Abstract 1: Apolipoprotein E2 and Apolipoprotein E4 Promote Macrophage Inflammation and Dysfunction Through Distinct Mechanisms
ApoE3 is the most common apoE variant in humans. Carriers of the apoE4 variant have increased risk of coronary artery disease, while apoE2 carriers have lower plasma lipid levels, reduced risk of coronary artery disease, but an increased risk of peripheral vascular disease. The differing effects of apoE2 and apoE4 on vascular disease manifestation have not been completely elucidated. ApoE2, apoE3 and apoE4 gene replacement (knock-in) mice were used to examine the influence apoE variants have on macrophage function and response to dietary challenge. Peritoneal macrophages isolated from apoE4 mice displayed less efferocytosis capacity and were more apoptotic even under basal conditions when compared to apoE3 and apoE2 macrophages. Stimulation with LPS or oxLDL increased apoptosis more robustly in apoE4 and apoE2 macrophages compared with apoE3 macrophages. Immunoblot analysis showed increased endoplasmic reticulum (ER) stress in apoE4 macrophages, with IRE1α and JNK being up-regulated 2- and 4-fold, respectively, compared with apoE2 and apoE3 macrophage. Phosphorylated PERK was 2-fold higher in apoE4 macrophages when stressed with LPS or oxLDL, compared with apoE3 or apoE2 macrophages. The pro-apoptotic protein, CHOP, was also up-regulated in apoE4 macrophages compared with apoE3 macrophages when exposed to oxLDL. When knock-in mice were challenged with a lipid-rich meal via gastric lavage, postprandial increase in neutrophil cell count was similar between apoE3 and apoE4 mice but was much higher in apoE2 mice. The robust postprandial neutrophil response in apoE2 mice coincided with their delay in postprandial clearance of triglyceride-rich lipoproteins. Interestingly, when the animals were maintained on a high fat, high cholesterol Western-type diet for 4 weeks, tissue macrophages were found to be more inflamed in both apoE2 and apoE4 mice compared to apoE3 mice. These results document distinct pro-inflammatory mechanisms: apoE2 increases postprandial inflammation through defective receptor binding and delayed lipid clearance, while apoE4 promotes inflammation by reducing macrophage function and increasing ER stress. These findings may help to elucidate the disparate influences on peripheral versus coronary artery diseases.
- © 2012 by American Heart Association, Inc.