Abstract 1727: Apolipoprotein E (ApoE) Induces an Anti-inflammatory Phenotype in Macrophages
Apolipoprotein E (apoE) exerts anti-atherogenic effects by promoting cholesterol efflux and hepatic lipoprotein clearance. However, apoE retains protective effects even under experimental settings, in which its influence on plasma cholesterol is negligible suggesting that this lipoprotein inhibits atherosclerosis independently from cholesterol transport. To gain further insight into mechanisms underlying apoE-mediated atheroprotection, we investigated its effect on the functional phenotype of RAW 264.7 macrophages overexpressing either of two apoE receptors: ApoER2/LRP8 or VLDL-R. Incubation of ApoER2/LRP8- or VLDL-R-expressing macrophages with apoE downregulated markers of pro-inflammatory M1 functional phenotype (expression and activity of iNOS, production of IL-12), whereas markers of anti-inflammatory M2 phenotype (expression and activity of arginase-I, production of IL-1RA) were upregulated. In addition, macrophage responses typical for M1 phenotype (migration, generation of reactive oxygen species, antibody-dependent cell cytotoxicity) were suppressed in ApoER2/LRP8- or VLDL-R-expressing cells in the presence of apoE. Finally, apoE prevented LPS- and IFN-γ-induced activation of ApoER2/LRP8- or VLDL-R-expressing macrophages as documented by reduced production of IL-12, TNF-α and MCP-1, reduced expression and activity of iNOS and COX2, and reduced activation and/or phosphorylation of NF-κB, IκB and STAT1. The modulatory effects of apoE on macrophage phenotype were inhibited by SB220025, a p38MAP kinase inhibitor, and PP1A, a tyrosine kinase inhibitor. Accordingly, apoE induced tyrosine kinase-dependent activation of p38MAP kinase in ApoER2/LRP8- or VLDL-R-expressing macrophages. Under in vivo conditions, apoE−/− mice transplanted with apoE-producing wild-type bone marrow presented with increased plasma IL-1RA levels. In addition, peritoneal macrophages from transplanted animals demonstrated enhanced M2 phenotype (increased IL-1RA production and CD206 expression). We conclude that apoE signalling over ApoER2/LRP8 or VLDL-R promotes macrophage conversion from pro-inflammatory M1 to anti-inflammatory M2 phenotype. This effect may represent a novel anti-atherogenic activity of apoE.