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 mechanisms and the role of cell-mediated pathways involved remain incompletely elucidated. We have recently demonstrated that brefeldin A or monensin, agents that disrupt Golgi apparatus structure and function, inhibit intracellular cholesterol efflux from cells to high density lipoproteins. In the present study we examined the effects of those agents on cell cholesterol and phospholipid efflux to purified apolipoprotein A-I (apoA-I) and apolipoprotein-depleted acceptors from cholesterol-loaded fibroblasts. Brefeldin A or monensin treatment of cells during incubation with apoA-I inhibited efflux of cellular cholesterol by greater than 80% compared with control cells, measured by changes in cellular cholesterol radioactivity, mass, and the substrate pool of cholesterol available for esterification by acyl coenzyme A:cholesterol acyltransferase. Inhibition of cholesterol efflux by these agents could not be overcome by increasing the apoA-I concentration and persisted during incubations up to 24 h. Similarly, brefeldin A and monensin inhibited up to 80% of apoA-I-mediated efflux of labeled phospholipids from cholesterol-loaded cells relative to controls. In contrast, lipid efflux mediated by apolipoprotein-depleted acceptors (trypsin-modified HDL and sonicated phospholipid vesicles) was not sensitive to these drugs. On the basis the known effects of brefeldin A and monensin on Golgi apparatus structure and function, these results are consistent with the notion that efflux of cell lipids by apolipoprotein-dependent mechanisms, but not by apolipoprotein-independent mechanisms, require active cellular processes involving an intact and functional Golgi apparatus.