APOLIPOPROTEIN-MEDIATED CELLULAR CHOLESTEROL AND PHOSPHOLIPID EFFLUX DEPEND ON A FUNCTIONAL GOLGI-APPARATUS

Several studies have demonstrated that lipid-free apolipoproteins can promote cholesterol and phospholipid efflux from cells; however, the m echanisms and the role of cell-mediated pathways involved remain incom pletely elucidated. We have recently demonstrated that brefeldin A or monensin, agents that disrupt Gels apparatus structure and function, i nhibit intracellular cholesterol efflux from cells to high density lip oproteins. In the present study we examined the effects of those agent s on cell cholesterol and phospholipid efflux to purified apolipoprote in A-I (apoA-I) and apolipoprotein-depleted accepters from cholesterol -loaded fibroblasts. Brefeldin A or monensin treatment of cells during incubation with apoA-I inhibited efflux of cellular cholesterol by gr eater than 80% compared with control cells, measured by changes in cel lular cholesterol radioactivity, mass, and the substrate pool of chole sterol available for esterification by acyl coenzyme A:cholesterol acy ltransferase. Inhibition of cholesterol efflux by these agents could n ot be overcome by increasing the apoA-I concentration and persisted du ring incubations up to 24 h. Similarly, brefeldin A and monensin inhib ited up to 80% of apoA-I-mediated efflux of labeled phospholipids from cholesterol-loaded cells relative to controls. In contrast, lipid eff lux mediated by apolipoprotein-depleated accepters (trypsin-modified H DL and sonicated phospholipid vesicles) was not sensitive to these dru gs. On the basis the known effects of brefeldin A and monensin on Golg i apparatus structure and function, these results are consistent with the notion that efflux of cell lipids by apolipoprotein-dependent mech anisms, but not by apolipoprotein-independent mechanisms, require acti ve cellular processes involving an intact and functional Golgi apparat us.