Differential effects in mice of infused native and MPO-modified ApoA-I on reverse cholesterol transport and atherosclerosis

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Purpose: Preclinical and clinical studies have shown beneficial effects of infusions of apolipoprotein A-I (ApoA-I, the major protein in HDL) on the progression and regression of atherosclerosis. However, ApoA-I is also a preferred and selective target for myeloperoxidase- (MPO) mediated oxidative modification, which leads in vitro to a loss of its ability to promote ABCA1-dependent cholesterol efflux from macrophages - a critical process for Reverse Cholesterol Transport (RCT). We therefore hypothesized that MPO-mediated oxidation of ApoA-I would impair its function to promote RCT in vivo and to mediate beneficial effects on atherosclerotic plaques. Methods: ApoE-/- mice were fed a Western diet for 16 weeks to develop advanced atherosclerosis and were then injected with native human ApoA-I, oxidized human ApoA-I (MPO/hydrogen peroxide/chloride treated; oxApoA-I) or PBS (carrier) over 1 week. Aortic root sections were used for analysis of markers (Oil Red O: lipids, CD68: macrophages, CCR7: plaque regression factor, monocyte chemoattractant protein-1 and IL-1β: M1 macrophages, mannose receptor and arginase-I: M2 macrophages) and for laser capture microdissection of macrophage (CD68+) plaque cells for molecular analysis. An in vivo RCT assay was performed by injecting ApoA-I (native or oxidized) into ApoA-I-/- mice. Results: Oxidation of ApoA-I significantly impaired RCT in vivo. Injection of native ApoA-I led to significant decreases in lipid content (by ∼20%), macrophage number (by ∼30%), and an increase in collagen content (by ∼40%) in atherosclerotic plaques, but oxApoA-I failed to mediate these changes. The decrease in macrophages in the native ApoA-I group was accompanied with an induction (∼5 fold) of their chemokine receptor CCR7. Furthermore, only native ApoA-I injections led to a significant reduction of inflammatory M1 macrophage markers MCP-1 and IL-1β, and an increase in anti-inflammatory M2 macrophage markers arginase-I and mannose receptor in the plaques. Conclusions: MPO-mediated oxidation renders ApoA-I dysfunctional and unable to promote reverse cholesterol transport and to mediate beneficial changes in the composition of atherosclerotic plaques and the inflammatory status of plaque macrophages.