Synthetic amphipathic helical peptides promote lipid efflux from cells by an ABCA1-dependent and an ABCA1-independent pathway

In order to examine the necessary structural features for a protein to promote lipid efflux by the ABCA1 transporter, synthetic peptides were tested on ABCA1-transfected cells (ABCA1 cells) and on control cells. L-37pA, an l amino acid peptide that contains two class-A amphipathic helices linked by proline, showed a 4-fold increase in cholesterol and phospholipid efflux from ABCA1 cells compared to control cells. The same peptide synthesized with a mixture of l and d amino acids was less effective than L-37pA in solubilizing dimyristoyl phosphatidyl choline vesicles and in effluxing lipids. In contrast, the 37pA peptide synthesized with all d amino acids (D-37pA) was as effective as L-37pA. Unlike apoA-I, L-37pA and D-37pA were also capable, although at a reduced rate, of causing lipid efflux independent of ABCA1 from control cells, Tangier disease cells, and paraformaldehyde fixed ABCA1 cells. The ability of peptides to bind to cells correlated with their lipid affinity. In summary, the amphipathic helix was found to be a key structural motif for peptide-mediated lipid efflux from ABCA1, but there was no stereoselective requirement. In addition, unlike apoA-I, synthetic peptides can also efflux lipid by a passive, energy-independent pathway that does not involve ABCA1 but does depend upon their lipid affinity.