Enhanced macrophage degradation of biologically modified low density lipoprotein.

Abstract

Low density lipoprotein (LDL) conditioned by incubation in the presence of rabbit aortic or human umbilical vein endothelial cells (endothelial cell-modified LDL) was degraded by macrophages three to five times more rapidly than LDL incubated in the absence of cells (control LDL). This enhanced degradation occurred mostly via a high affinity, saturable pathway related to the pathway for macrophage uptake of acetylated LDL. Conditioning LDL with cultured aortic smooth muscle cells had a qualitatively similar but smaller effect; conditioning with fibroblasts had no effect. Conditioning very low density lipoproteins or high density lipoproteins with endothelial cells did not affect subsequent metabolism of these lipoproteins by macrophages. Endothelial cell-modified LDL, while degraded more rapidly than control LDL by macrophages, was degraded more slowly by cultured smooth muscle cells and by human skin fibroblasts. Degradation of endothelial cell-modified LDL by macrophages was accompanied by stimulation of cholesterol esterification, inhibition of cholesterol synthesis, and a net increment in total cellular cholesterol content. Thus, a biologically generated modification of LDL is described that markedly alters cholesterol metabolism of macrophages and, consequently, may play a role in foam cell formation during atherogenesis. (Arteriosclerosis 3:149-159, March/April 1983) T he biological mechanisms leading to the formation of foam cells in atherosclerotic lesions have yet to be clarified. It is generally accepted that a significant fraction of foam cells is derived from cells of the mononuclear phagocyte system, 1 -3 and that the major portion of the cholesterol in atheromata is derived from plasma LDL. Yet, paradoxically, the capacity of macrophages to take up native LDL is relatively low, 4 suggesting the possibility that not LDL itself but some modified form of it is actually responsible for the delivery of cholesterol to the macrophage. Several chemically modified forms of LDL (acetylated LDL, 4 acetoacetylated LDL, 5 and malondialdehyde-conjugated LDL 6 ) are taken up by macrophages several times faster than is native LDL. There has, however, been no evidence for the occurrence of such chemically modified forms in vivo nor any evidence that they can be formed biologically. It has been reported that some malondialdehyde-conjugated LDL is formed in vitro from LDL during platelet aggregation. 6 We have previously reported that conditioning of human LDL with cultured rabbit aortic endothelial cells converts it to a form that is taken up and degraded by murine macrophages at rates three to five times the rate for control LDL. 7 The LDL conditioned by prior incubation with endothelial cells (endothelial cell-modified LDL; EC-modified LDL) was taken up by macrophages and degraded via a high affinity, saturable pathway related at least in part to the pathway for uptake of acetylated LDL. 7 The present studies show for the first time that EC-modified LDL can be produced using human umbilical vein endothelial cells....

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.…”

“…The purpose of the spin was to reisolate LDL. A solvent density of 1.100 was used to collect LDL that might have increased in density as indicated by Henriksen, et al 9 ' 10 The reisolated LDL was then assayed for cholesterol, protein, TBARS, electrophoretic mobility, and cytotoxicity.…”

“…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 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.…”

Transition metal ions within human atherosclerotic lesions can catalyse the oxidation of low density lipoprotein by macrophages

“…It has been demonstrated in vitro, however, that LDL can be oxidised by the main cells present in atherosclerotic lesions, namely endothelial cells [3], smooth muscle cells [4], monocyte-macrophages [5 7] and lymphocytes [8]. Most workers find that LDL oxidation by cells requires the presence of transition metal ions, usually iron [7,9], but copper ions will also catalyse the oxidation [9,10].…”