

mentioning

confidence: 99%

Liver X receptors inhibit human monocyte-derived macrophage foam cell formation by inhibiting fluid-phase pinocytosis of LDL

Buono

Waldo

et al. 2007

Self Cite

Liver X receptors (LXRs) are ligand-activated transcription factors involved in the control of lipid metabolism and inflammation. Several studies have recently shown that LXRs promote reverse cholesterol transport and inhibit atherosclerosis. Our study investigated whether LXRs affect macrophage uptake of LDL by human monocyte-derived macrophages. We have previously shown that human monocytes differentiated into macrophages with macrophagecolony-stimulating factor (M-CSF) constitutively take up large amounts of native LDL by receptor-independent, fluidphase pinocytosis. In the research reported here, human monocytes were differentiated to macrophages in the presence of M-CSF with or without the LXR agonists T0901317 or 22(R)-hydroxycholesterol. Then, macrophages were incubated with native 125 I-LDL to determine LDL uptake. T0901317 and 22(R)-hydroxycholesterol inhibited 125 I-LDL uptake by 68 6 1% and 69 6 2%, respectively, and decreased pinocytotic vacuoles in the macrophages. 125 I-BSA uptake, a measure of fluid-phase pinocytosis, and 125 I-LDL uptake were the same, and T0901317 treatment inhibited uptake of both to the same degree. T0901317 did not affect receptormediated uptake of acetylated LDL, showing that the LXR effect is specific for fluid-phase pinocytosis of lipoproteins. Our results show that LXRs downregulate macrophage pinocytosis of LDL. The findings reveal an additional new mechanism by which LXR agonists may inhibit macrophage cholesterol accumulation and atherosclerosis, namely, by inhibiting macrophage uptake of LDL.-Buono, C., Y. Li, S. W. Waldo, and H. S. Kruth. Liver X receptors inhibit human monocyte-derived macrophage foam cell formation by inhibiting fluid-phase pinocytosis of LDL. J. Lipid Res.