cAMP selectively promotes the efflux of cholesterol derived from cholesteryl ester hydrolysis in J774 foam cells

Although cAMP promotes the clearance of cholesteryl esters from J774 macrophage foam cells to high-density lipoproteins (HDL), the mechanisms underlying this effect are poorly understood. To determine the level at which cAMP affects the removal of cholesterol derived from the cholesteryl ester pool, the effect of a cAMP analog, (chlorophenylthio)-adenosine 3':5'-cyclic monophosphate, on efflux of cholesterol was examined. Because net removal of cholesterol depends on the balance between efflux of cellular cholesterol and influx of lipoprotein cholesterol, bidirectional flux of cholesterol was examined. Either J774 or mouse peritoneal macrophages enriched with radiolabeled cholesteryl ester were exposed to medium containing HDL radiolabeled with a different isotope, and the movements of cellular cholesterol to the medium (efflux) and lipoprotein cholesterol to the cells (influx) were measured. For both cell types, significantly greater efflux occurred when the cAMP analog was present, but no effect on influx was observed. Thus, the ability of cAMP to stimulate cholesterol clearance to HDL rests on increasing efflux of cholesterol. To determine whether the stimulation of cholesterol efflux is selectively directed at the removal of cholesterol derived from the hydrolysis of stored cholesteryl esters, a comparison was made of the efflux of cellular cholesterol from J774 cells with normal cholesterol levels, from cells enriched in free cholesterol, and from cells with cholesteryl ester stores. Stimulation of efflux in response to the cAMP analog occurred only in cells with esterified cholesterol stores, indicating that cAMP selectively promoted the efflux of cholesterol generated by intracellular hydrolysis of cholesteryl esters, presumably by stimulating transport of cholesterol to the plasma membrane for efflux.