Complement and Atherogenesis

Abstract

Abstract-Complement activation occurs in temporal correlation with the subendothelial deposition of LDL during early atherogenesis, and complement also plays a pathogenetic role in promoting lesion progression. Two lesion components have been identified that may be responsible for complement activation. First, enzymatic degradation of LDL generates a derivative that can spontaneously activate complement, and enzymatically degraded LDL (E-LDL) has been detected in the lesions. Second, C-reactive protein (CRP) colocalizes with complement C5b-9, as evidenced by immunohistological studies of early atherosclerotic lesions, so the possibility exists that this acute phase protein also fulfills a complement-activating function. Here, we report that addition of LDL and CRP to human serum did not result in significant C3 turnover. Addition of E-LDL provoked complement activation, which was markedly enhanced by CRP.Binding of CRP to E-LDL was demonstrated by sucrose flotation experiments. Binding was Ca 2ϩ -dependent and inhibitable by phosphorylcholine, and the complement-activating property of E-LDL was destroyed by treatment with phospholipase C. These results indicated that CRP binds to phosphorylcholine groups that become exposed in enzymatically degraded LDL particles. Immunohistological studies complemented these findings in showing that CRP colocalizes with E-LDL in early human atherosclerotic lesions. Thus enzymatic, nonoxidative modification of tissue-deposited LDL can be expected to confer CRP-binding capacity onto the molecule. The ensuing enhancement of complement activation may be relevant to the development and progression of the atherosclerotic lesion. Key Words: atherogenesis Ⅲ corrective protein Ⅲ complement Ⅲ LDL C omplement and C-reactive protein (CRP) are emerging as 2 components that may play important roles in atherogenesis. Early studies indicated that activated complement components 1,2 and CRP 3,4 are present in atherosclerotic lesions, and the demonstration followed that in situ C5b-9 generation occurred in temporal correlation with lipid deposition. 5 The search for a complement-activating entity led to the isolation of an LDL derivative, termed lesion complement activator (LCA), that had the capacity to activate the alternative complement pathway. 6 We are considering that the high content of free cholesterol in the LCA particles is important because unesterified cholesterol activates complement. 7 Similar lipidic moieties were isolated in other laboratories, 8,9 although their capacity to activate complement was not tested. Furthermore, fused LDL particles micromorphologically similar to LCA were visualized in extracellular location by deep freeze-etch electronmicroscopy in arteries of cholesterol-fed rabbits. 10 Collectively, these studies indicated that tissue-deposited LDL is modified extracellularly to yield lipid droplets with a high content of free cholesterol that have intrinsic complement-activating capacity.Subsequently, it was shown that LDL, but not HDL or VLDL, could be transformed...

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

“…Complement activation is a leading effect of eLDL in atherosclerotic lesions,10, 12, 13 and C3d and C5b‐9 deposits were also present in every grade 1 to 4 valvular cusp lesion examined. Grade 1 and grade 2 typically showed a predominant focal deposition of C3d in the pale insudative zone of the fibrosa below the layer of macrophage foam cells and in the  deeper part of the fibrosa adjacent to the spongiosa.…”

“…Enzymatic modification of LDL requires proteases,10, 23 and both binding to CRP and complement activation12, 13 are important capacities of eLDL in atherogenesis. We therefore performed expression studies by using Affymetrix HuGene 1‐0 st v1 arrays to determine genes differentially regulated in early valvular cusp lesions (grade 1) compared with healthy probe sets.…”

“…One of the hallmarks of atherosclerosis is the close spatial and functional association between eLDL, complement components and CRP within the vessel wall 11, 12, 13. It was thus self‐evident to investigate whether the same holds true for the 4 grades of aortic valve calcification and structural damage.…”

“…It is well known, however, that most of the CRP within atherosclerotic lesions derives from plasma CRP insudation 24. We therefore performed extensive immunohistochemical studies using a well‐established mouse monoclonal anti‐CRP antibody 12, 13, 24, 25. Surprisingly, besides increasing extracellular staining around calcified areas of grade 2 to 4 lesions (rather impressive in grade 4 lesions) as well as rare intracellular staining, we were not able to detect any extracellular staining in early lesions (Figure 5B, left panels).…”

“…Subsequent to cellular uptake of eLDL and foam cell formation,10 the physiological sequence of events is concluded by reverse cholesterol transport 7. If the capacity of the system is overburdened (eg, hypercholesterolemia), accumulation of eLDL is the consequence with subsequent generation of potentially harmful C5b‐9 complexes by both the classic and the alternative pathway 12, 13. Further, free fatty acids contained in the eLDL particle play multifaceted roles through their dual capacity to exert stimulatory and cytotoxic effects on neighboring cells 14, 15.…”