ROLE OF THE PLASMA-MEMBRANE IN CHOLESTEROL ESTERIFICATION IN RAT HEPATOMA-CELLS

The source of the cholesterol used for ester synthesis by cultured rat hepatoma cells was examined. The activities synthesizing and esterify ing cholesterol codistributed with RNA at a high buoyant density, pres umably in the rough endoplasmic reticulum (RER). Cholesterol mass was undetectable in the RER, and the transfer of cholesterol synthesized i n the RER to the cell surface was more than 100 times greater than was its esterification. Similarly, essentially all of the cholesterol lib erated from ingested intracellular lipoproteins was recovered at the c ell surface. The plasma membranes, which contained approximately 87% o f cell cholesterol, provided >100 times more cholesterol for esterific ation in the RER than did nascent cholesterol. The supply of cholester ol was rate-limiting for esterification in cell homogenates. Prior oxi dation of plasma membrane cholesterol in intact cells reduced the acyl -CoA:chlosterol acyltransferase activity in isolates proportionately. Finally, cholesterol in hepatoma plasma membranes was a far better sub strate for in vitro esterification than was that in fibroblast plasma membranes, red blood cell ghosts, or liposomes. We conclude that the l evel of saturation of acyl-CoA:cholesterol acyltransferase, controlled principally through the bidirectional movement of the substrate betwe en plasma membranes and RER, plays a major role in the regulation of c holesterol esterification.