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.

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....

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Mouse peritoneal macrophages incubated in serumless medium containing a 19:0 or trans-18:1 fatty acid complexed to bovine serum albu'min incorporate the exogenous fatty acid supplement into cellular phospholipids. Within 8 hr, 25% of the total phospholipid fatty acids are derived from the supplement, with cell viability remaining >95%. The incorporation of either of these supplements increases the saturated/ unsaturated fatty acid ratio in the phospholipids 2-fold over that of cells cultured in serum and effects striking changes in endocytic activities. The levels of both fluid-phase pinocytosis and receptor-mediated phagocytosis are decreased at all temperatures examined between 150 and 37°. The increased degree of saturation of cell phospholipids correlates with decreased endocytic rates for both processes and with increased activation energies (Eact) for phagocytosis. The Eact values for phagocytosis, which range from 54 to 90 kcal/mol, depend on the supplementation conditions used. Although the levels of pinocytosis are depressed, the Ea. values for pinocytosis (17-25 kcal/mol)

The dynamics of granule formation were studied using cells from two bench-scale UASB Reactors. The objective was to elucidate factors which influence formation and maintenance of highly active self-agglomerated microbial biomass. Simultaneous examination of biological and physical parameters was performed during the start-up of a calcium-positive (100 mg/1) reactor and a reactor without added calcium. The influence of carbon nutrients and Ca++ on the cell surface and microbial aggregation was studied. The granules formed in both reactors but were larger in the calcium-positive reactor in which they settled 3-4 times faster. A higher rate of biomass accumulation also was evident in the calcium-positive reactor and this allowed a more frequent increase in the substrate loading rate and earlier development of the granular sludge.