Báo cáo khoa học: Mechanisms of obesity and related pathologies: Role of apolipoprotein E in the development of obesity potx

After a lipid-rich meal, dietary Keywords ApoE receptors; ApoE3knock-in mice; ApoE4knock-in mice; ApoE-deficient mice; apolipoprotein E; glucose intolerance; insulin resistance; metaboli

Mechanisms of obesity and related pathologies: Role of apolipoprotein E in the development of obesity

Kyriakos E Kypreos1, Iordanes Karagiannides2, Elisavet H Fotiadou1, Eleni A Karavia1,

Maria S Brinkmeier1, Smaragda M Giakoumi1and Eirini M Tsompanidi1

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; ApoE3knock-in mice;

ApoE4knock-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.

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) Mutacircula-tions 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).

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

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 circulatindicat-ing 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 ApoE3knock-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, ApoE3knock-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 (ApoE3knock-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 ApoE3knock-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 ApoE3knock-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

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 ApoE3knock-in

and ApoE4knock-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 ApoE3knock-in mice had more total and subcutaneous fat than ApoE4knock-inmice at the end of the 8-week period However, although ApoE4knock-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 ApoE3knock-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 ApoE3knock-in and ApoE4knock-in mice contribute to the accelerated impairment of glucose tolerance in the ApoE4knock-inmice 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 ApoE3knock-in and ApoE4knock-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 ApoE3knock-in mouse develops as severe or even more severe metabolic dysfunctions compared to ApoE4knock-inmice, 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 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.

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] ApoE3knock-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] ApoE3knock-inversus

ApoE4knock-inmice

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 obesity b) Insulin resistance c) Glucose intolerance

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.

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 In summary, the recently acquired

kno-wledge reported in the literature identifies ApoE

expression as a key peripheral contributor to the

development of obesity and related metabolic

dysfunc-tions

Acknowledgements

This work was supported by the European

Commu-nity’s Seventh Framework Programme [FP7⁄

2007-2013] grant agreement PIRG02-GA-2007-219129 and

The University of Patras Karatheodoris research grant,

both awarded to K E Kypreos We would like to

thank our statistician Mr E E Kypreos for his

assis-tance in the preparation of the manuscript

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