The evolution of the atherosclerotic plaque is regulated by the ability of its cells to dispose of excess cholesterol before apoptosis ensues. Macrophages are the dominant cell type of early plaques in humans and mice, and can utilize several pathways for lipoprotein internalization and for cholesterol efflux. Because cholesterol efflux is activated by delivery of lipoprotein-derived oxysterols to LXR, it is possible that different internalizing receptors may induce different macrophage responses to the same cholesterol load. Apolipoprotein (apo)E, a multifunctional protein highly expressed by macrophages and under LXR control, has strong anti-atherogenic effects that can be mediated by its interactions with internalizing receptors, such LDLR and LRP1, or with agents of cholesterol efflux, such as ABCA1 and PLTP. In the previous cycle of this grant, we discovered that LRP1 deletion from macrophages leads to increased apoE synthesis and, surprisingly, increased lesion formation. This suggests that apoE cannot exert its beneficial effects without LRP1. We also identified a functional interaction between apoE and LRP1 controlling the secretory efficiency of macrophage apoE, and determined that human apoE variants expressed by macrophages associate differently with LRP1 and have profoundly different effects on atherogenesis. Finally, we provided evidence for a role of apoE in macrophage cholesterol efflux even in the presence of excess amounts of apoAI, the canonical activator of the HDL pathway. Our results are compatible with a scenario where apoE and LRP1 work in a functional axis to maximize the adaptive responses of atheroma foam cells to the tremendous cholesterol burden in the arterial intima. In this new proposal, we plan to: 1) Demonstrate that the interaction between apoE and LRP1 in macrophages influences lesion formation in mouse models of atherosclerosis;2) Test the hypothesis that LRP1-mediated internalization of apoE-enriched lipoproteins improves trafficking of cholesterol to efflux- accessible compartments and enhances LXR response;3) Determine whether LRP1 signaling is responsible for anti-inflammatory and pro-survival counter-regulation in macrophages through cholesterol-dependent and independent mechanisms. Our studies aim at unlocking the fundamental regulation of cholesterol release from lesion macrophages into the HDL pathway as a platform for the development of therapeutics for plaque regression and control of ischemic heart disease. Our studies focus on the mechanisms leading to the accumulation and removal of cholesterol from the atherosclerotic plaque, which is the common cause of heart attacks and strokes. Whereas current medications can reduce the accumulation of cholesterol in the plaque by reducing plasma LDL levels, no therapies are available to induce exit of cholesterol from the plaque. We have discovered a functional connection between two proteins (apoE and LRP1) that improves cholesterol disposal in plaque macrophages, and may lead to development of new drugs for regression of coronary plaques.
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