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Atherosclerosis is a chronic inflammatory disease that results from the accumulation of cholesterol-containing LDL particles in the vessel wall, and is the underlying pathology for many cardiovascular diseases. Current clinical guidelines focus on LDL-cholesterol lowering as a means of cardiovascular disease prevention. The immune system also plays a major role in the pathogenesis of this disease, and emerging evidence points to the therapeutic potential of targeting inflammation to combat cardiovascular diseases. Studies have identified dual roles of apolipoprotein E (apoE) and apoE receptors in modulating plasma lipids and inflammation, and have implicated these proteins as key players in atherogenesis. Moreover, genome-wide association studies have linked polymorphisms in the genes encoding these proteins with altered cardiovascular disease risk in humans. ApoE was first identified as a component of circulating lipoproteins, and the importance for ligand-receptor interactions between lipoprotein-associated apoE and hepatic endocytic receptors for maintaining healthy cholesterol levels has been since well established. However, the roles of apoE and apoE receptors expressed in other tissues and their involvement in the pathogenesis of atherosclerosis have not been fully elucidated. The goals of this dissertation research are to (1) investigate the impact of human APOE polymorphisms on macrophage-driven mechanisms of atherosclerosis, and (2) evaluate the effect of global and immune-cell specific expression of a novel mutation of the apoE receptor, LRP1, on development of metabolic disease and atherosclerosis.The functions of macrophage expression of the different human apoE isoforms – apoE2, apoE3, and apoE4 – in modulating atherogenesis was assessed by transplantation of donor bone marrow expressing the apoE isoforms into recipient ApoE-/- mice. Despite the similar plasma cholesterol levels between groups, we observed significant reduction in atherosclerosis in the mice receiving apoE3-expressing bone marrow, but not apoE2- or apoE4-expressing bone marrow, indicating that apoE3 is anti-atherogenic, whereas apoE2 and apoE4 are pro-atherogenic in immune cells. Further experiments revealed differential mechanisms by which apoE2 and apoE4 promote macrophage inflammation. ApoE2 increased intracellular cholesterol accumulation in myeloid cells, which was associated with enhanced myelopoiesis and inflammasome-mediated inflammation. Conversely, apoE4 increased inflammatory signaling pathways by promoting oxidative stress. Taken together, these results showed that APOE polymorphisms can influence atherosclerosis risk independently of plasma cholesterol levels by increasing macrophage inflammation. This research also investigated the impact of mutation of the proximal dileucine motif of the cytoplasmic tail of LRP1 on development of high-fat diet-induced cardiometabolic disease. This mutation was hypothesized to cause LRP1 dysfunction in various tissues by impairing its association with endosomal recycling adaptor protein, SNX17. We showed that Ldlr-/- mice expressing the LRP1 dileucine mutation displayed increased hypercholesterolemia that resulted in accelerated atherosclerosis progression. Interestingly, the same phenotype was observed in Ldlr-/- mice transplanted with LRP1 dileucine mutant-expressing bone marrow, suggesting significant contribution of LRP1 dysfunction in immune cells to increasing atherosclerosis risk. This study was the first to demonstrate the functional role of the proximal dileucine motif for LRP1 anti-atherogenic functions in vivo, and may indicate the therapeutic potential of improving LRP1 function by targeting pathways that enhance its intracellular trafficking. |
