AGGREGATED LOW-DENSITY-LIPOPROTEIN INDUCES AND ENTERS SURFACE-CONNECTED COMPARTMENTS OF HUMAN MONOCYTE-MACROPHAGES - UPTAKE OCCURS INDEPENDENTLY OF THE LOW-DENSITY-LIPOPROTEIN RECEPTOR

Aggregation of low density lipoprotein (LDL) stimulates its uptake by macrophages. We have now shown by electron microscopic and chemical ex periments that aggregated LDL (produced by vortexing (VxLDL) or treatm ent with phospholipase C) induced and became sequestered in large amou nts within surface-connected compartments (SCC) of human monocyte-deri ved macrophages. This occurred through a process different from phagoc ytosis, Formation of SCC and accumulation of aggregated LDL in SCC are cell-mediated processes that were temperature-dependent (10 x greater cell as association at 37 degrees C than at 4 degrees C) and blocked by cytochalasin D but not by nocodazole, Because of the surface connec tions of SCC, trypsin could release aggregated EDL from SCC, Degradati on of I-125-VxLDL through the SCC pathway showed delayed and a lower r ate of degradation (10-55%) compared with nonaggregated I-125-acetylat ed LDL that did not enter SCC. However, similar to I-125-acetylated LD L degradation, I-125-VxLDL degradation occurred through a chloroquine- sensitive pathway. Uptake of VxLDL into SCC was not mediated by the LD L receptor, Methylation of LDL prevents its binding to the LDL recepto r, However, methylated LDL still entered SCC after it was aggregated b y vortexing, On the other hand, degradation of I-125-VxLDL was substan tially decreased by methylation of LDL and by cholesterol enrichment o f 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 SC C to lysosomes may depend in part on LDL receptor function. Sequestrat ion into SCC is a novel endocytosis pathway for uptake of aggregated L DL that allows the macrophage to store large amounts of this lipoprote in before it is further processed.