Abstract 15239: ER Stress May Underlie Differences in Foam Cell Lipid Droplet Autophagy in AKR and DBA/2 Mouse Macrophages
Atherosclerosis in various mouse models is sensitive to the genetic background, and we have previously shown that apoE-deficient DBA/2 mice have 10-fold larger lesion areas than apoE-deficient AKR mice. As an early step in the formation of atherosclerotic lesions is the transformation of arterial wall macrophages into lipid-loaded foam cells, we studied cholesterol metabolism after cholesterol loading with acetylated low-density lipoprotein (AcLDL) in bone marrow-derived macrophages from apoE-deficient AKR and DBA/2 mice in vitro. We found that DBA/2 versus AKR macrophages accumulated more cholesterol esters (CE) in lipid droplets (LD) attributable to a decreased rate of LD-stored CE hydrolysis through autophagy. The defect in DBA/2 mice appeared to be at the step of fusion of autophagosomes with lysosomes. The focus of the present study is to identify signaling pathways involved in the differential regulation of autophagic flux between AKR and DBA/2 cells. We report here that CHOP protein and mRNA were up regulated upon AcLDL loading in AKR macrophages (4.3-fold increase, p<0.01 and 3-fold increase, p<0.001, respectively), but not DBA/2 cells, suggesting that endoplasmic reticulum (ER) stress was induced by cholesterol loading only in AKR cells. As ER stress is a well known activator of autophagy, we looked at downstream effectors of that pathway and found that TRIB3 protein and mRNA were similarly up regulated by AcLDL loading only in AKR cells (2.7-fold increase and 1.4-fold increase, p<0.001, respectively). However, we were not able to find changes in activation of Akt or p70-S6K, two downstream members of Trib3 signaling suggesting that another pathway is responsible for autophagy activation through ER stress. Trib3 has previously been shown to increase PPARgamma levels. We found that PPARgamma mRNA was 59% higher in AKR vs. DBA/2 macrophages (p<0.001), and that upon AcLDL loading, PPARgamma mRNA increased only in AKR cells (35% increase, p<0.001). Thus, mouse strain effects mediated by ER stress may signal through PPARgamma to alter autophagic flux at the step of autophagosome fusion with the lysosome.
- © 2013 by American Heart Association, Inc.