title: Phagocytosis of lipase-aggregated low density lipoprotein promotes macrophage foam cell formation. Sequential morphological and biochemical events.
abstract: Macrophages internalize aggregated low density lipoprotein (LDL) by LDL receptor-dependent phagocytosis. To investigate this model of foam cell formation, we have used human and mouse macrophages to characterize biochemically and morphologically the fate of ingested phospholipase C-modified low density lipoprotein (PLC-LDL). When LDL was digested with phospholipase C, it lost phospholipid and aggregated. Human monocyte-derived macrophages rapidly ingested and degraded 125I-PLC-LDL. The degraded PLC-LDL released free cholesterol, measured either as free sterol mass or by the stimulation of [14C]oleate incorporation into cellular cholesteryl ester. Esterification was blocked by chloroquine, a weak base that inhibits lysosomal degradation. Macrophages exposed to PLC-LDL exhibited a 30-fold to a 50-fold increase in esterified sterol: by light microscopy, cytoplasmic inclusions were abundant. The inclusions were stained with oil red O, indicating that they were neutral lipid droplets. By electron microscopy, mouse peritoneal macrophages incubated with PLC-LDL contained numerous membrane-bounded vacuoles and cytoplasmic inclusions that were not surrounded by a limiting membrane. Pulse-chase experiments demonstrated that vacuoles filled with particulate material appeared first. Subsequently, the macrophages exhibited vacuoles containing multivesicular bodies. Last, inclusions that were homogeneously electron-dense and that lacked a tripartite membrane accumulated in the cytoplasm of the cells. These results are consonant with the following model of foam cell formation. Cultured macrophages rapidly ingest PLC-LDL that is initially localized in phagosomes. The aggregated lipoprotein subsequently is digested in secondary lysosomes, thus releasing free cholesterol that is reesterified, forming cytoplasmic cholesteryl ester droplets lacking a tripartite membrane.
authors.affiliation: Department of Medicine, University of Washington, Seattle.
authors.forename: J W
authors.initials: JW
authors.lastname: Heinecke
authors.forename: A G
authors.initials: AG
authors.lastname: Suits
authors.forename: M
authors.initials: M
authors.lastname: Aviram
authors.forename: A
authors.initials: A
authors.lastname: Chait
mesh_terms: D005487:Foam Cells / Q000166:cytology / Q000502:physiology*; D006493:Heparin / Q000494:pharmacology; D006801:Humans; D002479:Inclusion Bodies / Q000648:ultrastructure; D008077:Lipoproteins, LDL / Q000378:metabolism*; D008264:Macrophages / Q000166:cytology / Q000502:physiology* / Q000648:ultrastructure; D010587:Phagocytosis; D013997:Time Factors; D010738:Type C Phospholipases / Q000494:pharmacology*; D014617:Vacuoles / Q000648:ultrastructure
chemical_list: D008077:Lipoproteins, LDL; D006493:Heparin; D010738:Type C Phospholipases