Inhibition of cholesterol efflux by 7-ketocholesterol: Comparison between cells, plasma membrane vesicles, and liposomes as cholesterol donors

Cholesterol removal from lipid-loaded macrophages is an important, potentia lly antiatherogenic process, and we have previously shown that an oxysterol , 7-ketocholesterol (7K), can impair efflux to lipid-free apoprotein A-1 (a poA-1). This publication investigates whether incorporation of 7K into memb ranes could account for this impairment of cholesterol efflux. Cholesterol efflux was studied from lipoprotein-loaded THP-1 cells, from plasma membran e vesicles obtained from these cells, and from artificial, protein-free lip osomes. Impairment of cholesterol efflux by 7K was observed for all cholest erol donor systems whether measured as declined in cholesterol removal rate s or as the percentage mass of total cellular cholesterol exported. 7-Ketoc holesterol itself was not removed by apoA-1 from any of the cholesterol don or systems. Increasing membrane cholesterol content increased the rate of c holesterol removal by apoA-1 (as seen with plasma membrane vesicles), the q uantity of cholesterol removed at equilibrium (liposomes), or both (whole c ells), Although the minimum inhibitory 7K concentrations varied between the cholesterol donor systems, 7K inhibited cholesterol efflux in all systems. It was concluded that 7K induces alteration in membranes which decreased t he efficiency of cholesterol efflux and the quantity of removed cholesterol induced by apoA-1. As, cell membrane proteins are not essential for choles terol efflux in these systems, the impairment of such by 7K suggests that i ts effect on membrane lipid composition and its structure are key regulator y elements in this efflux process.