Enzymatically Degraded LDL Preferentially Binds to CD14 ^high CD16 ⁺ Monocytes and Induces Foam Cell Formation Mediated Only in Part by the Class B Scavenger-Receptor CD36

Abstract

Abstract-Heterogeneity of peripheral blood monocytes is characterized by specific patterns in the membrane expression of Fc ␥-receptor III (Fc␥RIII/CD16) and the lipopolysaccharide receptor (LPS receptor CD14), allowing discrimination of distinct subpopulations. The aim was to analyze the correlation of these phenotypic differences to the early interaction of freshly isolated monocytes with modified lipoproteins by the use of either enzymatically degraded low density lipoprotein (E-LDL), acetylated low density lipoprotein (ac-LDL), oxidized low density lipoprotein (ox-LDL), or native low density lipoprotein. Highest E-LDL binding was observed on CD14 high CD16 ϩ monocytes as determined by flow cytometry, suggesting a selective interaction of E-LDL with distinct subpopulations of monocytes. E-LDL induced rapid foam cell formation both in predifferentiated monocyte-derived macrophages and, in contrast to ac-LDL or ox-LDL, also in freshly isolated peripheral blood monocytes. This was accompanied by upregulation of the 2 class B scavenger receptors CLA-1/SR-BI (CD36 and LIMPII Analogous-1/scavenger receptor type B class I) and CD36. Cellular binding and uptake of E-LDL was neither competed by ac-LDL nor the class A scavenger-receptor inhibitor polyinosinic acid but was partially inhibited by an excess of ox-LDL. In predifferentiated monocyte-derived macrophages, an anti-CD36 antibody inhibited cellular binding and uptake of E-LDL by Ϸ20%, suggesting that recognition of these hydrolasemodified low density lipoprotein particles is mediated only in part by the class B scavenger receptor CD36. Peripheral blood monocytes are phenotypically different with respect to membrane expression of Fc ␥-receptor III (Fc␥RIII/CD16) and the lipopolysaccharide receptor (LPS receptor/CD14), allowing discrimination of distinct subpopulations. 1 The impact on atherogenicity is as yet unknown. Within the vessel wall, the transformation of monocytes to macrophage foam cells may derive from the cellular uptake of different forms of chemically modified lipids and lipoproteins. Partial hydrolysis of lipoproteins by the hydrolytic host defense machinery, such as enzymatically degraded LDL (E-LDL), transforms lipoproteins to an atherogenic moiety. [2][3][4] Other lipoprotein modifications considered as relevant in atherogenesis include oxidized LDL (ox-LDL), 5 advanced glycation end products, 6 LDL modified by phospholipase A 2 , 7 and aggregated LDL. 8 Cellular uptake of these lipids and lipoproteins is considered to be mediated by charge and motif receptors directly recognizing nonopsonized ligands.Despite increasing knowledge about the mechanisms involved in foam cell formation of predifferentiated monocytederived macrophages, little is known about the interaction of freshly isolated monocytes with modified lipoproteins. In the present study, we demonstrate the correlation of blood monocyte heterogeneity to the cellular interaction with E-LDL. Furthermore, the present study shows that E-LDL, compared with acetylated LDL (ac-LDL) and ox-LDL, ...

High-Density Lipoprotein Inhibits the Uptake of Modified Low- Density Lipoprotein and the Expression of CD36 and FcγRI

“…In the case of mLDL, the uptake and subsequent formation of foam cells can be either a rapid process involving already present receptors or a delayed process with the participation of mLDL-inducible receptors [13][14][15][16] . Modifications of LDL involve both oxidative modifications, caused by oxygen reactive radical species, and enzymatic alterations processed by serumor cell-associated proteolytic enzymes 4,[17][18][19][20] . These enzyme-mediated processes are likely to occur concomitantly with or independently of oxidation.…”

“…In an attempt to further delineate other possible effects that can explain the well-known anti-atherogenic role of HDL, we aimed to analyze the influence of HDL on the uptake of mLDL and the expression of two known receptors, CD36 and Fc RI, in this process 19,23) . This was accomplished using LDL molecules modified by either transition-metal oxidation (Cu 2 ) or trypsin proteolysis 14,19,24) .…”

“…This was accomplished using LDL molecules modified by either transition-metal oxidation (Cu 2 ) or trypsin proteolysis 14,19,24) . Since studies based on human monocytes/macrophages often face the problem of a limited amount of cells, and further and most importantly their different stages of maturation, we used two monocytic cell lines, THP-1 and U-937.…”

“…CD36 expression in monocytes/macrophages is directly related to the increased uptake of oxLDL or enzLDL particles 13,19) . While CD36 expression increased with oxLDL, enzLDL or LDL, the presence of HDL induced the reduction of CD36 expression in both cell lines.…”

“…We consider it important to analyze enzyme-modified LDL because enzLDL is generated in an inflammatory environment where proteolytic enzymes are abundant in all parts of the body 2,20) . Even when oxygen reactive radical species are absent, LDL may be altered by inflammation and taken up by macrophages through mechanisms similar to those involved in the uptake of oxLDL 19,20) . Our diverse results emphasize how oxLDL and enzLDL may have different roles in foam cell formation.…”