Blood lipoproteins carry cholesterol into blood vessel walls where the cholesterol can accumulate causing atherosclerotic plaques. Massive accumulation of cholesterol by monocyte-derived macrophages is a prominent feature of these atherosclerotic plaques. Because macrophages may trap or help remove lipoprotein cholesterol from lesions, it is important to understand the process by which these cells take up blood-derived lipoproteins such as low density lipoprotein (LDL). Aggregation of LDL stimulates its uptake by macrophages. We have now shown that aggregated LDL induced and became sequestered in large amounts within tortuous, interconnected, membranous surface-connected compartments (SCC) of human monocyte-derived macrophages. Endocytosis into SCC is different from phagocytosis, where particles are taken into the macrophage within vacuoles that form from pinched off regions of the plasma membrane, and do not maintain any connection to the extracellular space. Degradation of aggregated LDL that accumulated in SCC was slow and incomplete. On the other hand, a protease activity of blood could release aggregated LDL from SCC. This showed that uptake of aggregated LDL into SCC was reversible. Uptake of aggregated LDL into SCC did not depend on the previously characterized LDL receptor because sequestration was not decreased by heparin, cholesterol-enrichment of macrophages, or methylation of LDL, treatments known to block LDL receptor action. Uptake of LDL into SCC could explain how LDL is accumulated by macrophages in atherosclerotic lesions where activity of the LDL receptor is down-regulated. SCC represent a novel pathway for endocytosis of lipoproteins. Macrophages can store large amounts of aggregated LDL within SCC, and possibly release these LDL back into the blood when macrophages emigrate from the vessel wall.
Showing the most recent 10 out of 23 publications
