Interaction with proteoglycans enhances the sterol efflux produced by endogenous expression of macrophage apoE

Endogenous expression of apolipoprotein (apo)E in macrophages facilitates c holesterol efflux in the presence and absence of extracellular sterol accep ters. A proteoglycan-associated pool of apoE has also been described. The r elationship between a proteoglycan-associated pool of apoE and enhanced cho lesterol efflux was investigated in these studies. Inhibition of proteoglyc an expression reduced cholesterol efflux from apoE,expressing cells (J774E( +)) in the presence and absence of HDL, but did not do so from non expressi ng cells (J774E(-)). The effect of proteoglycan depletion on sterol efflux from J774E(+) cells was confirmed by measuring differences in cell sterol m ass, secreted sterol mass, and sterol efflux rates. Furthermore, apoE-conta ining particles secreted from proteoglycan-depleted J774E(+) cells were den ser than those secreted from J774E(+) cells with intact proteoglycan expres sion. Also, in J774E(+) cells with intact proteoglycans, apoE particles iso lated from the cell surface proteoglycan layer were denser than secreted pa rticles. The apoE-lipid particles isolated from the cell surface proteoglyc an layer had a lower lipid-to-apoE and cholesterol-to-apoE ratio compared w ith secreted particles. In distinction, proteoglycan depletion of J774E(-) cells did not reduce sterol efflux produced by the exogenous addition of ap oE.jlr These observations indicate that one mechanism by which endogenous e xpression of apoE facilitates effective cholesterol efflux from macrophages is related to its retention at the cell surface in a proteoglycan-associat ed pool. Further, our data suggest that apoE arrives at the cell surface in a relatively lipid-poor state, and that a proximate source of lipid availa ble to the proteoglycan-bound apoE at the cell surface resides in the plasm a membrane.