STEROL CARRIER PROTEIN-2 OVEREXPRESSION ENHANCES STEROL CYCLING AND INHIBITS CHOLESTEROL ESTER SYNTHESIS AND HIGH-DENSITY-LIPOPROTEIN CHOLESTEROL SECRETION

Recent data indicate that sterol carrier protein-a (SCP-S) functions i n the rapid movement of newly synthesized cholesterol to the plasma me mbrane (Puglielli, L., Rigotti, A, Greco, A. V., Santos, M, J., and Ne rvi, F. (1995) J. Biol, Chem. 270, 18723-18726), In order to further c haracterize the cellular function of SCP-2, we transfected McA-RH7777 rat hepatoma cells with a preSCP-8 cDNA expression construct. In stabl e transfectants, pre-SCP-a processing resulted in an 8 fold increase i n peroxisomal levels of SCP-S, SCP-2 overexpression increased the rate s of newly synthesized cholesterol transfer to the plasma membrane and plasma membrane cholesterol internalization by 4-fold, There was no e ffect of SCP-2 overexpression on the microsomal levels of acyl-CoA:cho lesterol acyltransferase and neutral cholesterol ester (CE) hydrolase; however, in the intact cell, CE synthesis and mass were reduced by 50 % SCP 8 overexpression also reduced high density lipoprotein-cholester ol secretion and apoA-I gene expression by 70% and doubled the rate of plasma membrane desmosterol conversion to cholesterol, We conclude th at SCP-2 overexpression enhances the rate of cholesterol cycling, whic h reduces the availability of cholesterol for CE synthesis and alters the activity of a cellular cholesterol pool involved in regulating apo A-I-mediated high density lipoprotein cholesterol secretion, The net r esult of these changes in cholesterol metabolism is a 46% increase in plasma membrane cholesterol content, the implications of which are dis cussed.