The Receptor Binding Domain of Apolipoprotein E, Linked to a Model Class A Amphipathic Helix, Enhances Internalization and Degradation of LDL by Fibroblasts

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Abstract
Human apolipoprotein E (apo E) consists of two distinct domains, the lipid-associating domain (residues 192−299) and the globular domain (residues 1−191) which contains the LDL receptor (LDLR) binding site (residues 129−169). To test the hypothesis that an arginine-rich apo E receptor binding domain (residues 141−150) is sufficient to enhance low-density lipoprotein (LDL) uptake and clearance when covalently linked to a class A amphipathic helix, a peptide in which the receptor binding domain of human apo E, LRKLRKRLLR (hApoE[141−150]), is linked to 18A, a well-characterized high-affinity lipid-associating peptide (DWLKAFYDKVAEKLKEAF), we synthesized the peptide hApoE[141−150]-18A (hE18A) and its end-protected analogue, Ac-hE18A-NH2. The importance of positively charged residues and the role of the hydrophobic residues in the receptor binding domain were also studied using four analogues. Ac-LRRLRRRLLR-18A-NH2 [Ac-hE(R)18A-NH2] and Ac-LRKMRKRLMR-18A-NH2 (Ac-mE18A-NH2) contained an extended hydrophobic face, including the receptor binding region. Control peptides, Ac-LRLLRKLKRR-18A-NH2 [Ac-hE(Sc)18A-NH2], had the amino acid residues of the apo E receptor binding domain scrambled to disrupt the extended hydrophobic face, and Ac-RRRRRRRRRR-18A-NH2 (Ac-R1018A-NH2) had only positively charged Arg residues as the receptor binding domain. The effect of the dual-domain peptides on the uptake and degradation of human LDL by fibroblasts was determined in murine embryonic fibroblasts (MEF1). LDL internalization was enhanced 3-, 5-, and 7-fold by Ac-mE18A-NH2, Ac-hE18A-NH2, and Ac-hE(R)18A-NH2, respectively, whereas the control peptides had no significant biological activity. All three active peptides increased the level of degradation of LDL by 100%. The LDL binding and internalization to MEF1 cells in the presence of these peptides was not saturable over the LDL concentration range that was studied (1−10 μg/mL). Furthermore, a similar enhancement of LDL internalization was observed independent of the presence of the LDL receptor-related protein (LRP), LDLR, or both. Pretreatment of cells with heparinase and heparitinase abolished more than 80% of the enhanced peptide-mediated LDL uptake and degradation by cells. We conclude that the dual-domain peptides enhanced LDL uptake and degradation by fibroblasts via a heparan sulfate proteoglycan (HSPG)-mediated pathway.