Molecular interactions between apoE and ABCA1: impact on apoE lipidation

Apolipoprotein E (apoE)/ABCA1 interactions were investigated in human intact fibroblasts induced with 22(R)-hydroxycholesterol and 9-cis-retinoic acid (stimulated cells). Here, we show that purified human plasma apoE3 forms a complex with ABCA1 in normal fibroblasts. Lipid-free apoE3 inhibited the binding of (125)I-apoA-I to ABCA1 more efficiently than reconstituted HDL particles (IC(50) = 2.5 +/- 0.4 microg/ml vs. 12.3 +/- 1.3 microg/ml). ApoE isoforms showed similar binding for ABCA1 and exhibited identical kinetics in their abilities to induce ABCA1-dependent Cholesterol efflux. Mutation of ABCA1 associated with Tangier disease (C1477R) abolished both apoE3 binding and apoE3-mediated Cholesterol efflux. Analysis of apoE3-containing particles generated during the incubation of lipid-free apoE3 with stimulated normal cells showed nascent apoE3/Cholesterol/phospholipid complexes that exhibited prebeta-electrophoretic mobility with a particle size ranging from 9 to 15 nm, whereas lipid-free apoE3 incubated with ABCA1 mutant (C1477R) cells was unable to form such particles. These results demonstrate that 1). apoE association with lipids reduced its ability to interact with ABCA1; 2). apoE isoforms did not affect apoE binding to ABCA1; 3). apoE-mediated ABCA1-dependent Cholesterol efflux was not affected by apoE isoforms in fibroblasts; and 4). the lipid translocase activity of ABCA1 generates apoE-containing high density-sized lipoprotein particles. Thus, ABCA1 is essential for the biogenesis of high density-sized lipoprotein containing only apoE particles in vivo.