Delineation of a novel hepatic route for the selective transfer of unesterified sterols from high-density lipoproteins to bile: Studies using the perfused rat liver

Cholesterol is principally excreted from the body in bile as unesterified c holesterol (UC), Using the unesterified plant sterol, sitostanol (SIT), as a nonexchangeable analog for UC, we have found that high-density lipoprotei ns (HDL), but not low-density lipoproteins, provide a vehicle for the direc t delivery of cholesterol to bile. To determine the mechanism for preferent ial cholesterol transport from HDL to bile, isolated rat livers were perfus ed with a reconstituted HDL, made with radiolabeled unesterified SIT, UC, a nd cholesteryl esters (CE), Total biliary sterol secretion was independent of the concentration of HDL added to perfusions, but with increasing HDL-SI T perfused, the proportion of SIT to cholesterol in bile was linearly incre ased. Biliary SIT secretion was rapid (detected within 2 to 4 minutes after reconstituted HDL was added to perfusions) and was dependent on the immedi ate presence of SIT in the perfusate, but independent of the amount of SIT that had accumulated in the liver. The ratio of SIT to UC was seven- to nin efold greater in bile than in the liver, consistent with preferential mobil ization of membrane sterols delivered from HDL. Although radiolabeled UC as well as SIT was taken up from HDL by the liver and secreted in bile, net U C uptake could not be quantitated because of both UC exchange acid a sizabl e enrichment of HDL with UC mass that approximated the SIT removed during t he passage of HDL through the liver. These results are consistent with ster ol transport to bile from HDL by a direct plasma membrane pathway and by a mechanism that appears to involve substitution of unesterified (exogenous) sterol from HDL for plasma membrane UC during transport. By this process, H DL can promote reverse cholesterol transport from peripheral tissues to bil e, even without an increase in biliary cholesterol secretion.