ATP-binding cassette transporter A1 (ABCA1) functions as a cholesterol efflux regulatory protein

ABCA1, an ATP-binding cassette transporter mutated in Tangier disease, prom otes cellular phospholipid and cholesterol efflux by loading free apoA-I wi th these lipids. This process involves binding of apoA-I to the cell surfac e and phospholipid translocation by ABCA1, The goals of this study were to examine the relationship between ABCA1-mediated lipid efflux and apolipopro tein binding and to determine whether phospholipid and cholesterol efflux a re coupled. Inhibition of lipid efflux by glybenclamide treatment or by mut ation of the ATP-binding cassette of ABCA1 showed a close correlation betwe en lipid efflux, the binding of apoA-I to cells, and cross-linking of apoA- I to ABCA1, The data suggest that a functionally important apoA-I binding s ite exists on ABCA1 and that the binding site could also involve lipids. Af ter using cyclodextrin preincubation to deplete cellular cholesterol, ABCA1 -mediated cholesterol efflux was abolished but phospholipid efflux and the binding of apoA-I were unaffected, The conditioned media hom cyclodextrin-p retreated, ABCA1-expressing cells readily promoted cholesterol efflux when added to fresh cells not expressing ABCA1, indicating that cholesterol effl ux can be dissociated from phospholipid efflux, Further, using a photoactiv atable cholesterol analog, we showed that ABCA1 did not bind cholesterol di rectly, even though several other cholesterol-binding proteins specifically bound the cholesterol analog. The data suggest that the binding of apoA-I to ABCA1 leads to the formation of phospholipid-apoA-I complexes, which sub sequently promote cholesterol efflux in an autocrine or paracrine fashion.