Importance of different pathways of cellular cholesterol efflux.

The removal of excess free cholesterol from cells by HDL or its apolipoproteins is important for maintaining cellular cholesterol homeostasis. This process is most likely compromised in the atherosclerotic lesion because the development of atherosclerosis is associated with low HDL cholesterol. Multiple mechanisms for efflux of cell cholesterol exist. Efflux of free cholesterol via aqueous diffusion occurs with all cell types but is inefficient. Efflux of cholesterol is accelerated when scavenger receptor class-B type I (SR-BI) is present in the cell plasma membrane. Both diffusion-mediated and SR-BI-mediated efflux occur to phospholipid-containing acceptors (ie, HDL and lipidated apolipoproteins); in both cases, the flux of cholesterol is bidirectional, with the direction of net flux depending on the cholesterol gradient. The ATP-binding cassette transporter AI (ABCA1) mediates efflux of both cellular cholesterol and phospholipid. In contrast to SR-BI-mediated flux, efflux via ABCA1 is unidirectional, occurring to lipid-poor apolipoproteins. The relative importance of the SR-BI and ABCA1 efflux pathways in preventing the development of atherosclerotic plaque is not known but will depend on the expression levels of the two proteins and on the type of cholesterol acceptors available.