Abstract
Cultured human fibroblasts were labelled with {4-('14)C} cholesterol. When the labelled cells were incubated in a medium containing serum lipoproteins, there was an efflux of {4-('14)C} cholesterol from the cells into the medium. High density lipoprotein (HDL) promoted the greatest amount of {4-('14)C} cholesterol efflux, 49.9%, followed by low density lipoprotein (LDL), 25.6%, and very low density lipoprotein (VLDL), 12.3%.When all three lipoproteins were present in the acceptor medium, there was a redistribution of {4-('14)C} cholesterol from HDL to VLDL and LDL.These data based on ('14)C gave information on the movement of cholesterol, but not on the net change of cholesterol contents, as there was exchange of {('14)C} cholesterol and non-radioactive cholesterol. Gas liquid chromatography was utilized to measure changes in the cholesterol content of cultured fibroblasts after incubation with an acceptor medium containing lipoproteins. HDL was the only lipoprotein which was effective in promoting cholesterol efflux from the cells (22%).In another experiment, combinations of lipoproteins were present in the acceptor medium. The combination of HDL and VLDL was more effective in promoting cholesterol efflux than HDL alone, although VLDL by itself was ineffective in promoting cholesterol efflux.When human serum was incubated at 37(DEGREES)C with rotation, there was a decrease in HDL cholesterol for the first few hours. This decrease was due to a transfer of cholesterol from HDL to VLDL and LDL. Free and esterified cholesterol decreased in HDL, while the gain by VLDL-LDL was only in cholesteryl esters, suggesting esterification of cholesterol before the transfer.HDL, deficient in cholesterol after transfer of cholesterol to VLDL and LDL, was tested for the capacity to facilitate cholesterol efflux from cultured fibroblasts. The cholesterol-deficient HDL promoted 50% more cholesterol efflux than native HDL. It is proposed that HDL functions as a shuttle for cholesterol transport between the cells and other lipoproteins. After HDL transfers cholesterol to VLDL and LDL, the HDL is able to remove additional cholesterol from the cells.It appears possible that in vivo free cholesterol is picked up from the cells by HDL. The cholesterol is esterified by lecithin:cholesterol acyl transferase. HDL transfers the cholesteryl ester to VLDL and LDL, which transport the cholesterol to the liver for removal as bile acids and biliary cholesterol. This mechanism of cholesterol efflux would operate to prevent cholesterol accumulation in cells, thus preventing the development of atherosclerosis.