PROGESTERONE BLOCKS INTRACELLULAR TRANSLOCATION OF FREE-CHOLESTEROL DERIVED FROM CHOLESTERYL ESTER IN MACROPHAGES

Macrophage foam cells must accommodate continuing fluxes of free chole sterol in spite of a greatly expanded store of cholesteryl ester. Thou gh endogenous free cholesterol synthesis is suppressed, free cholester ol continues to enter the cell via endocytosis of oxidized/modified li poproteins. It has been shown previously that this free cholesterol is released into the lysosomal compartment and rapidly transported to th e plasma membrane prior to its esterification. A substantial amount of free cholesterol is also presented via the continuous hydrolysis of c holesteryl ester during the cholesteryl ester cycle. We addressed the question of whether the intracellular free cholesterol derived from th e hydrolysis of cholesteryl eater formed a protected pool for rapid re -esterification. Incubation of macrophage foam cells with cyclic AMP t o enhance cholesteryl ester hydrolysis, and with S58035 to inhibit acy l-CoA:cholesterol acyltransferase (ACAT) activity, led to conversion o f cellular cholesteryl ester to free cholesterol and transport of this free cholesterol to the plasma membrane. Addition of progesterone, pr eviously demonstrated to be an inhibitor of free cholesterol transport in other cell types, also led to conversion of cholesteryl ester to f ree cholesterol even though progesterone was only a weak inhibitor of ACAT activity. Free cholesterol in the plasma membrane was an importan t source of ACAT substrate to balance the constitutive hydrolysis of c holesteryl ester in cholesterol-loaded macrophages. Treatment of cells with progesterone, however, prevented free cholesterol derived from c holesteryl ester hydrolysis from moving to the plasma membrane. The se questration of free cholesterol by progesterone could be reversed by i ncubation with human HDL(3). Our data indicate that free cholesterol d erived from cholesteryl eater hydrolysis requires translocation throug h the cell prior to becoming available for re-esterification. Disrupti ng free cholesterol transport to the plasma membrane by treatment with progesterone disrupts the cholesteryl ester cycle in cholesteryl este r-loaded macrophages.