Physiological expression of macrophage apoE in the artery wall reduces atherosclerosis in severely hyperlipidemic mice

Abstract

We have previously reported that the introduction of macrophage apoE into mice lacking both apoE and the LDL receptor (apoE ؊ / ؊ /LDLR ؊ / ؊ ) through bone marrow transplantation (apoE ؉ / ؉ /LDLR ؊ / ؊ → apoE ؊ / ؊ /LDLR ؊ / ؊ ) produces progressive accumulation of apoE in plasma without affecting lipid levels. This model provides a tool to study the effects of physiologically regulated amounts of macrophage apoE on atherogenesis in hyperlipidemic animals. Ten-week-old male apoE ؊ / ؊ /LDLR ؊ / ؊ mice were transplanted with either apoE ؉ / ؉ /LDLR ؊ / ؊ (n ‫؍‬ 11) or apoE ؊ / ؊ / LDLR ؊ / ؊ (n ‫؍‬ 14) marrow. Although there were no differences between the two groups in lipid levels at baseline or at 5 and 9 weeks after transplantation, apoE levels in the apoE ؉ / ؉ LDLR ؊ / ؊ → apoE ؊ / ؊ /LDLR ؊ / ؊ mice increased to 4 times the apoE levels of normal mice. This resulted in a 60% decrease in aortic atherosclerosis in the apoE Although apolipoprotein E (apoE) of macrophage origin is fully functional in its ability to induce the clearance of plasma lipoproteins, substantial evidence supports the concept of a direct anti-atherogenic effect of apoE in the arterial wall. Strong support of a direct effect of apoE in the artery wall comes from bone marrow transplantation studies (1, 2). We have reported that the lack of macrophage apoE in C57BL/6 mice increases the size of diet-induced arterial lesions by 10-fold in the absence of differences in plasma lipoprotein levels (3). We have also shown that apoE Ϫ / Ϫ macrophages increase lesion size in apoA-I deficient and apoA-I over-expressing mice (4). In addition, using a retroviral transduction system, we have shown that low level expression of apoE from macrophages has a strong anti-atherogenic effect during foam cell lesion formation, but this effect was lost when the plaques grew larger and more complex, an indication that the protective capacity of apoE is eventually overwhelmed by the severe hyperlipidemia of apoE deficiency (5). Evidence in favor of an anti-atherogenic role of apoE also comes from studies showing that low level expression of human apoE-3 in the arterial wall of apoE Ϫ / Ϫ mice was accompanied by decreased growth of lesions without affecting plasma lipids (6), and that the expression of low levels of human apoE-3 in the macrophages of apoE Ϫ / Ϫ mice resulted in decreased atherogenesis independent of a decrease in plasma cholesterol (7). Interestingly, recent studies from our laboratory have shown that retroviral transfer of receptor-binding defective mutant forms of human apoE, such as apoE-2 and apoE-cys142, do not protect apoE null mice against atherosclerosis development, suggesting that the local function of apoE is linked to its ability to bind receptors Abbreviations: BMT, bone marrow transplant; HSPG, heparan sulfate proteoglycans; LDLR, low density lipoprotein receptor; LXR, liver X receptor; TNF ␣ , tumor necrosis factor ␣ .

The recycling of apolipoprotein E in macrophages

“…A large body of published data suggest an atheroprotective role of apoE secreted by macrophages (14,15,(34)(35)(36)(37)(38). We have shown that reconstitution of apoE Ϫ/Ϫ mice with wild-type marrow results in a near-normalization of plasma cholesterol levels and a Ͼ50-fold reduction in atherosclerotic lesion area (35).…”

Reduced ABCA1-Mediated Cholesterol Efflux and Accelerated Atherosclerosis in Apolipoprotein E–Deficient Mice Lacking Macrophage-Derived ACAT1

“…We have previously reported that the absence of either macrophage apoE or ACAT1 increases atherosclerosis in LDLR Ϫ/Ϫ mice in the presence of a more severe hypercholesterolemia than that observed in the present study. 13,16 We interpret this finding to mean that the increased effect on atherogenesis resulting from absence of both macrophage ACAT1 and apoE in the apoE Ϫ/Ϫ model is most likely not mediated by the degree of plasma cholesterol changes.…”

“…Apolipoprotein (apo)E is a strong driver of cholesterol efflux, 14 and the increased atherogenicity of apoE-deficient macrophages may be due to defective elimination of cholesterol. 15,16 We and others have shown that macrophage apoE influences atherosclerosis both systemically, by controlling plasma cholesterol levels, and locally, by regulating cholesterol homeostasis in the macrophage. 15,17 Thus, in the present study, we used bone marrow transplantation (BMT) to generate apoE Ϫ/Ϫ mice with or without macrophage ACAT1 and apoE to address the effects of ACAT1 deletion on the development of atherosclerosis under both hypercholesterolemic and normocholesterolemic conditions, and we investigated the underlying molecular mechanisms of the accelerated atherosclerosis resulting from ACAT1 deficiency.…”

Macrophages, inflammation, and atherosclerosis

“…25 Moreover, we have recently shown that a physiologically normal expression of macrophage apoE has antiatherogenic effects that are not limited to the foam cell lesion but extend into the more advanced stages of plaque growth. 26 Macrophage apoE expression is regulated by cellular cholesterol. 27 Two distal enhancers that specify apoE gene expression from macrophages contain conserved liver X receptor (LXR) response elements.…”