Aggregated Low Density Lipoprotein Induces and Enters Surface-connected Compartments of Human Monocyte-Macrophages

Aggregation of low density lipoprotein (LDL) stimulates its uptake by macrophages. We have now shown by electron microscopic and chemical experiments that aggregated LDL (produced by vortexing (VxLDL) or treatment with phospholipase C) induced and became sequestered in large amounts within surface-connected compartments (SCC) of human monocyte-derived macrophages. This occurred through a process different from phagocytosis. Formation of SCC and accumulation of aggregated LDL in SCC are cell-mediated processes that were temperature-dependent (10 × greater cell association at 37 °C than at 4 °C) and blocked by cytochalasin D but not by nocodazole. Because of the surface connections of SCC, trypsin could release aggregated LDL from SCC. Degradation of125I-VxLDL through the SCC pathway showed delayed and a lower rate of degradation (10–55%) compared with nonaggregated125I-acetylated LDL that did not enter SCC. However, similar to 125I-acetylated LDL degradation,125I-VxLDL degradation occurred through a chloroquine-sensitive pathway. Uptake of VxLDL into SCC was not mediated by the LDL receptor. Methylation of LDL prevents its binding to the LDL receptor. However, methylated LDL still entered SCC after it was aggregated by vortexing. On the other hand, degradation of125I-VxLDL was substantially decreased by methylation of LDL and by cholesterol enrichment of macrophages, which decreases macrophage LDL receptor expression. The results suggest that whereas uptake of aggregated LDL into SCC occurs independently of the LDL receptor, movement of aggregated LDL from SCC to lysosomes may depend in part on LDL receptor function. Sequestration into SCC is a novel endocytosis pathway for uptake of aggregated LDL that allows the macrophage to store large amounts of this lipoprotein before it is further processed.