Apolipoprotein B and E Basic Amino Acid Clusters Influence Low-Density Lipoprotein Association with Lipoprotein Lipase Anchored to the Subendothelial Matrix

Abstract

Abstract Lipoprotein accumulation in the subendothelial matrix is an important step in atherogenesis. We have previously shown that addition of lipoprotein lipase (LPL) markedly increased binding of apolipoprotein B (apoB)–containing lipoproteins to an endothelial cell–derived matrix, and this enhanced lipoprotein binding was inhibited by apoE. In the present studies we examined the role of various regions of apoB in the binding of LDL to LPL-containing endothelial cell matrix and the ability of various apoE domains to decrease lipoprotein retention. We studied three apoB epitope-specific monoclonal antibodies for their ability to block the binding of 125 I-LDL to LPL-containing matrix. Of these, monoclonal antibody 4G3, which recognizes an arginine-containing epitope in apoB, was the most effective in reducing LDL binding. Chemical modification of LDL apoB lysines or arginines markedly reduced the ability of the lipoprotein to block the binding of 125 I-LDL to LPL-containing matrix, suggesting that apoB positively charged amino acids are involved in the interaction. Furthermore, polyarginine or polylysine markedly decreased 125 I-LDL binding to LPL-containing matrix, whereas polyleucine was ineffective. These data suggest that apoB positively charged regions are important in LDL binding. To explore the role of charge modifications on apoE by single arginine-cysteine interchanges, we examined the effects of the three major human apoE isoforms (apoE2, apoE3, and apoE4). ApoE3 was the most effective in decreasing 125 I-LDL retention, followed by apoE4; apoE2 was the least effective. Similarly, apoE2-containing HDL was much less effective than apoE3-containing HDL in decreasing 125 I-LDL retention. Therefore, both cysteine for arginine substitutions at amino acids 112 and 158, known to markedly reduce apoE binding to the LDL receptors, also had significant effects on the ability of this apoE isoform to displace LDL bound to LPL. Two peptides generated by thrombin cleavage of apoE3 both were able to decrease 125 I-LDL binding, indicating the presence of multiple sites within apoE that could participate in the inhibitory effect. We conclude that positively charged regions on apoB are responsible for the binding of LDL to LPL-containing matrix and that similar regions of positive charge in apoE allow it to compete and decrease the retention of LDL.