Activating Transcription Factor 1 Directs Mhem Atheroprotective Macrophages Through Coordinated Iron Handling and Foam Cell Protection

Rationale: Intraplaque hemorrhage (IPH) drives atherosclerosis through the dual metabolic stresses of cholesterol-enriched erythrocyte membranes and pro-oxidant heme/iron. When clearing tissue hemorrhage, macrophages are typically seen storing either iron or lipid. We have recently defined hemorrhage-associated macrophages (HA-mac) as a plaque macrophage population that responds adaptively to IPH. Objective: This study aimed to define the key transcription factor(s) involved in HO-1 induction by heme. Methods and Results: To address this question, we used microarray analysis and transfection with siRNA and plasmids. To maintain physiological relevance, we focused on human blood-derived monocytes. We found that heme stimulates monocytes through induction of activating transcription factor 1 (ATF-1). ATF-1 coinduces heme oxygenase-1 (HO-1) and Liver X receptor beta (LXR-beta). Heme-induced HO-1 and LXR-beta were suppressed by knockdown of ATF-1, and HO-1 and LXR-beta were induced by ATF-1 transfection. ATF-1 required phosphorylation for full functional activity. Expression of LXR-beta in turn led to induction of other genes central to cholesterol efflux, such as LXR-alpha and ABCA1 This heme-directed state was distinct from known macrophage states (M1, M2, Mox) and, following the same format, we have designated them Mhem. Conclusions: These results show that ATF-1 mediates HO-1 induction by heme and drives macrophage adaptation to intraplaque hemorrhage. Our definition of an ATF-1 mediated pathway for linked protection from foam cell formation and oxidant stress may have therapeutic potential. (Circ Res. 2012;110:20-33.)