Tangier disease is characterized by HDL hypercatabolism and increased deposition of cholesterol in tissues. Tangier disease skin fibroblasts have decreased apoA-I-mediated cholesterol and phospholipid efflux, which may lead to the excess accumulation of cellular cholesterol. The mechanism of apolipoprotein-mediated cholesterol efflux and the apolipoprotein acceptor specificity for cholesterol efflux from normal and Tangier disease fibroblasts was investigated. Normal cells readily effluxed cholesterol and phospholipid to apoA-I and to all of the other apolipoproteins tested (apoA-II, AIV, C-I, C-II, C-III). In contrast, Tangier cells were almost completely defective in cholesterol efflux to apoA-I and to all of the other apolipoproteins tested. HDL was also less effective, by approximately 50%, in stimulating cholesterol efflux from Tangier cells compared with normal cells. In addition, Tangier cells also showed significantly reduced phospholipid efflux to both apolipoproteins and HDL. A similar rate of cholesterol efflux, however, was observed from normal and Tangier cells when phospholipid vesicles or cyclodextrin were used as acceptors. In contrast to normal cells, only phospholipid vesicles and cyclodextrin and not apoA-I or HDL depleted intracellular cholesteryl esters from Tangier cells. Brefeldin, an inhibitor of intracellular vesicular trafficking, decreased HDL-mediated cholesterol efflux by approximately 40% but almost completely blocked both cholesterol and phospholipid efflux to apoA-I from normal cells. Brefeldin also inhibited cholesteryl ester depletion by apoA-I and HDL from normal cells. Brefeldin, however, had no significant effect on cholesterol efflux from Tangier cells to HDL. In summary, Tangier cells were found to be defective in both cholesterol and phospholipid efflux to HDL and apoA-I. The defect in apolipoprotein-mediated lipid efflux was not specific for apoA-I but also occurred for other apolipoproteins, and brefeldin blocked HDL-mediated lipid efflux from normal but not Tangier disease cells. On the basis of these results, a model is proposed whereby decreased cholesterol efflux by apolipoproteins in Tangier cells is the result of a defect in a brefeldin-sensitive pathway of lipid efflux.

Epithelial cells contain two distinct membrane surfaces, the apical and basolateral plasma membranes, which have different lipid and protein compositions. In order to assess the effect of the compositional differences of the apical and basolateral membranes on their ability to undergo cholesterol efflux, MDCK cells were radiolabeled with [ 3 H]cholesterol and grown as a polarized monolayer on filter inserts, that separate the upper apical compartment from the lower basolateral compartment. The rate of cholesterol efflux from the basolateral membrane into media containing HDL in the basolateral compartment was 6.3%/h Ϯ 0.7, whereas HDL-mediated efflux from the apical membrane was approximately 3-fold slower (1.9%/h Ϯ 0.3). In contrast, Fu5AH cells, which do not form distinct polarized membrane domains, had a similar rate of HDLmediated cholesterol efflux into the apical and basolateral compartments. Similar to HDL, other cholesterol acceptors, namely LDL, bovine serum albumin, and a lipid emulsion, also showed a decreased rate of cholesterol efflux from the apical membrane surface versus the basolateral membrane. Compared to the basolateral membrane, the apical membrane was also found to be more resistant to cholesterol oxidase treatment, to bind less HDL, and to take up less cholesterol from the medium. In conclusion, cholesterol efflux occurred less readily from the apical membrane than from the basolateral membrane for all types of acceptors tested. These results suggest that differences in the composition of the apical and basolateral membrane lead to a relative decrease in cholesterol desorption from the apical membrane and hence a reduced rate of cholesterol efflux.-