High-density lipoproteins (HDL) are believed to protect against
cardiovascular disease by removing excess
cholesterol from cells.
Lipid-free HDL
apolipoproteins remove cellular
cholesterol and
phospholipids by an active, Golgi-dependent process that is still poorly understood. Here we characterized the morphology of
apolipoprotein binding sites on cultured cells by immunogold electron microscopy. After 6 h incubations with
lipid-free
apoA-I or
apoE, immunogold-labeled
apolipoproteins were distributed sparsely along the planar surface of human fibroblasts and THP-1 macrophages. Overloading these cells with
cholesterol led to a several-fold increase in the concentration of immunogold-labeled
apoA-I and
apoE on the cell surface, and over 80% of these
gold particles were associated with novel electron-opaque structures protruding from the plasma membrane. Protrusions binding
apoE were larger (100-200 nm) than those binding
apoA-I (10-60 nm), and similar
apoA-I-binding structures appeared when cells were incubated with either purified
apoA-I or HDL particles. These structures were formed and enlarged by a time-dependent process inhibited by the Golgi disruptor brefledin A, the energy
poison NaF, and low temperature. Moreover, formation of these structures was nearly absent in fibroblasts from a subject with
Tangier disease, cells that lack a functioning
apolipoprotein-mediated
lipid removal pathway. Thus, formation of novel
apolipoprotein binding structures protruding from the cell surface is an intermediate step in the cellular pathway by which
apolipoproteins remove excess
cholesterol.