Direct observation of rapid internalization and intracellular transport of sterol by macrophage foam cells
Transport of the fluorescent cholesterol analog dehydroergosterol (DHE) from the plasma membrane was studied in J774 macrophages (Mphis) with normal and elevated cholesterol content. Cells were labeled with DHE bound to methyl-beta-cyclodextrin. In J774, Mphis with normal cholesterol, intracellular DHE became enriched in recycling endosomes, but was not highly concentrated in the trans-Golgi network or late endosomes and lysosomes. After raising cellular cholesterol by incubation with acetylated low-density lipoprotein (AcLDL), DHE was transported to lipid droplets, and less sterol was found in recycling endosomes. Transport of DHE to droplets was very rapid (t1/2 = 1.5 min after photobleaching) and did not require metabolic energy. In cholesterol-loaded J774 Mphis, the initial fraction of DHE in the plasma membrane was reduced, and rapid DHE efflux from the plasma membrane to intracellular organelles was observed. This rapid sterol transport was not related to plasma membrane vesiculation, as DHE did not become enriched in endocytic vesicles formed after sphingomyelinase C treatment of cells. When cells were incubated with DHE ester incorporated into AcLDL, fluorescence of the sterol was first found in punctate endosomes. After a chase, this DHE colocalized with transferrin in a distribution similar to cells labeled with DHE delivered by methyl-beta-cyclodextrin. Our results indicate that elevation of sterol levels in Mphis enhances transport of sterol from the plasma membrane by a non-vesicular pathway.