Myeloperoxidase-generated reactive nitrogen species convert LDL into an atherogenic form in vitro
Eugene A. Podrez, … , Henry F. Hoff, Stanley L. Hazen
Eugene A. Podrez, … , Henry F. Hoff, Stanley L. Hazen
Published June 1, 1999
Citation Information: J Clin Invest. 1999;103(11):1547-1560. https://doi.org/10.1172/JCI5549.
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Article

Myeloperoxidase-generated reactive nitrogen species convert LDL into an atherogenic form in vitro

Abstract

Oxidized LDL is implicated in atherosclerosis; however, the pathways that convert LDL into an atherogenic form in vivo are not established. Production of reactive nitrogen species may be one important pathway, since LDL recovered from human atherosclerotic aorta is enriched in nitrotyrosine. We now report that reactive nitrogen species generated by the MPO-H2O2-NO2– system of monocytes convert LDL into a form (NO2-LDL) that is avidly taken up and degraded by macrophages, leading to massive cholesterol deposition and foam cell formation, essential steps in lesion development. Incubation of LDL with isolated MPO, an H2O2-generating system, and nitrite (NO2–)— a major end-product of NO metabolism—resulted in nitration of apolipoprotein B 100 tyrosyl residues and initiation of LDL lipid peroxidation. The time course of LDL protein nitration and lipid peroxidation paralleled the acquisition of high-affinity, concentration-dependent, and saturable binding of NO2-LDL to human monocyte–derived macrophages and mouse peritoneal macrophages. LDL modification and conversion into a high-uptake form occurred in the absence of free metal ions, required NO2–, occurred at physiological levels of Cl–, and was inhibited by heme poisons, catalase, and BHT. Macrophage binding of NO2-LDL was specific and mediated by neither the LDL receptor nor the scavenger receptor class A type I. Exposure of macrophages to NO2-LDL promoted cholesteryl ester synthesis, intracellular cholesterol and cholesteryl ester accumulation, and foam cell formation. Collectively, these results identify MPO-generated reactive nitrogen species as a physiologically plausible pathway for converting LDL into an atherogenic form.

Authors

Eugene A. Podrez, David Schmitt, Henry F. Hoff, Stanley L. Hazen

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Figure 3

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NO2– concentration dependence of MPO-mediated modification of LDL and co...
NO2– concentration dependence of MPO-mediated modification of LDL and conversion into a high-uptake form for macrophages. [125I]LDL (0.2 mg/mL) was incubated with isolated human MPO (30 nM), glucose (100 μM), glucose oxidase (20 ng/mL), and the indicated concentrations of NO2–, in the absence (filled circles) and presence (filled squares) of Cl– (100 mM) in sodium phosphate buffer (50 mM, pH 7.0) supplemented with DTPA (100 μM) overnight at 37°C, as described in Methods. Uptake of the modified lipoproteins was then assessed using hMDMs (a) and thioglycollate-elicited MPMs (b) as described in Methods. Data represent the mean ± SD of triplicate determinations. Similar results were observed in 3 independent experiments.