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Given the multiple differences between mice and men, it was once thought that mice could not be used to model atherosclerosis, principally a human disease. Apolipoprotein E-deficient (apoEKO) mice have convincingly changed this view, and the ability to model human-like plaques in these mice has provided scientists a platform to study multiple facets of atherogenesis and to explore potential therapeutic interventions. In addition to its well-established role in lipoprotein metabolism, recent observations of reduced adiposity and improved glucose homeostasis in apoEKO mice suggest that apoE may also play a key role in energy metabolism in peripheral organs, including adipose tissue. Finally, along with apoEKO mice, knockin mice expressing human apoE isoforms in place of endogenous mouse apoE have provided insights into how quantitative and qualitative genetic alterations interact with the environment in the pathogenesis of complex human diseases. Given the multiple differences between mice and men, it was once thought that mice could not be used to model atherosclerosis, principally a human disease. Apolipoprotein E-deficient (apoEKO) mice have convincingly changed this view, and the ability to model human-like plaques in these mice has provided scientists a platform to study multiple facets of atherogenesis and to explore potential therapeutic interventions. In addition to its well-established role in lipoprotein metabolism, recent observations of reduced adiposity and improved glucose homeostasis in apoEKO mice suggest that apoE may also play a key role in energy metabolism in peripheral organs, including adipose tissue. Finally, along with apoEKO mice, knockin mice expressing human apoE isoforms in place of endogenous mouse apoE have provided insights into how quantitative and qualitative genetic alterations interact with the environment in the pathogenesis of complex human diseases. Apolipoprotein E (apoE) plays a central role in lipoprotein metabolism and is required for the efficient clearance of diet-derived chylomicrons and liver-derived VLDL remnants by the liver (1Mahley R.W. Apolipoprotein E: cholesterol transport protein with expanding role in cell biology.Science. 1988; 240: 622-630Crossref PubMed Scopus (3336) Google Scholar). Consequently, mice lacking apoE (apoEKO) provided the first practical model of hyperlipidemia and atherosclerosis. In this review, we revisit the primary features of lipoprotein metabolism and atherosclerosis in apoEKO mice and the contributions of human apoE isoforms using the apoE knock-in mice. We then extend our discussion to more recent observations that suggest an important role for apoE in peripheral energy metabolism and consequently in metabolic syndrome (MetS) and its components, mainly obesity and diabetes. Plasma cholesterol in wild-type mice on a regular chow diet is ∼80 mg/dl, primarily carried in HDL particles. Mice have a small amount of LDL and other atherogenic lipoproteins, such as VLDL remnants. This high HDL-to-LDL ratio is maintained even when mice are fed diets similar to those consumed by humans in Western society. This is in marked contrast with humans who carry the majority of their plasma cholesterol in LDL (110 mg/dl) (2Havel R.J. Kane J.P. Introduction: structure and metabolism of plasma lipoproteins. In The Metabolic Basis of Inherited Disease. C. S. Scriver, A. L. Beaudet, W. S. Sly, and D. Valle. McGraw-Hill, New York1989: 1129-1138Google Scholar). It is well established in humans that a low ratio of HDL to LDL cholesterol confers a high risk of atherosclerosis and subsequent cardiovascular disease (3Miller N.E. Coronary atherosclerosis and plasma lipoproteins: epidemiology and pathophysiologic consideons.J. Cardiovasc. Pharmacol. 1982; 4: S190-S195Crossref PubMed Google Scholar). Thus, the natural atheroprotective lipoprotein profile in mice could account for the absence of these pathologic conditions. Despite the different plasma lipid profiles, cholesterol transport and metabolism are sufficiently similar in the two species, suggesting that inducing suitable disturbances in plasma lipoprotein metabolism would also lead to atherosclerosis in mice. Gene targeting in embryonic stem cells developed during the 1980s (4Smithies O. Gregg R.G. Boggs S.S. Koralewski M.A. Kucherlapati R.S. Insertion of DNA sequences into the human chromosomal beta-globin locus by homologous recombination.Nature. 1985; 317: 230-234Crossref PubMed Scopus (654) Google Scholar, 5Thomas K.R. Capecchi M.R. Site-directed mutagenesis by gene targeting in mouse embryo-derived stem cells.Cell. 1987; 51: 503-512Abstract Full Text PDF PubMed Scopus (1790) Google Scholar, 6Evans M.J. Kauffman M.H. Establishment in culture of pleuripotent cells from mouse embryos.Nature. 1981; 292: 154-156Crossref PubMed Scopus (6305) Google Scholar) opened the door to test this concept, and mice homozygous for a defective apoE gene were produced by us and independently by Plump et al. in 1992 (7Piedrahita J.A. Zhang S.H. Hagaman J.R. Oliver P.M. Maeda N. Generation of mice carrying a mutant apolipoprotein E gene inactivated by gene targeting in embryonic stem cells.Proc. Natl. Acad. Sci. USA. 1992; 89: 4471-4475Crossref PubMed Scopus (742) Google Scholar, 8Plump A.S. Smith J.D. Hayek T. Walsh K.Aalto-Setala, A. Verstuyft J.G. Rubin E.M. Breslow J.L. Severe hypercholesterolemia and atherosclerosis in apolipoprotein E-deficient mice created by homologous recombination in ES cells.Cell. 1992; 71: 343-353Abstract Full Text PDF PubMed Scopus (1847) Google Scholar, 9Zhang S.H. Reddick R.L. Piedrahita J.A. Maeda N. Spontaneous hypercholesterolemia and arterial lesions in mice lacking apolipoprotein E.Science. 1992; 258: 468-471Crossref PubMed Scopus (1818) Google Scholar). Although extremely rare, humans lacking apoE are reported to have elevated remnant cholesterol in plasma (10Schaefer E.J. Gregg R.E. Ghiselli G. Forte T.M. Ordovas J.M. Zech L.A. Brewer Jr., H.B. Familial apolipoprotein E deficiency.J. Clin. Invest. 1986; 78: 1206-1219Crossref PubMed Scopus (240) Google Scholar). Similar to these individuals, apoEKO mice accumulate cholesterol-rich remnant particles with plasma cholesterol levels reaching 400 mg/dl, even when fed a regular low-fat, low-cholesterol diet. Although atherosclerosis is not a distinguishing feature described in apoE-deficient humans (10Schaefer E.J. Gregg R.E. Ghiselli G. Forte T.M. Ordovas J.M. Zech L.A. Brewer Jr., H.B. Familial apolipoprotein E deficiency.J. Clin. Invest. 1986; 78: 1206-1219Crossref PubMed Scopus (240) Google Scholar), apoE deficiency alone proved to be sufficient for aortic atherosclerotic plaques to develop in mice. In addition, diets high in fat and cholesterol markedly accelerate plaque development in these mice. Thus, apoEKO mice, and subsequently mice lacking the LDL receptor (LDLR) that develop severe atherosclerosis on a Western-type diet (11Ishibashi S. Brown M.S. Goldstein J.L. Gerard R.D. Hammer R.E. Herz J. Hypercholesterolemia in low density lipoprotein receptor knockout mice and its reversal by adenovirus-mediated gene delivery.J. Clin. Invest. 1993; 92: 883-893Crossref PubMed Scopus (1252) Google Scholar), have demonstrated that hyperlipidemia is an essential prerequisite for the development of atherosclerotic lesions. The lesion development and plaque composition in apoEKO mice is also similar to that in humans, establishing it as an excellent animal model for studying the pathogenesis of atherosclerosis. A small collection of foam cells that are tightly adhered to the aortic valve begin to appear in mice at about 2 months of age. With time the complexity of the lesion increases to have fibrous caps, a necrotic core of foam cell debris, cholesterol crystals, and calcifications. Large advanced plaques are often associated with the thickening of medial and adventitial tissue, accompanied by chronic inflammation. Lesions with spontaneous hemorrhage and rupture, the features associated with clinical symptoms of human atherosclerosis, have been observed in some studies of older, cholesterol-fed mice (12Rosenfeld M.E. Polinsky P. Virmani R. Kauser K. Rubanyi G. Schwartz S.M. Advanced atherosclerotic lesions in the innominate artery of the ApoE knockout mouse.Arterioscler. Thromb. Vasc. Biol. 2000; 20: 2587-2592Crossref PubMed Scopus (334) Google Scholar, 13Williams H. Johnson J.L. Carson K.G. Jackson C.L. Characteristics of intact and ruptured atherosclerotic plaques in brachiocephalic arteries of apolipoprotein E knockout mice.Arterioscler. Thromb. Vasc. Biol. 2002; 22: 788-792Crossref PubMed Scopus (196) Google Scholar). However, the occurrence of these events in apoEKO mice is not sufficiently reliable, leaving room for improvement in studying the plaque rupture process. ApoEKO mice have been used extensively for several years to study the impact of various genetic and environmental risk factors on atherosclerotic susceptibility and resistance and to evaluate the effects of various therapeutic means. These studies have been comprehensively reviewed elsewhere (14Maeda N. Givens R.C. Reddick R.L. Cardiovascular disease: mouse models of atherosclerosis. In The Mouse in Medical Research. 2nd edition. J. G. Fox, S. W. Barthold, M. T. Davisson, C. E. Newcomer, F. W. Quimby, and A. L. Smith. Academic Press, Burlington, MA2007: 535-563Google Scholar). While the apoEKO mouse has been established as an excellent model of atherosclerosis, the lack of apoE is extremely rare in the human population. However, apoE is polymorphic in humans, and plasma LDL cholesterol levels and atherosclerosis risk are both strongly associated with the three common apoE isoforms in the order of apoE4 > apoE3 > apoE2. This association is rather counterintuitive because apoE4 (Arg-112 and Arg-158) binds to LDLR with a slightly higher affinity than apoE3 (Cys-112 and Arg-158), while apoE2 (Cys-112 and Cys-158) binds to the receptor with much reduced affinity (15Knouff C. Hinsdale M.E. Mezdour H. Altenburg M.K. Watanabe M. Quarfordt S.H. Sullivan P.M. Maeda N. Apo E structure determines VLDL clearance and atherosclerosis risk in mice.J. Clin. Invest. 1999; 103: 1579-1586Crossref PubMed Scopus (227) Google Scholar). Unlike in humans, the plasma lipoprotein profiles in apoE knockin mice expressing the human apoE proteins in place of mouse apoE are reflective of their different LDLR affinities. Thus, mice with apoE3 and apoE4 are normolipidemic and do not develop atherosclerosis even on a Western-type diet (15Knouff C. Hinsdale M.E. Mezdour H. Altenburg M.K. Watanabe M. Quarfordt S.H. Sullivan P.M. Maeda N. Apo E structure determines VLDL clearance and atherosclerosis risk in mice.J. Clin. Invest. 1999; 103: 1579-1586Crossref PubMed Scopus (227) Google Scholar). In contrast, mice with human apoE2, which binds to LDLR with less affinity, accumulate plasma remnants with high cholesterol and triglycerides (TG) and develop atherosclerosis (16Sullivan P.M. Mezdour H. Quarfordt S.H. Maeda N. Type III hyperlipoproteinemia and spontaneous atherosclerosis in mice resulting from gene replacement of mouse Apoe with human Apoe*2.J. Clin. Invest. 1998; 102: 130-135Crossref PubMed Scopus (186) Google Scholar). Surprisingly, however, mice with human apoE2, E3, or E4 recapitulate the lipoprotein profiles of their human counterparts when they also express a high amount of the human LDLR and are fed a Western-type diet (17Malloy S.I. Altenburg M.K. Knouff C. Parks L.Lanningham-Foster, J.S. Maeda N. Harmful effects of increased LDLR expression in mice with human APOE*4 but not APOE*3.Arterioscler. Thromb. Vasc. Biol. 2004; 24: 91-97Crossref PubMed Scopus (41) Google Scholar). Thus, an increased expression of LDLR in mice with human apoE4 causes an accumulation of cholesterol-rich, apoE-poor remnants in plasma, a reduction of HDL, and severe atherosclerosis. In marked contrast, the same increase in LDLR in apoE2 mice ameliorates their hyperlipidemia and diet-induced atherosclerosis. These results raise the possibility that apoE4, by binding strongly to excess LDLR, is prevented from transferring to nascent lipoproteins, a step necessary for their subsequent clearance. This in turn leads to an increase in the plasma concentration of apoE-poor remnants. Indeed, we found that primary hepatocytes from apoE4 mice secrete less apoE into the medium than hepatocytes from apoE2 mice. Increased LDLR expression leads to a localization of apoE4 on the hepatocyte surface and enhances sequestration of apoE-deficient VLDL remnants injected into apoE4 mice. However, these surface-bound VLDL were poorly internalized compared with apoE2 mice (18Altenburg M. Johnson J.Arbones-Mainar, L. Wilder J. Maeda N. Human LDL receptor enhances sequestration of ApoE4 and VLDL remnants on the surface of hepatocytes but not their internalization in mice.Arterioscler. Thromb. Vasc. Biol. 2008; 28: 1104-1110Crossref PubMed Scopus (28) Google Scholar). ApoE isoform-dependent changes in cholesterol uptake and efflux from macrophages have been reported (19Cullen P. Cignarella A. Brennhausen B. Mohr S. Assmann G. Eckardstein A.von Phenotype-dependent differences in apolipoprotein E metabolism and in cholesterol homeostasis in human monocyte-derived macrophages.J. Clin. Invest. 1998; 101: 1670-1677Crossref PubMed Scopus (103) Google Scholar, 20Hara M. Matsushima T. Satoh H. Noto N.Iso-o, H. Togo M. Kimura S. Hashimoto Y. Tsukamoto K. Isoform-dependent cholesterol efflux from macrophages by apolipoprotein E is modulated by cell surface proteoglycans.Arterioscler. Thromb. Vasc. Biol. 2003; 23: 269-274Crossref PubMed Scopus (40) Google Scholar). Cholesterol delivery to macrophages in culture increases as LDLR expression increases, and the effect was more prominent in apoE4 macrophages than those with apoE3 (21Lucic D. Huang Z.H. de S.Gu Altenburg M.K. Maeda N. Mazzone T. Regulation of macrophage apoE secretion and sterol efflux by the LDL receptor.J. Lipid Res. 2007; 48: 366-372Abstract Full Text Full Text PDF PubMed Scopus (21) Google Scholar). Conversely, increased LDLR expression reduces cholesterol efflux from macrophages expressing apoE4 but not apoE3 (22Altenburg M. Johnson L. Wilder J. Maeda N. Apolipoprotein E4 in macrophages enhances atherogenesis in a low density lipoprotein receptor-dependent manner.J. Biol. Chem. 2007; 282: 7817-7824Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar). Consequently, in mice with human apoE4 that lack the LDLR (LDLRKO), the replacement of bone marrow cells with cells expressing LDLR increased atherosclerosis in a dose-dependent manner compared with mice transplanted with LDLRKO cells. In contrast, atherosclerosis in LDLRKO mice expressing human apoE3 was not affected by the bone marrow with varying levels of LDLR expression (22Altenburg M. Johnson L. Wilder J. Maeda N. Apolipoprotein E4 in macrophages enhances atherogenesis in a low density lipoprotein receptor-dependent manner.J. Biol. Chem. 2007; 282: 7817-7824Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar). Although further tests are required to extrapolate these findings in mice to humans, interactions between apoE isoforms and LDLR in macrophages likely contribute to the association of apoE4 with an increased cardiovascular risk in humans. In addition to its primary site of synthesis in the liver, apoE is also synthesized in peripheral tissues, including adipose tissue (23Driscoll D.M. Getz G.S. Extrahepatic synthesis of apolipoprotein E.J. Lipid Res. 1984; 25: 1368-1379Abstract Full Text PDF PubMed Google Scholar, 24Zechner R. Moser R. Newman T.C. Fried S.K. Breslow J.L. Apolipoprotein E gene expression in mouse 3T3–L1 adipocytes and human adipose tissue and its regulation by differentiation and lipid content.J. Biol. Chem. 1991; 266: 10583-10588Abstract Full Text PDF PubMed Google Scholar). 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ApoE expression and PubMed Scopus Google Scholar), who that apoE synthesized by adipocytes uptake in culture and that the lack of endogenous apoE to a marked in uptake from VLDL even when the VLDL The uptake was by expression of The further that a increased apoE expression and accumulation in wild-type but the same produced less synthesis and accumulation in apoEKO Thus, apoE may play a role in lipid in an studies are necessary to the of apoE and apoE synthesized by adipose tissue in metabolic studies have that the apoE a association between increased and increased plasma VLDL in humans J.P. S. A. C. Apolipoprotein E the association between and plasma in Lipid Res. Full Text PDF PubMed Google Scholar). in the a quantitative locus for and adiposity was found on is T. A. T. A.S. J.S. J. C. on for triglycerides and the Full Text Full Text PDF PubMed Scopus Google Scholar). ApoE isoforms were associated with in the order of apoE4 apoE3 apoE2 in from the in study R.G. G. E. 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Maeda N. of diet-induced obesity and by human apolipoprotein and E4 in J. 2008; PubMed Scopus Google Scholar). expression of apoE3 in adipocytes expression for in a dose-dependent but the was in cells expressing ApoE4 expression increased for but not in suggesting that apoE4 may be with These apoE isoform-dependent effects on fat are a that in addition to fat the of fat cells is also an important of disease Cardiovascular as a of atherosclerosis are increased in atherosclerosis studies have apoEKO mice to with which the in the The resulting atherosclerosis development in apoEKO mice in association with an increase in plasma cholesterol levels Y. atherosclerosis mouse 2007; Full Text Full Text PDF PubMed Scopus Google Scholar). However, the of atherosclerosis is a in plasma lipid by the of the receptor for advanced L. K.G. Y. Jr., D. of atherosclerosis by the receptor for advanced 1998; 4: PubMed Scopus Google Scholar), J.M. Smith M. 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While different conditions, such as diet and for atherosclerosis may be to these different of these differences is of because and resistance in MetS the metabolic of apoE in lipoprotein uptake in liver and adipose tissue plasma contrast, lipoprotein uptake by the liver, in the absence of leads to an increase in VLDL and remnants in the plasma and increased of foam cells in the and atherosclerosis the other apoE deficiency also reduces adiposity and which may have an atheroprotective In the studies of atherosclerosis, such as those described apoEKO mice have provided an for atherosclerosis, but the possibility that apoE may also have in the pathogenesis of and its has not been The potential of apoE isoforms in MetS and the development and of the of the apoE knockin mice in the by which apoE of apoE in energy metabolism its role in lipoprotein metabolism is from The in metabolic have a potential to atherosclerosis and cardiovascular disease Similar to their contributions the of lipoprotein metabolism and pathogenesis of atherosclerosis, apoEKO mice and mice with apoE be in these in The J. 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