HDL receptors and cholesterol efflux from parenchymal cells

High-density lipoproteins are thought to play an important role in the initial steps of reverse cholesterol transport in which cholesterol is removed from peripheral tissues to the liver for ultimate excretion. Since the cell types involved have been shown to possess high-affinity binding sites for HDL, it has been of considerable interest to find out whether or not these bindings sites are also involved in efflux of cholesterol from cells. Studies conducted in several laboratories have consistently shown that the binding site does not promote or enhance efflux of cholesterol from cellular plasma membrane. Binding of HDL to its cell surface binding site does, however, appear to promote efflux of cholesterol from intracellular compartments. The activity of the HDL binding sites has also been shown to be metabolically regulated in response to changes in the growth state and cholesterol homeostasis of the cell. It is therefore apparent that the binding sites are involved in a physiologically regulated process of cholesterol removal from cells, as part of reverse cholesterol transport