Hemoglobin Directs Macrophage Differentiation and Prevents Foam Cell Formation in Human Atherosclerotic Plaques

• Type of Material: Article
• Authors: Aloke V Finn, Emory University; Masataka Nakano, CVPath Institute, Inc.; Rohini Polavarapu, Emory University; Vinit Karmali, Emory University; Omar Saeed, Emory University; XiaoQing Zhao, CVPath Institute, Inc.; Saami Yazdani, CVPath Institute, Inc.; Fumiyuki Otsuka, CVPath Institute, Inc.; Talina Davis, Emory University; Anwer Habib, Emory University; Jagat Narula, University of California; Frank D. Kolodgie, CVPath Institute, Inc.; Renu Virmani, CVPath Institute, Inc.
• Language: English
• Date: 2012-01-10
• Publisher: Elsevier: 12 months
• Publication Version: Author Accepted Manuscript (After Peer Review)
• Copyright Statement: © 2011 American College of Cardiology Foundation. Published by Elsevier Inc. All rights reserved.
• License: Creative Commons Attribution-NonCommerical-NoDerivs 3.0 Unported
• Final Published Version (URL): http://dx.doi.org/10.1016%2Fj.jacc.2011.10.852
• Title of Journal or Parent Work: Journal of the American College of Cardiology
• ISSN: 0735-1097
• Volume: 59
• Issue: 2
• Start Page: 166
• End Page: 177
• Grant/Funding Information: This study was supported by the Carlyle Fraser Heart Center, CVPath Inc., and US National Institutes of Health grant RO1 HL096970-01A.
• Keywords: macrophages; atherosclerosis; ABC transporters; hemoglobin; inflammation; reactive oxygen species
• Research Categories: Health Sciences, Medicine and Surgery; Biophysics, Medical

Abstract

Objectives To examine selective macrophage differentiation occurring in areas of intraplaque hemorrhage in human atherosclerosis. Background Macrophage subsets are recognized in atherosclerosis but the stimulus for and importance of differentiation programs remains unknown. Methods We used freshly isolated human monocytes, a rabbit model, and human atherosclerotic plaques to analyze macrophage differentiation in response to hemorrhage. Results Macrophages characterized by high expression of both mannose and CD163 receptors preferentially exist in atherosclerotic lesions at sites of intraplaque hemorrhage. These hemoglobin (Hb)-stimulated macrophages, M(Hb), are devoid of neutral lipids typical of foam cells. In vivo modeling of hemorrhage in the rabbit model demonstrated that sponges exposed to red cells showed an increase in mannose receptor positive macrophages only when these cells contained hemoglobin (Hb). Cultured human monocytes exposed to hemoglobin:haptoglobin complexes (Hb:Hp), but not IL-4, expressed the M(Hb) phenotype and were characterized by their resistance to cholesterol loading and upregulation of ABC transporters. M(Hb) demonstrated increased ferroportin (FPN) expression, reduced intracellular iron, and reactive oxygen species (ROS). Degradation of FPN using hepcidin increased ROS, inhibited ABCA1 expression, and cholesterol efflux to ApoAI, suggesting reduced ROS triggers these effects. Knockdown of liver x receptor alpha (LXRα) inhibited ABC transporter expression in M(Hb) and macrophages differentiated in the anti-oxidant superoxide dismutase. Lastly, liver X receptor α (LXR) luciferase reporter activity was increased in M(Hb) and significantly reduced by overnight treatment with hepcidin. Collectively, these data suggest reduced ROS triggers LXRα activation and macrophage reverse cholesterol transport (RCT). Conclusions Hb is a stimulus for macrophage differentiation in human atherosclerotic plaques. A reduction of macrophage intracellular iron plays an important role in this non- foam cell phenotype by reducing ROS, which drives transcription of ABC transporters through activation of LXRα. Reduction of macrophage intracellular iron may be a promising avenue to increase macrophage RCT.