Cholesteryl ester transfer between high density lipoprotein and phospholipid bilayers

High density lipoprotein-associated cholesteryl esters (HDL CE) are taken up by many cells without parallel uptake of HDL apoproteins, a pathway we have termed "selective uptake." The first step in this pathway, the reversible incorporation of HDL CE into the plasma membrane, is the subject of the present study. To examine the role of membrane proteins, the rate of HDL CE incorporation into isolated rat liver plasma membrane was compared with the rate of incorporation into synthetic membranes devoid of protein. Both membrane systems exhibited saturable uptake of CE, and at rates that were similar [t(1/2) is approximately equal to 2 h, measured with 50 micrograms of HDL protein]. Addition of unlabeled HDL to "chase" CE tracer from the biological and synthetic membranes revealed two kinetically distinct CE pools [t(1/2) is approximately equal to 0.5 h and t(1/2) approximately equal to 30 h]. Both biological and synthetic membranes accepted similar amounts of CE into both pools, with a maximum incorporation of 2-4 mol % relative to membrane phospholipids. CE transfer between HDL and membranes was kinetically second-order, in contrast to the first-order transfer of unesterified cholesterol. There was no evidence for direct participation of any apolipoprotein in CE uptake; CE in HDL or in protein-free microemulsions of similar particle size transferred to membranes at similar rates. To examine the possibility that CE transfer requires transient fusion of the HDL amphipathic coat with the membrane outer leaflet, radio-labeled cardiolipin was incorporated into either HDL particles or into synthetic membranes as an amphipathic coat marker that does not diffuse through the aqueous phase; transfer between HDL and membranes was not observed. Thus, CE are transferred between HDL and cell membranes in a collision-mediated process that does not involve amphipathic coat fusion and is not dependent on either membrane protein or apolipoproteins