Abstract 16344: Hemoglobin Directs Macrophage Differentiation and Prevents Foam Cell Formation in Human Atherosclerotic Plaques
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 cells 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 and inhibited expression of ABCA1, indicating the importance of FPN-mediated reduction in intracellular iron and ROS in ABC transporter expression. Staining of hemorrhagic plaques with the redox sensitive dye dihydrorhodamine revealed lower expression of ROS compared to foam cells which consisted primarily of hydrogen peroxide, not the fenton reaction product *OH. Knockdown of LXRα inhibited ABC transporter expression in Hb:Hp differentiated macrophages while chromatin immunoprecipitation revealed increased binding of LXRα to the ABCA1 and ABCG1 promoters in M(Hb) and macrophages treated with superoxide dismutase, indicating reduced ROS triggers LXRα activation.
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 increasing expression of ABC transporters.
- © 2011 by American Heart Association, Inc.