MINIREVIEW Mechanisms of obesity and related pathologies: Role of apolipoprotein E in the development of obesity Kyriakos E. Kypreos 1 , Iordanes Karagiannides 2 , Elisavet H. Fotiadou 1 , Eleni A. Karavia 1 , Maria S. Brinkmeier 1 , Smaragda M. Giakoumi 1 and Eirini M. Tsompanidi 1 1 Department of Medicine, Pharmacology Unit, University of Patras Medical School, Rio, Greece 2 Department of Medicine, David Geffen School of Medicine, UCLA, Los Angeles, CA, USA Introduction Apolipoprotein E (ApoE) is a major protein of the lipid and lipoprotein transport system mainly involved in the metabolism of dietary lipids and the removal of atherogenic lipoproteins, such as chylomi- cron remnants and very low density lipoproteins (VLDL), from the circulation [1,2]. In humans, ApoE is a polymorphic 34.5 kDa glycoprotein synthesized primarily by the liver, although it is also synthesized by other tissues, such as brain and adipose tissue. Human ApoE has three natural isoforms, ApoE2, ApoE3 and ApoE4 [3]. These isoforms differ in their amino acid compositions at positions 112 and 158, where ApoE2 has Cys at both sites, ApoE4 has Arg at both sites, and ApoE3 has Cys112 and Arg158 [3]. Epidemiological studies have linked ApoE4 to elevated LDL cholesterol levels and an increased risk of the development of cardiovascular disease [4,5]. Lipoprotein-bound ApoE is the natural ligand for the LDL-receptor (LDLr) [6,7], which is the main receptor involved in the clearance of ApoE-containing lipoproteins in vivo [8]. After a lipid-rich meal, dietary Keywords ApoE receptors; ApoE3 knock-in mice; ApoE4 knock-in mice; ApoE-deficient mice; apolipoprotein E; glucose intolerance; insulin resistance; metabolic syndrome; obesity Correspondence K. E. Kypreos, Department of Medicine, University of Patras Medical School, Pharmacology Unit, Panepistimioupolis, Rio, TK 26500, Greece Fax: +30 2610994720 Tel: +30 2610969120 E-mail: kkypreos@med.upatras.gr (Received 18 February 2009, revised 1 August 2009, accepted 11 August 2009) doi:10.1111/j.1742-4658.2009.07301.x Apolipoprotein E is a polymorphic glycoprotein in humans with a molecu- lar mass of 34.5 kDa. It is a component of chylomicron remnants, very low density lipoprotein, low density lipoprotein and high density lipopro- tein, and is primarily responsible for maintaining plasma lipid homeostasis. In addition to these well-documented functions, recent studies in experi- mental mouse models, as well as population studies, show that apolipo- protein E also plays an important role in the development of obesity and insulin resistance. It is widely accepted that disruption in homeostasis between food intake and energy expenditure, and the subsequent deposition of excess fatty acids into fat cells in the form of triglycerides, leads to the development of obesity. Despite the pivotal role of obesity and dyslipide- mia in the development of the metabolic syndrome and heart disease, the functional interactions between adipose tissue and components of the lipo- protein transport system have not yet been investigated thoroughly. In this minireview, we focus on the current literature pertinent to the involvement of apolipoprotein E in the development of pathologies associated with the metabolic syndrome. Abbreviations ABCA1, ATP-binding cassette A1; ApoE, apolipoprotein E; ApoE ) ⁄ ) , ApoE-deficient; HDL, high density lipoprotein; LCAT, lecithin:cholesterol acyl transferase; LDLr, low density lipoprotein receptor; LDLr ) ⁄ ) , LDLr-deficient; LpL, lipoprotein lipase; LRP1, LDLr related protein 1; VLDL, very low density lipoprotein; VLDLr, very low density lipoprotein receptor. 5720 FEBS Journal 276 (2009) 5720–5728 ª 2009 The Authors Journal compilation ª 2009 FEBS lipids are packaged into chylomicrons, which, subse- quent to partial lipolysis by lipoprotein lipase (LpL), are converted into chylomicron remnants and acquire ApoE [2] (Fig. 1A). Then, lipid bound ApoE interacts with the LDLr, which mediates the removal of ApoE- containing atherogenic lipoproteins from the circula- tion (Fig. 1A). Mutations in ApoE or LDLr that prevent their physical interactions are associated with high plasma cholesterol levels and predispose to pre- mature atherosclerosis in humans and experimental animals [9,10]. In addition, ApoE also promotes cholesterol efflux [11] and the de novo biogenesis of spherical ApoE-con- taining high density lipoprotein (HDL)-like particles with the participation of the lipid transporter ATP bind- ing cassette A1 (ABCA1) and the plasma enzyme leci- thin:cholesterol acyl transferase (LCAT) (Fig. 1B) [12]. Thus, ApoE may also contribute to the maintenance of plasma and tissue cholesterol homeostasis and the pro- tection from atherosclerosis [13–20] via mechanisms that are independent of its interactions with the LDLr [18]. It is widely accepted that disruption in the homeo- stasis between food intake and energy expenditure, and the subsequent deposition of excess fatty acids into fat cells in the form of triglycerides, leads to the development of obesity [21]. A lipid-rich diet and sed- entary lifestyle, physical inactivity and an imbalance in caloric load are the most common contributors to the development of central obesity and the metabolic syndrome [22,23]. Aging, hormonal imbalance and genetic predisposition may also contribute to obesity [24–35]. Epidemiological and population studies have established a direct correlation between obesity and the development of cardiovascular disease [36,37]. Despite the pivotal role of obesity and dyslipidemia in the development of the metabolic syndrome and heart disease, the functional interactions between adipose tis- sue and the lipid and lipoprotein transport system have only recently started to be investigated. ApoE in adipocyte differentiation and lipid loading In vitro experiments using cultures of primary prea- dipocytes, adipocytes, adipose tissue explants or Peripheral tissues or liver ABCA1 N C Plasma apoE Minimally lipidated apoE Discoidal apoE-HDL LCAT Spherical apoE-HDL Chylomicrons ApoE-containing chylomicron remnants LpL-mediated lipolysis Interactions of remnant-bound apoE with LDLr Secretion of lipid-rich chylomicrons in the circulation Clearance of dietary lipids from the circulation Lipid-rich meal Intestine ApoE LDLr 1 2 4 A B 3 Fig. 1. (A) Summary of the role of ApoE in the clearance of chylomicron remnants and VLDL from the circulation. Dietary lipids are packaged into chylomicrons, which are then partially lipolyzed by plasma lipoprotein lipase on the surface of vascular endothelial cells. Subsequent to lipolysis, chylomicrons acquire ApoE and are converted into chylomicron remnants. ApoE-containing chylomicron remnants are then taken up by the liver and other peripheral tissues mainly via the LDLr, which appears to be the major physiological receptor for remnant clearance. (B) Depicting the pathway of de novo biogenesis of ApoE-containing HDL with the participation of the lipid transporter ABCA1 and plasma enzyme LCAT. Minimally lipidated ApoE in plasma interacts with ABCA1 (step 1) that is present in the liver or other peripheral tissues. This interaction promotes the lipidation of ApoE (step 2), which is then converted into a discoidal HDL-like particle through a sequence of steps that are not yet well understood (step 3). Then, ApoE containing discoidal HDL-like particles are converted into spherical HDL by the action of the plasma enzyme LCAT (step 4). K. E. Kypreos et al. ApoE and obesity FEBS Journal 276 (2009) 5720–5728 ª 2009 The Authors Journal compilation ª 2009 FEBS 5721 3T3-L1 cells provide some information on the role of ApoE in preadipocyte differentiation and on ApoE expression from mature adipocytes. A study by Chiba et al. [38] provided the first direct evidence that lipid-bound ApoE promotes preadipo- cyte differentiation in a dose-dependent manner. Using bone marrow stromal cells from ApoE-deficient (ApoE ) ⁄ ) ) mice and 3T3-L1 cells, these investigators showed that ApoE-deficient VLDL failed to induce adipogenesis, whereas normal VLDL promoted differ- entiation of these cells into fat cells. Incubation of ApoE-deficient VLDL with recombinant human ApoE partially restored its ability to stimulate adipogenesis, whereas the selective removal of ApoE from VLDL by trypsin abolished the adipogenic activity of human VLDL. When tetrahydrolipstatin, a potent lipoprotein lipase inhibitor, was used in these experiments, it did not alter the ability of ApoE-containing VLDL to pro- mote adipogenesis, suggesting that hydrolysis of VLDL triglycerides does not play a major role in the adipogenic effects of ApoE-containing VLDL. Simi- larly, individual lipid components of the VLDL or free fatty acids alone induced the expression of adipocyte- specific genes but failed to generate adipocytes filled with large lipid droplets, and this finding was inter- preted as partial adipogenesis compared to the effects of ApoE-containing VLDL. Along the same lines, a study by Huang et al. [39] suggested that the endogenous expression of ApoE promotes lipid accumulation and adipocyte differentia- tion in cell cultures. Specifically, adipocytes isolated from ApoE-deficient mice contained lower levels of tri- glyceride and free fatty acids compared to adipocytes isolated from wild-type mice, and these differences were also maintained in cultured adipocytes derived from preadipocytes. During incubation with ApoE- containing triglyceride-rich lipoproteins, ApoE-defi- cient adipose tissue accumulated less triglycerides than adipose tissue isolated from wild-type mice. Similarly, a lack of ApoE expression in primary cultured adipo- cytes led to changes in the expression of genes involved in the metabolism ⁄ turnover of fatty acids and the tri- glyceride droplet, whereas peroxisome proliferator-acti- vated receptor gamma-mediated changes in lipid content and gene expression were markedly altered in cultured ApoE-deficient adipocytes. Interestingly, when human ApoE3 was expressed by adenovirus-mediated gene transfer in cultured adipocytes from ApoE-defi- cient mice, it promoted the accumulation of triglyce- rides and fatty acids in the infected cells. This finding is in agreement with a study by Zechner et al. [40] who showed that ApoE expression in differentiating 3T3-L1 cells increases linearly with time in differentiation, whereas the inhibition of lipid accumulation in differ- entiated cells by biotin deprivation decreased ApoE expression. Interestingly, another set of experiments conducted by Huang et al. [41] suggested that ApoE expression in adipocytes was affected by the feeding state of the mice that the tissue was derived from. ApoE expres- sion was induced by fasting, whereas diet-induced obesity or hyperphagia was associated with the reduced expression of ApoE in the adipose tissue. Because other studies showed that ApoE-expression in the adipose tissue promoted lipid accumulation and adipocyte differentiation [39], one interpretation of the results obtained by Huang et al. [41] is that intrinsic defense mechanisms in adipose tissue limit adipogene- sis by reducing the expression of ApoE in the fed state. Certainly, additional studies are required to determine the role of adipocyte-synthesized ApoE, and to distin- guish between the functions of peripherally expressed ApoE versus adipocyte expressed ApoE. Studies in experimental mouse models Despite the differences in anatomy, pathology, physiol- ogy and metabolism between mice and humans, studies in mice during the last few decades have provided important leads with respect to the pathogenesis and genetics of human metabolic diseases, including obes- ity. A number of studies in experimental mouse models have provided a definitive link between ApoE and obesity. Work by Chiba et al. [38] demonstrated that leptin deficient (ob ⁄ ob) mice that are also deficient in apoE (ob ⁄ ob · ApoE ) ⁄ ) ) did not show an increased body weight or an increased amount of adipose tissue when fed a high-fat ⁄ high-cholesterol diet, despite an increase in their plasma VLDL levels. By contrast, control ob ⁄ ob mice fed a high-fat ⁄ high-cholesterol diet for the same period of time showed an increased body weight and amount of adipose tissue, suggesting that ApoE is a key modulator of adipogenesis in vivo. In agreement with that study, Huang et al. [39] reported that ApoE ) ⁄ ) mice have less body fat content and smaller adipocytes compared to wild-type C57BL ⁄ 6 controls. A study by Hofmann et al. [42] fur- ther extended this observation by showing that ApoE ) ⁄ ) mice fed a high-fat-high-sucrose diabetogenic diet for 24 weeks were resistant to diet-induced obesity and exhibited improved glucose tolerance and uptake by muscle and brown adipose tissue, whereas their plasma insulin levels were lower compared to control wild-type C57BL ⁄ 6 mice. The reduced body weight and improved glycemic control of the ApoE ) ⁄ ) mice ApoE and obesity K. E. Kypreos et al. 5722 FEBS Journal 276 (2009) 5720–5728 ª 2009 The Authors Journal compilation ª 2009 FEBS was accompanied by impaired plasma triglyceride clearance and lipid uptake by adipose tissue. Direct calorimetry studies did not reveal any significant differ- ences in energy expenditure and respiratory quotient between ApoE ) ⁄ ) and wild-type C57BL ⁄ 6 mice fed a high-fat, high-sucrose diet for 24 weeks, suggesting that, in the absence of ApoE, decreased plasma lipid delivery to insulin sensitive tissues improves insulin sensitivity and prevents the development of diet induced obesity. Using an approach similar to Chiba et al. [38], Gao et al. [43] established that ApoE deficiency in Ay ⁄ + mice prevented the development of obesity, with decreased fat accumulation in the liver and adipose tis- sues. Ay (also known as lethal yellow) is a mutation at the mouse agouti locus in chromosome 2 that results in a number of dominant pleiotropic effects, including a yellow coat color, obesity, an insulin-resistant type II diabetic condition, and an increased propensity to develop a variety of spontaneous and induced tumors [44]. The Ay mutation is the result of a 170 bp deletion that removes all but the promoter and noncoding first exon of the Raly gene, which lies in the same transcrip- tional orientation as agouti and maps 280 kb proximal to the 3¢ end of the agouti gene [44]. Gao et al. [43] generated ApoE-deficient Ay (ApoE ) ⁄ ) · Ay ⁄ + ) mice and found that ApoE ) ⁄ ) · Ay ⁄ + mice exhibited better glucose tolerance than ApoE + ⁄ + · Ay ⁄ + mice, whereas insulin tolerance testing and hyperinsulinemic- euglycemic clamp analysis revealed a marked improve- ment of insulin sensitivity in ApoE ) ⁄ ) · Ay ⁄ + mice compared to ApoE + ⁄ + · Ay ⁄ + mice, despite an increase in their plasma free fatty acid levels. When these investigators used adenovirus-mediated gene expression of ApoE in ApoE ) ⁄ ) · Ay ⁄ + mice, ApoE protein expression in the plasma of these mice wors- ened the glucose tolerance and insulin sensitivity of the ApoE ) ⁄ ) · Ay ⁄ + mice, and triggered obesity, indicat- ing that circulating ApoE is involved in increased adiposity and obesity-related metabolic disorders. Of note, the uptake of ApoE-lacking VLDL into the liver and adipocytes was markedly inhibited, although adipocytes in ApoE ) ⁄ ) · Ay ⁄ + mice exhibited normal differentiation. In a recent study from our laboratory [45], we established that ApoE3 knock-in mice fed the standard Western-type diet for 24 weeks were more sensitive to diet-induced obesity and related metabolic dys- functions than wild-type C57BL ⁄ 6 mice, whereas ApoE ) ⁄ ) mice were resistant to the development of these conditions. Furthermore, deficiency in the LDLr resulted in reduced sensitivity towards obesity in response to a Western-type diet (Harlan-Teklad, catalogue number TD 88137, Indianapolis, IN, USA), raising the possibility that the effects of ApoE may be mediated, at least in part, via its interactions with the LDLr. Of note, ApoE3 knock-in mice had lower steady-state plasma ApoE levels than C57BL ⁄ 6 mice, establishing that the difference in the ability of human ApoE3 and murine ApoE to promote obesity in response to a high-fat diet may be the result of intrinsic differences between these two peptides. Inter- estingly, in our experiments, we did not observe sig- nificant differences in plasma free fatty-acid levels among mouse groups (ApoE3 knock-in versus C57BL ⁄ 6 versus LDLr ) ⁄ ) versus ApoE ) ⁄ ) ), although previous studies suggested that increased plasma levels of free fatty acids are closely associated with obesity-induced insulin resistance [46,47]. Daily food consumption of the ApoE3 knock-in , C57BL ⁄ 6 and ApoE ) ⁄ ) mice was similar among groups, suggesting that different responses to a Western type diet could not be attrib- uted to differences in appetite. It is quite interesting that, in all our experiments, plasma cholesterol levels correlated inversely with body weight gain and body fat accumulation. In the ApoE ) ⁄ ) mice, failure to clear chylomicron remnants because of a deficiency in ApoE resulted in a steady increase in plasma cho- lesterol levels and rendered these mice resistant to diet-induced obesity. By contrast, in the ApoE3 knock- in mice, the efficient catabolism of chylomicron rem- nants resulted in only slightly elevated plasma choles- terol levels, but promoted obesity, insulin resistance and glucose intolerance. Similar to the ApoE3 knock-in mice, C57BL ⁄ 6 mice, which express the mouse ApoE, developed only mild hypercholesterolemia, but became obese and insulin resistant after consuming a Western-type diet for 24 weeks. Direct measurements of dietary lipid delivery to hepatic and adipose tissue raised the possibility that chylomicron and VLDL remnants containing the human ApoE3 isoform are taken up more avidly by adipose tissue than the lipo- proteins containing mouse ApoE. There has been much discussion in the medical com- munity concerning the role of inflammation in obesity. In particular, although some studies suggest that inflammation causes obesity, others present data supporting the idea that inflammation is simply a metabolic side-effect of the obese state. ApoE is long- known to be an anti-inflammatory molecule [48], and a deficiency in ApoE is considered to induce general inflammation that leads to spontaneous atherosclerosis in the ApoE ) ⁄ ) mice [49]. Thus, the resistance of ApoE ) ⁄ ) mice to developing diet-induced obesity may support the theory that inflammation does not trigger obesity, but rather it is the result of it. K. E. Kypreos et al. ApoE and obesity FEBS Journal 276 (2009) 5720–5728 ª 2009 The Authors Journal compilation ª 2009 FEBS 5723 In our studies, we also found that LDLr ) ⁄ ) mice became more obese than ApoE ) ⁄ ) mice, yet less obese than C57BL ⁄ 6 mice, raising the possibility that, in addition to the LDLr, other ApoE-recognizing recep- tors may also promote the deposition of postprandial lipids to adipose tissue, thus contributing to diet- induced obesity and related metabolic dysfunctions. Thus, in the absence of LDLr, other ApoE-recognizing ‘scavenger’ receptors, such as LDLr-related protein (LRP1) and very low density lipoprotein receptor (VLDLr) may promote, to some extent, delivery of ApoE-containing chylomicron remnants to adipose tis- sue. However, in the case of the ApoE ) ⁄ ) mice that lack the endogenous ApoE, all these ApoE-mediated receptor processes may be blocked, and ApoE ) ⁄ ) mice become more resistant to body fat gaining compared to LDLr ) ⁄ ) mice. Indeed, Hofmann et al. [50] showed that adipocyte-specific inactivation of the multifunc- tional receptor LRP1 in mice resulted in delayed post- prandial lipid clearance, reduced body weight, smaller fat stores, lipid-depleted brown adipocytes, improved glucose tolerance and elevated energy expenditure as a result of enhanced muscle thermogenesis. Furthermore, inactivation of adipocyte LRP1 resulted in resistance to dietary fat-induced obesity and glucose intolerance. In another study by Gourdiaan et al. [51] VLDLr-defi- cient mice were found to be resistant to diet-induced obesity when fed a high-fat, high-calorie diet. Thus, it is possible that, in the absence of LDLr, remnant- bound ApoE interacts with VLDLr or LRP1 present on the surface of adipocytes [52,53] to facilitate the lipolysis of VLDL-triglycerides by LpL [53] and possi- bly the subsequent uptake of remnant particles by ApoE-recognizing receptors [50]. In humans, ApoE has three natural isoforms: ApoE2, ApoE3 and ApoE4 [3]. In vitro receptor binding studies have established that lipid bound ApoE3 and ApoE4 have a similar affinity for the LDLr, whereas lipid bound ApoE2 has a much lower affinity [54]. If the effects of ApoE3 on obesity are mediated solely by its lipid lowering potential via the LDLr and possibly other ApoE recognizing receptors, it would be expected that both ApoE3 and ApoE4 will predispose to a similar extent to diet-induced obesity and insulin resistance in mice, whereas ApoE2 may have a much lower potential to promote these conditions. One study [55] investigated the contribution of the natural human ApoE3 and ApoE4 phenotypes in the development of obesity and other metabolic abnormalities in mice. ApoE3 knock-in and ApoE4 knock-in mice were fed Western-type diet for 8 weeks and, during this time, the sensitivity of these mice towards the development of obesity and glucose tolerance was assessed. Analysis of total fat content showed that ApoE3 knock-in mice had more total and subcutaneous fat than ApoE4 knock-in mice at the end of the 8-week period. However, although ApoE4 knock-in mice gained 30% less weight during the period on high- fat diet compared to ApoE3 mice, they showed impaired insulin-stimulated glucose uptake. Further- more, epididymal adipocytes derived from ApoE4 knock- in mice were larger in size than those derived from ApoE3 knock-in mice. When ApoE3 and ApoE4 were expressed by adenovirus-mediated gene transfer in cul- tures of ApoE-deficient adipocytes, only ApoE3 expres- sion was able to significantly induce adiponectin mRNA expression, and mobilize the glucose transporter GLUT4, suggesting that ApoE3 but not ApoE4 expres- sion interferes with insulin sensing pathways. On the basis of these findings, it was concluded that, even though ApoE3 expression leads to higher adipose tissue mass in mice compared to ApoE4, qualitative differ- ences in the epididymal adipose tissue between the ApoE3 knock-in and ApoE4 knock-in mice contribute to the accelerated impairment of glucose tolerance in the ApoE4 knock-in mice fed a Western-type diet for 8 weeks. Although this study did not address the question of how differences in receptor-mediated clearance of ApoE-containing lipoproteins and possibly holoparticle uptake may contribute to an ApoE isoform-dependent sensitivity towards obesity, it raised the interesting pos- sibility that metabolic dysfunctions such as impaired glucose tolerance and insulin sensitivity may be the result of qualitative differences in fat depots present in mice expressing different ApoE isoforms. Of course, obesity and its related complications are chronic dys- functions that develop over long periods of time. It is possible that 8 weeks on a high-fat diet was too short a period for ApoE3 knock-in and ApoE4 knock-in mice to develop obesity and its related metabolic dysfunctions. Thus, in future studies, it would be interesting to inves- tigate whether the more obesity-prone ApoE3 knock-in mouse develops as severe or even more severe metabolic dysfunctions compared to ApoE4 knock-in mice, when fed a Western-type diet for 24 weeks or longer. Shen et al. [56] suggested that brain apoE expression reduces food intake in rats. Specifically, the intrecere- broventricular injection of ApoE in rats decreased their food intake, whereas intrecerebroventricular infu- sion of ApoE anti-serum stimulated feeding. However, in previous studies [38,43,45,55] that compared ApoE- deficient with ApoE-expressing mice, there were no significant changes in daily food intake between these mouse groups. One possibility is that the peripheral effects of ApoE predisposing to obesity in those studies offset the brain-specific effects that reduced food-intake in the study by Shen et al. [56]. ApoE and obesity K. E. Kypreos et al. 5724 FEBS Journal 276 (2009) 5720–5728 ª 2009 The Authors Journal compilation ª 2009 FEBS ApoE expression and obesity in epidemiological studies To date, there is no established link between ApoE- deficiency and obesity in humans. Specifically, there are no epidemiological studies comparing the sensi- tivity towards obesity of ApoE-expressing versus ApoE-deficient human subjects because ApoE-defi- ciency is an extremely rare condition in humans. However, mutations in ApoE that affect its func- tions, including the natural ApoE polymorphism (ApoE4, ApoE3 and ApoE4), appear to modulate the ability of the protein to predispose to obesity. Few studies have attempted to link different human ApoE-isoforms to obesity and related metabolic dysfunctions, although they have produced somewhat conflicting results. Data from the Atherosclerosis Risk in Communities (ARIC) study, which included 15 000 individuals, showed that ApoE-isoforms in humans were associated with body mass index in the order ApoE4 < ApoE3 < ApoE2 [57]. However, another epidemiological study showed that, in older women with a family history of diabetes, ApoE4 ⁄ 4 and ApoE3 ⁄ 4 phenotypes were correlated with increased waist circumference and obesity [58]. Simi- larly, in a Romanian epidemiological study compar- ing control healthy individuals with obese patients suffering from the metabolic syndrome, a higher frequency of the epsilon 4 allele was found in patients with metabolic syndrome [59]. Table 1. Studies in mouse models. Study Animal models used Observed phenotype Chiba et al. [38] ApoE ) ⁄ ) · Ob ⁄ Ob versus Ob ⁄ Ob ApoE-deficiency renders genetically predisposed leptin-deficient (ob ⁄ ob) mice resistant to diet-induced obesity, mainly because ApoE-containing VLDL promotes adipogenesis Huang et al. [39] Hofmann et al. [42] C57BL ⁄ 6 versus ApoE ) ⁄ ) ApoE-deficient mice are leaner than their wild-type counterparts, and resistant to diet-induced obesity, after 24 weeks of being fed a Western-type diet Gao et al. [43] ApoE ) ⁄ ) · Ay ⁄ + versus Ay ⁄ + ApoE-deficiency renders genetically predisposed Ay ⁄ + mice resistant to obesity mainly by limiting uptake of VLDL by adipose tissue Karagiannides et al. [45] ApoE3 knock-in versus C57Bl ⁄ 6 versus LDLr ) ⁄ ) versus ApoE ) ⁄ ) ApoE promotes diet-induced obesity and insulin resistance, at least in part, through its interactions with the LDLr, after 24 weeks of being fed a Western-type diet. Human ApoE3 is more potent than mouse ApoE in promoting diet-induced obesity Hofmann et al. [50] Adipose tissue-specific LRP1 ) ⁄ ) versus wild-type mice Adipose tissue-specific deletion of LRP1 renders mice resistant to diet-induced obesity by limiting postprandial lipid clearance Gourdiaan et al. [51] VLDLr ) ⁄ ) versus wild-type mice Deletion of VLDLr renders mice resistant to diet-induced obesity possibly by limiting LpL-mediated lipolysis of postprandial triglycerides Arbones-Mainar et al. [55] ApoE3 knock-in versus ApoE4 knock-in mice ApoE3-expressing mice appear to be more sensitive to diet-induced obesity but less prone to insulin resistance than ApoE4-expressing mice, after 8 weeks of being fed a Western-type diet Chylomicrons ApoE-containing chylomicron remnants LpL-mediated lipolysis A. Interactions with apoE-recognizing receptors B. Delivery of dietary lipids to the adipose tissue Secretion of lipid-rich chylomicrons in the circulation Development of : a) Diet-induced obesit y b) Insulin resistance c) Glucose intolerance 12 3 Fat cells 4 ApoE Lipid-rich meal Intestine Fig. 2. Model for the role of ApoE in the development of diet-induced obesity in mice. Dietary lipids are packaged into chylomicrons in the intestine and then secreted into the circulation (step 1) where they are partially lipolysed by plasma lipoprotein lipase and acquire ApoE (step 2). ApoE-containing chylomicron remnants then interact with receptors, such as LDLr, LRP1 and VLDLr, present on the surface of a number of cells, including hepatocytes and adipocytes (step 3). This interaction promotes the delivery of dietary lipids to adipose tissue and leads to diet-induced obesity and related metabolic dysfunctions (step 4). In the absence of the expression of ApoE or ApoE-recognizing receptors, the delivery of dietary lipids to the adipose tissue is blocked (steps 3 and 4), resulting in resistance to diet-induced obesity. K. E. Kypreos et al. ApoE and obesity FEBS Journal 276 (2009) 5720–5728 ª 2009 The Authors Journal compilation ª 2009 FEBS 5725 In addition to its direct relation to body mass index and obesity, the ApoE4 phenotype also appears to be the link between obesity and abnormalities related to glucose metabolism and diabetes. In obese men, the expression of the ApoE4 isoform was correlated with higher plasma insulin and glucose levels than in obese men expressing the other ApoE phenotypes [60,61]. However, no such association between ApoE pheno- type and insulin or glucose levels was observed in non- obese men [60], whereas the association between ApoE4 phenotype and insulin and glucose levels became stronger with increasing body mass index [60,61]. These findings again raise the interesting possi- bility that, although hyperplastic types of obesity may be more extreme in individuals expressing other ApoE- phenotypes, it is the hypertrophic adipocytes in indi- viduals expressing ApoE4 that may lead to metabolic dysfunctions, in terms of responses to insulin. ApoE and obesity ApoE has long been known to be atheroprotective, mainly because of its ability to promote the removal of atherogenic lipoproteins from the circulation and the formation of ApoE-containing HDL particles (Fig. 1). However, recent data on ApoE and obesity (Table 1) show that, if excess dietary lipids are pres- ent in the circulation, this atheroprotective property of ApoE may be counter-acted by the enhanced depo- sition of dietary lipids to adipose tissue (Fig. 2), which may be the result, at least in part, of the pres- ence of ApoE-recognizing receptors on the surface of adipocytes. 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Kypreos et al. 5728 FEBS Journal 276 (2009) 5720–5728 ª 2009 The Authors Journal compilation ª 2009 FEBS . studies have established a direct correlation between obesity and the development of cardiovascular disease [36,37]. Despite the pivotal role of obesity and dyslipidemia in the development of the. lipo- protein transport system have not yet been investigated thoroughly. In this minireview, we focus on the current literature pertinent to the involvement of apolipoprotein E in the development of. abnormalities related to glucose metabolism and diabetes. In obese men, the expression of the ApoE4 isoform was correlated with higher plasma insulin and glucose levels than in obese men expressing the