Complement Protein C1q Recognizes Enzymatically Modified Low-Density Lipoprotein through Unesterified Fatty Acids Generated by Cholesterol Esterase

Abstract

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 C1q and activate the C1 complex of complement. The objective of this study was to identify the E-LDL component(s) recognized by C1q. In addition to trypsin, plasmin, thrombin, tryptase, and matrix metalloprotease-2 each yielded E-LDL particles with high C1-activating efficiency, and the C1 activation extent was strictly dependent on cholesterol esterase treatment in all cases. When incorporated into vesicles, the lipid fraction of E-LDL, but not of native LDL, triggered C1 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 C1-activating ability of the particles. Incorporation of linoleic acid into phosphatidylcholine-containing model vesicles enabled them to interact with the C1q globular domain and to trigger C1 activation, and cholesterol enhanced both processes by facilitating incorporation of the fatty acid into the vesicles. Direct evidence that C1q binds E-LDL through its globular domains was obtained by electron microscopy. This study demonstrates that C1 binding to E-LDL particles involves recognition by the C1q globular domain of the unesterified fatty acids generated by cholesterol esterase. The potential implications of these findings in atherogenesis are discussed.

Enzymatically Modified Low‐Density Lipoprotein Is Present in All Stages of Aortic Valve Sclerosis: Implications for Pathogenesis of the Disease

“…The analysis of the lipid composition of early valvular cusp lesions (grade 1) compared with internal control tissues by mass spectrometry detecting both unesterified cholesterol and linoleic acid corroborated our immunohistochemical findings. While it is known that free cholesterol has an intrinsic complement‐activating capacity (see later),28 free fatty acids contained in the eLDL particle not only bind and activate C129 but also play multifaceted roles through their dual capacity to exert stimulatory and cytotoxic effects on neighboring cells 14, 15…”

Molecular Biology of Atherosclerosis

“…C1q is also activated by enzymatically modified LDL containing free fatty acids together with phospholipid (151). C1q enhanced the uptake of oxLDL into macrophages with subsequent activation of the cells including increased CD80, PECAM1, and MCP-1 release but also induction of ABCA1 and ABCG1 with enhanced cholesterol efflux to HDL (543).…”

Investigations on the C1q–Calreticulin–Phosphatidylserine Interactions Yield New Insights into Apoptotic Cell Recognition

“…Besides its role in the activation of the classical complement pathway in response to pathogen infection, C1q plays a crucial role in the detection and scavenging of a wide variety of noxious alteredself substances, such as β-amyloid fibrils, the pathological form of the prion protein, apoptotic and necrotic cells, or modified forms of the lowdensity lipoprotein. [5][6][7][8][9][10][11] Interestingly, unlike the other complement proteins, C1q is produced by macrophages and immature dendritic cells. Numerous studies have shown that C1q influences the phagocyte "status" through regulation of cytokine expression 12,13 and that it may be involved in the nonimmunogenic presentation of self-antigens through its ability to modulate maturation of dendritic cells.…”