Complement Protein Clq Recognizes Enzymatically Modified Low-Density Lipoprotein through Unesterified Fatty Acids Generated by Cholesterol Esterase

We previously reported that enzymatically modified low-density lipoprotein (E-LDL) particles obtained by LDL treatment with trypsin and then cholesterol esterase are recognized by Clq and activate the Cl complex of complement. The objective of this study was to identify the E-LDL component(s) recognized by Clq. In addition to trypsin, plasmin, thrombin. tryptase, and matrix metalloprotease-2 each yielded E-LDL particles With high Cl-activating efficiency, and the Cl activation extent was strictly dependent oil cholesterol esterase treatment in all cases. When incorporated Into vesicles, the lipid fraction of E-LDL, but not of native LDL, triggered Cl activation, and activation correlated with the amount of unesterified cholesterol generated by cholesterol esterase. Whereas treatment of E-LDL particles with human serum albumin reduced their fatty acid content, both cholesterol and unesterified Fatty acids were decreased by methyl-beta-cyclodextrin, both treatments resulting in dose-dependent inhibition of the Cl-activating ability of the particles. Incorporation of linoleic acid into phosphatidylcholine-containing model vesicles enabled them to interact with the Clq globular domain and to trigger Cl activation, and cholesterol enhanced both processes by facilitating incorporation of the fatty acid into the vesicles. Direct evidence that Clq binds E-LDL through its globular domains was obtained by electron microscopy. This study demonstrates that Cl binding to E-LDL particles Involves recognition by the Clq globular domain of the unesterified fatty acids generated by cholesterol esterase. The potential implications of these Findings in atherogenesis are discussed.