Enhanced macrophage degradation of low density lipoprotein previously incubated with cultured endothelial cells: recognition by receptors for acetylated low density lipoproteins.

Abstract

Human low density lipoprotein (LDL) was incubated with an established line of rabbit aortic endothelial cells. Density gradient fractionation showed a time-, concentration-, and temperature-dependent increase in the average density ofthe LDL (from about 1.036 to as high as 1.070 g/ml). Incubation without cells or with other types of cultured cells (fibroblasts, henatocytes, 3T3-L1 cells) caused no significant change in density. '2I-Labeled LDL ('25I-LDL) recovered after incubation with endothelial cells (EC-modified LDL) was taken up and degraded 3 to 4 times more rapidly than control LDL by resident mouse peritoneal macrophages and by an established tumor line of mouse macro hages (J774 cells). Macrophage degradation of EC-modified 5I-LDL exhibited saturation kinetics (>85% inhibited by excess unlabeled EC-modified LDL). Degradation was also inhibited by unlabeled acetylated LDL and, conversely, unlabeled ECmodified LDL inhibited degradation of acetylated '2'I-LDL. Incubation of LDL with conditioned medium removed from endothelial cell cultures modified neither its density nor its rate of degradation by macrophages. These studies show that endothelial cells have the potential to metabolically modify the LDL molecule, generating a form that is more rapidly degraded by macrophages and that is recognized by the macrophage receptor for acetylated LDL. This process may play a significant role in the pathogenesis of atherosclerosis.

Endothelial and smooth muscle cells alter low density lipoprotein in vitro by free radical oxidation.

“…Our results show that incubation of LDL in serum-free medium with human umbilical vein EC or bovine aortic SMC, but not human fibroblasts or bovine aortic EC, resulted in a modified LDL with increased electrophoretic mobility. These findings are in general agreement with the results of Henriksen et al 8 " 10 Like these authors, 10 we observed that the electrophoretic mobility change was accompanied by a variable decrease in the LDL sterol. A decreased total cholesterol/protein ratio of up to 20% was observed after 48 hours of incubation of LDL with human umbilical vein EC or bovine SMC.…”

“…5 Although oxygen-free radicals such as these have been implicated as effectors of tissue damage, 6 ' 7 the toxic action of oxidized LDL is effected by an oxidized lipid producad by free radicals rather than by free radicals generated during propagation of the oxidation reaction. 5 From Henriksen, et al 8 " 10 have reported that cultured human umbilical vein endothelial cells (EC) and vascular smooth muscle cells (SMC), but not human fibroblasts or bovine aortic EC, can modify LDL. The changes in LDL produced by incubation with human umbilical vein EC and SMC include increased anodic electrophoretic mobility, a reduced ratio of total cholesterol to protein, and increased degradation by macrophages.…”

“…The changes in LDL produced by incubation with human umbilical vein EC and SMC include increased anodic electrophoretic mobility, a reduced ratio of total cholesterol to protein, and increased degradation by macrophages. 8 " 10 Bowman et al 11 have reported that pulmonary artery EC, but not lung fibroblasts, produce superoxide anion in vitro. Since superoxide anion appeared to be involved in spontaneous oxidation of LDL during its isolation, 5 we chose to determine whether cultured EC and SMC could oxidize LDL and render it cytotoxic and to identify the relationship between oxidation and certain other EC modifications of LDL, including the changes in electrophoretic mobility and the decreased steroi content reported by Henriksen et al prepared from citrated bovine whole-blood serum.…”

Complement C6 Deficiency Protects Against Diet-Induced Atherosclerosis in Rabbits

“…In order to offer an explanation, we depart from the conventional view that oxidative modification of LDL represents the single most important event in the development of atherosclerosis. [23][24][25][26][27][28][29][30][31] Instead, we propose that cholesterol contained within the LDL molecule itself is endowed with proinflammatory potential, because exposure of free cholesterol to the extracellular environment will activate complement. Normally, LDL degradation and deesterification of cholesterol occurs within cells after they take up the lipoprotein.…”

Dualism of Oxidized Lipoproteins in Provoking and Attenuating the Oxidative Burst in Macrophages: Role of Peroxisome Proliferator-Activated Receptor-γ

“…OxLDL has been shown to be a powerful regulator of cell signaling in provoking various responses, among others, expression of adhesion molecules on endothelial cells (5), production of proinflammatory cytokines and growth factors by vascular cells (6), or proliferation and migration of vascular cells (7). In addition, oxLDL is both a potent chemoattractant for circulating monocytes (8) and a differentiating agent that promotes transition of macrophages to lipid-loaded foam cells (9). In close association, receptor-mediated endocytosis of oxLDL by several scavenger receptor family members including macrophage class A scavenger receptor (10,11), CD36 (12), scavenger receptor that binds phosphatidylserine and oxidized lipoprotein (13), or lectin-like oxidized low density lipoprotein receptor-1 (14) is implicated in the process of atherogenesis.…”