Apolipoprotein E-dependent cholesterol efflux from macrophages: kinetic study and divergent mechanisms for endogenous versus exogenous apolipoproteinE

In these studies, we have utilized a J774 macrophage model in order to comp are phospholipid and cholesterol efflux kinetics in macrophage cells that d o not express endogenous apoE to cells transfected to express physiologic l evels of human apoE, This model was also used to compare the effect of exog enously added versus endogenously expressed apoE on cholesterol efflux kine tics from macrophages, ApoE expression increased free cholesterol and phosp holipid efflux into the medium, but did not change the free cholesterol/pho spholipid molar ratio of secreted Lipids. Kinetic examination showed that f ree cholesterol and phospholipid appeared simultaneously in the medium, and that cholesterol loading widened the difference in the rate of cholesterol efflux between apoE-expressing and nonexpressing macrophages, Addition of exogenous lipid-free apoE added to non-expressing cells, at a >2-fold highe r medium concentration than that produced by endogenous expression, produce d less cholesterol efflux: than that observed from apoE-expressing cells. T he addition of phosphatidylcholine Liposomes substantially increased choles terol efflux from apoE-expressing and nonexpressing J774 cells. Addition of these liposomes eliminated the enhanced cholesterol efflux produced by add ition of exogenous apoE. On the other hand, even in the presence of phospha tidylcholine liposomes, cholesterol efflux rates remained significantly hig her from apoE-expressing macrophages than nonexpressing cells. Similar resu lts were obtained when efflux was studied in the presence of cyclodextrin. These results suggest that endogenous expression of apoE by macrophages alt ers cell cholesterol balance via mechanisms distinct from those utilized by the extracellular addition of apoE, and may involve intracellular or peric ellular mechanisms.