In contrast, apolipoprotein E-deficient mice fed western-type diet for the same period were resistant to diet-induced obesity, had normal plasma glucose, leptin and insulin levels, and ex
Trang 1Apolipoprotein E predisposes to obesity and related
metabolic dysfunctions in mice
Iordanes Karagiannides1,2, Rami Abdou1, Aikaterini Tzortzopoulou1, Peter J Voshol3
and Kyriakos E Kypreos1,4
1 Whitaker Cardiovascular Institute, Boston University School of Medicine, MA, USA
2 Beth Israel Deaconess Medical Center, Gastrointestinal Neuropeptide Center, Division of Gastroenterology, Harvard Medical School, Boston, MA, USA
3 Department of Endocrinology, Leiden University Medical Center, The Netherlands
4 Department of Medicine, Pharmacology Unit, University of Patras Medical School, Rio, Greece
Obesity and its related pathologies constitute a major
cause of death, with rates increasing at an alarming
pace [1] By the beginning of the millennium,
over-weight adults accounted for over 15% of the world’s
population (body mass index > 30, World Health
Organization) [2], with this number increasing to 50% within the USA and Europe [3] Obesity develops as a result of disruption of the homeostasis between food intake and energy expenditure, and therefore factors affecting these processes are the focus of extensive
Keywords
ApoE3knock-inmice; apolipoprotein E;
glucose intolerance; insulin resistance;
obesity
Correspondence
K E Kypreos, Department of Medicine,
Pharmacology Unit, University of Patras
Medical School, Panepistimioupolis, Rio,
TK 26500, Greece
Fax: +30 2610996103
Tel: +30 2610969120
E-mail: kkypreos@med.upatras.gr
(Received 23 February 2008, revised 25 July
2008, accepted 30 July 2008)
doi:10.1111/j.1742-4658.2008.06619.x
Obesity is a central feature of the metabolic syndrome and is associated with increased risk for insulin resistance and type II diabetes Here, we investigated the contribution of human apoliprotein E3 and mouse apoli-protein E to the development of diet-induced obesity in response to western-type diet Our data show that apolipoprotein E contributes to the development of obesity and other related metabolic disorders, and that human apolipoprotein E3 is more potent than mouse apolipoprotein E in promoting obesity in response to western-type diet Specifically, we found that apolipoprotein E3 knock-in mice fed western-type diet for 24 weeks became obese and developed hyperglycemia, hyperinsulinemia, hyperleptin-emia, glucose intolerance and insulin resistance that were more severe than
in C57BL/6 mice In contrast, apolipoprotein E-deficient mice fed western-type diet for the same period were resistant to diet-induced obesity, had normal plasma glucose, leptin and insulin levels, and exhibited normal responses to glucose tolerance and insulin resistance tests Furthermore, low-density lipoprotein receptor-deficient mice were more sensitive to the development of diet-induced obesity and insulin resistance than apolipo-protein E-deficient mice, but were still more resistant than C57BL/6 mice, raising the possibility that low-density lipoprotein receptor mediates, at least in part, the effects of apolipoprotein E on obesity Taken together, our findings suggest that, in addition to other previously identified mecha-nisms of obesity, apolipoprotein E and possibly the chylomicron pathway are also important contributors to the development of obesity and related metabolic dysfunctions in mice
Abbreviations
ApoE, apolipoprotein E; ApoE)/),ApoE-deficient; ApoE3knock-inmice, mice containing a targeted replacement of the mouse ApoE gene for the human ApoE3 gene; GTT, glucose tolerance test; IST, insulin sensitivity test; LDLr, low-density lipoprotein receptor; LDLr)/), LDLr-deficient; LpL, lipoprotein lipase; VLDL, very low-density lipoprotein.
Trang 2research for the development of effective antiobesity
drugs, with only limited success being achieved thus
far [4] Aging, hormonal imbalance and genetic
predis-position may also contribute to obesity [5–16]
How-ever, very few cases of human obesity are reported to
be caused by genetic factors [17], leaving western-type
diet and sedentary lifestyle, physical inactivity and
imbalance in caloric load as the most common
contrib-utors to the development of central obesity and the
metabolic syndrome [2,18]
The risk of developing all other components of the
metabolic syndrome increases with obesity, supporting
the hypothesis that obesity is the central feature of the
syndrome [19] It is well established that abdominal
obesity may result in insulin resistance and
hyperinsu-linemia [19,20] Epidemiological and population studies
have established a direct correlation between obesity
and the development of cardiovascular disease [21,22]
Despite the pivotal role of obesity and dyslipidemia in
the development of the metabolic syndrome and heart
disease, the functional interactions between adipose
tissue and the lipid and lipoprotein transport system
have not yet been investigated thoroughly
Apolipoprotein E (ApoE) is a 34.2 kDa glycoprotein
synthesized by the liver and other peripheral tissues In
humans, there are three major natural isoforms, ApoE2,
ApoE3, and ApoE4 [23], with ApoE3 being the most
common [23–29] ApoE is a major protein component
of chylomicron remnants and very low-density
lipopro-tein (VLDL) [23] The importance of this prolipopro-tein in the
maintenance of plasma lipid homeostasis and
athero-protection was first established with the generation of
the ApoE-deficient mouse [30,31], which develops
hypercholesterolemia and spontaneous atherosclerosis
[30,31] Lipid-bound ApoE is the natural ligand of the
low-density lipoprotein receptor (LDLr) [32–34], a cell
surface receptor that is responsible for the catabolism of
atherogenic lipoproteins [32,35–37]
In this study, we sought to determine the role of
ApoE in the development of diet-induced obesity,
glu-cose intolerance and insulin resistance in vivo To
address this question, female ApoE3knock-in, wild-type
C57BL/6, LDLr-deficient (LDLr)/)) and
ApoE-defi-cient (ApoE)/)) mice were fed western-type diet for a
period of 15 or 24 weeks, during which time their
plasma lipid and glucose levels, body weight, body
com-position, glucose tolerance and insulin sensitivity were
monitored We chose to study ApoE3 because it is the
most common ApoE isoform in humans [24–29] Our
data establish that expression of ApoE predisposes mice
to diet-induced obesity, hyperglycemia and insulin
resis-tance, whereas deficiency in ApoE renders mice resistant
to these conditions Human ApoE3 appeared to be more
potent than mouse ApoE in promoting obesity in response to western-type diet Furthermore, LDLr)/) mice were more sensitive to the development of diet-induced obesity and insulin resistance than ApoE)/) mice, but still more resistant than wild-type C57BL/6 mice in response to western-type diet Gavage adminis-tration of olive oil containing the nonhydrolyzable [3H]cholesteryl-hexadecyl-ether to mice suggested that deficiency in LDLr and ApoE reduces the direct delivery
of postprandial nonhydrolyzed lipids to the liver, one of the major tissues involved in glucose uptake from the circulation A similar trend was also observed in the delivery of nonhydrolyzed dietary lipids to adipose tissue Taken together, our data establish that ApoE is
a key mediator of diet-induced obesity in response to western-type diet
Results
ApoE promotes diet-induced weight gain in mice, whereas ApoE deficiency prevents it
To test the effects of ApoE on weight gain in mice, groups of 4–6 weeks old female ApoE3knock-in, ApoE)/) and wild-type C57BL/6 mice were placed on western-type or normal chow diet for a total period of
24 weeks Mice in each group were weighed immedi-ately before the initiation of the experiment (week 0) and every 6 weeks thereafter up to week 24, using a Mettler precision microscale
It became apparent that as early as 6 weeks on high-fat diet, ApoE3-expressing mice gained weight and were significantly heavier than the wild-type C57BL/6 mice on the same diet (Fig 1A) The weight of the ApoE3knock–in mice was 31.37 ± 3.98 g (115 ± 34% higher than their initial weight of 17.03 ± 0.94 g,
P < 0.05) (Fig 1A) During the same period, C57BL/
6 mice on a high-fat diet had an average body weight
of 26.08 ± 1.89 g (66 ± 8% higher than their initial weight of 16.26 ± 0.61 g, P < 0.05) (Fig 1A) There were no statistical differences between the weights of the ApoE3knock-in and C57BL/6 control groups fed chow diet for 6 weeks (data not shown)
At week 24 on high-fat diet, ApoE3-expressing mice showed a dramatic increase in body weight, with an average value of 50.23 ± 2.22 g (235 ± 32% higher than their starting weight at week 0, P < 0.05) (Fig 1A) The body weight of the C57BL/6 mice was 43.10 ± 0.94 g (164 ± 9% higher than their starting weight at week 0, P < 0.05) (Fig 1A) The control ApoE3knock-inand C57BL/6 mouse groups on chow diet showed a much smaller increase in body weight, ranging between 22 and 24 g (29 ± 6% increase as compared to
Trang 3their starting weight at week 0, P < 0.05) (data not
shown) In contrast to the ApoE3knock-in and the
C57BL/6 mice, ApoE)/)mice that were fed western-type
diet showed only a modest increase in body weight
dur-ing the course of the experiment (Fig 1A) At week 6 of
the experiment, the ApoE)/)mouse group had an
aver-age body weight of 20.36 ± 1.37 g (32 ± 6% increase
as compared to their starting weight of 16.26 ± 0.61 g
at week 0, P < 0.05) At week 24, their body weight
was 24.58 ± 1.07 g (41 ± 4% increase as compared to
their starting weight at week 0, P < 0.05) (Fig 1A)
Similar increases in body weight were observed in the
control ApoE)/)mice fed chow diet (data not shown)
To compare the steady-state plasma ApoE levels
between ApoE3knock-in and C57BL/6 mice, at week 0
of the experiment plasma samples were isolated from
three mice from each group and 5 lL of plasma was analyzed by western blotting using a polyclonal antibody that recognizes both human and mouse ApoE (Santa-Cruz Biotech, Santa Cruz, CA, USA; cat no sc-31821) This analysis showed that C57BL/6 mice had approximately four-fold higher steady-state plasma ApoE levels than ApoE3knock-in mice, suggest-ing that the increased sensitivity of ApoE3knock-in mice
to obesity is not due to higher plasma ApoE levels in these mice compared to C57BL/6 mice (Fig 1B)
To determine whether body weight differences among groups fed western-type diet could be explained
by differences in their average daily food consumption,
at week 12 of the experiment we determined the average daily food consumption for each mouse group
It was found that ApoE3knock-in mice consumed
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% of initial body-weight
Weeks
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Fig 1 Percentage of initial body weight (A), plasma ApoE levels (B), plasma cholesterol levels (C) and plasma triglyceride levels (D) of C57BL/6, ApoE3knock-inand ApoE)/)mice fed western-type diet for a period of 24 weeks (E) Percentage body fat content of ApoE3knock-in, ApoE)/)and C57BL/6 mice at week 24 (F) Fasting plasma glucose levels of ApoE3 knock-in , ApoE)/)and C57BL/6 mice at weeks 0 and 24 Each point on the graphs represents the mean value of the group, and error bars indicate the SEM The statistical significance of the observed differences among groups at each time-point is as indicated (*P < 0.05; **P < 0.005) In (B), plasma ApoE levels in ApoE3knock-in and ApoE)/)mice were determined by western blot analysis using an antibody that recognizes equally mouse ApoE and human ApoE Ponceau S staining of the nitrocellulose membrane was used to confirm equal loading and efficient transfer of proteins to the membrane.
Trang 43.04 ± 0.13 g per mouse per day, C57BL/6 mice
con-sumed 3.31 ± 0.15 g per mouse per day, and ApoE)/)
mice consumed 3.19 ± 0.17 g per mouse per day, and
there was no statistical difference among groups
(P > 0.05)
Plasma lipid levels of the mice fed western-type
diet for 24 weeks
During the 24-week period of feeding mice with
western-type diet, fasting plasma samples were taken
every 6 weeks, and cholesterol, triglyceride and free
fatty acid levels were measured as described in
Experimental procedures As shown in Fig 1C, at
week 24 both ApoE3knock in and C57BL/6 mice on
high-fat diet had slightly elevated fasting cholesterol
levels (138 ± 10 mgÆdL)1 and 188 ± 14 mgÆdL)1
respectively) as compared to their starting cholesterol
levels at week 0 (50 ± 3 mgÆdL)1 and 54 ±
6 mgÆdL)1, respectively), whereas their plasma
triglyc-eride levels remained normal (24 ± 7 mgÆdL)1 and
24 ± 5 mgÆdL)1, respectively) (Fig 1D) FPLC
anal-ysis of plasma from these mice showed that the
small increases in the cholesterol levels of these mice
at week 24 were due to a minor accumulation of
chylomicron and VLDL remnants (Fig 2A,B) In
contrast, ApoE)/) mice showed a dramatic increase
in their plasma cholesterol levels during the course
of the experiment (Fig 1C) At week 24 of the experiment, plasma cholesterol levels of the ApoE)/) mice were 1064 ± 198 mgÆdL)1 (Fig 1C), whereas their plasma triglyceride levels remained normal (52 ± 28 mgÆdL)1 at week 24) (Fig 1D) FPLC analysis showed that the hypercholesterolemia of these mice was due to increased accumulation of triglyceride-containing cholesterol-rich chylomicron remnants (Fig 2A,B) No significant difference in the plasma free fatty acid levels among groups was observed during the course of the experiment At week 24, plasma free fatty acid levels of the ApoE3knock-in, C57BL/6 and ApoE)/) mice were 0.89 ± 0.08 mmol equiv., 0.81 ± 0.05 mmol equiv., and 0.99 ± 0.08 mmol equiv., respectively
Body composition analysis of the mice fed western-type diet for 24 weeks
At the end of the 24-week period on western-type diet,
at least six mice from each group (ApoE3knock-in, ApoE)/) and C57BL/6 mice) were killed The total weight, dry weight, water content, lipid content and lean body mass of the mice were determined as described in Experimental procedures As shown in Fig 1E, this analysis established that ApoE3knock-in mice had a total body lipid content of 39 ± 4% The wild-type C57BL/6 mice had a significantly lower total body lipid content of 32 ± 3% (P < 0.05) Thus, the increased body weight of the ApoE3knock-in and C57BL/6 mice reflects excess accumulation of adipose tissue in these mice In contrast, ApoE)/) mice remained lean during the course of the experiment, with a total body fat content of 11 ± 1% (Fig 1E,
P < 0.005) The complete body composition analysis
of the mice fed western-type diet for 24 weeks is sum-marized in Table 1
Diet-induced obesity in ApoE3knock-inand C57BL/6 mice is associated with elevated plasma glucose, insulin and leptin levels
Epidemiological and animal studies have established that central obesity is associated with glucose intoler-ance and insulin resistintoler-ance [20] In addition, obesity is accompanied by increased levels of leptin [38], a hor-mone that reduces appetite and may function as the link between obesity and hypertension in individuals with the metabolic syndrome [39,40]
To determine whether the obesity observed in ApoE3knock-in and C57BL/6 mice is associated with
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Fig 2 Representative FPLC cholesterol (A) and triglyceride (B)
pro-files of C57BL/6, ApoE3 knock-in and ApoE)/)mice fed western-type
diet for a period of 24 weeks.
Trang 5hyperglycemia, at weeks 0 and 24 of the experiment
mice were fasted for 16 h and plasma glucose levels
were then measured Immediately prior to switching
mice to western-type diet (week 0), the fasting plasma
glucose levels of the ApoE3knock-in, ApoE)/) and
C57BL/6 mice were 84.2 ± 3.7 mgÆdL)1,
60.7 ± 2.8 mgÆdL)1, and 75.2 ± 3.5 mgÆdL)1,
respec-tively (Fig 1F, P< 0.05) After 24 weeks on
western-type diet, ApoE3knock-in and control C57BL/6
mice developed hyperglycemia with fasting
glucose levels of 146.5 ± 9.1 mgÆdL)1 (P < 0.05) and
135.4 ± 7.9 mgÆdL)1 (P < 0.05) respectively
(Fig 1F) In contrast, ApoE)/) mice, which remained
lean during the course of the experiment, had only
slightly elevated fasting glucose levels that were within
the physiological range (94.3 ± 3.22 mgÆdL)1,
P< 0.005) To determine plasma insulin and leptin
levels in these mice, serum samples were isolated at
weeks 0 and 24 of the experiment and analyzed for
insulin and leptin At week 0, all three mouse groups
had similar plasma insulin levels (0.17 ± 0.05
ngÆmL)1 for ApoE3knock-in mice, 0.12 ± 0.05 ngÆmL)1
for ApoE)/) mice, and 0.16 ± 0.01 ngÆmL)1 for
C57BL/6 mice) At week 24, ApoE3knock-in and
C57BL/6 mice had elevated plasma insulin levels, with
concentrations in the range of 4.73 ± 1.03 ngÆmL)1
(P < 0.05) and 1.28 ± 0.32 ngÆmL)1 (P < 0.05),
respectively In contrast, ApoE)/) mice fed
western-type diet for 24 weeks had insulin levels of
0.23 ± 0.07 ngÆmL)1, which were similar to the levels
of ApoE)/) mice (0.317 ± 0.17 ngÆmL)1) on chow
diet for the same period of time
Analysis of plasma leptin levels showed that at
week 0, mice had similar leptin levels: 5.90 ± 0.40
ngÆmL)1 for ApoE3knock-in mice, 4.51 ± 0.32 ngÆmL)1
for ApoE)/) mice, and 9.2 ± 0.30 ngÆmL)1 for
C57BL/6 mice At week 24, the plasma leptin levels
of the ApoE)/) mice were reduced to 2.1 ± 0.4
ngÆmL)1 In contrast, in ApoE3knock-in mice fed
western-type diet for 24 weeks, leptin levels increased
dramatically to 41.14 ± 1.20 ngÆmL)1 (P < 0.005) A
similar but lower increase was also observed in
the plasma leptin levels of C57BL/6 mice fed
western-type diet for 24 weeks (34.70 ± 1.50 ngÆmL)1,
P < 0.005)
Diet-induced obesity in ApoE3knock-inand C57BL/6 mice is associated with reduced glucose tolerance and insulin sensitivity
To determine the role of ApoE in the development of obesity-associated insulin resistance and glucose intol-erance, we performed the standard glucose tolerance test (GTT) and insulin sensitivity test (IST) The GTT established that at week 0 all three mouse groups (ApoE3knock-in, ApoE)/) and C57BL/6 mice) had simi-lar normal responses to intraperitoneal administration
of glucose (Fig 3A) However, at week 24 of the experiment, ApoE3knock-in mice showed a significant deterioration in their ability to clear plasma glucose, as compared to C57BL/6 and ApoE)/) mice (Fig 3B;
P < 0.05) A similar but less severe effect was also observed in the C57BL/6 mice (Fig 3A,B; P < 0.05) Remarkably, however, ApoE)/) mice (which are resis-tant to diet-induced obesity) fed western-type diet for
24 weeks cleared glucose from the circulation more efficiently than the two other groups, and there was no significant difference in their response to intraperito-neal glucose load between weeks 0 and 24 on western-type diet (compare Fig 3A,B, P > 0.05)
In a similar fashion, when an IST was performed at week 0, all three mouse groups exhibited a similar response to intraperitoneal administration of insulin (Fig 3C) However, at week 24, ApoE3knock-in mice fed western-type diet for 24 weeks displayed the poor-est response to insulin administration as compared to C57BL/6 and ApoE)/) mice (Fig 3D; P < 0.05) C57BL/6 mice also exhibited reduced insulin sensitivity
at week 24 that was less severe than in ApoE3knock-in mice (Fig 3D; P < 0.05) In contrast, ApoE)/) mice fed western-type diet for 24 weeks exhibited the highest sensitivity to insulin of all three mouse groups (P < 0.05) In addition, there was no significant differ-ence in their insulin sensitivity curves between weeks 0 and 24 of the experiment (compare Fig 3C,D;
P > 0.05)
Table 1 Body composition of ApoE3 knock-in , C57BL/6 and ApoE)/)mice fed western-type diet for 24 weeks Values are in grams expressed
as mean ± SEM.
Trang 6LDLr)/)mice are more sensitive to diet-induced
obesity and hyperglycemia than ApoE)/)mice
but less sensitive than C57BL/6 mice
Low-density lipoprotein receptor is the major receptor
involved in the clearance of ApoE-containing
lipopro-teins from the circulation [41] Therefore, one
mecha-nistic explanation for the role of ApoE in the
development of obesity would be that LDLr-mediated
uptake of ApoE-containing chylomicron remnants
promotes the direct deposition of dietary fat into the
adipose tissue If this was the case, deficiency in LDLr
would prevent obesity and hyperglycemia To address
this possibility, groups of 8–10 female LDLr)/) or
C57BL/6 or ApoE)/) mice were placed on
western-type diet for a period of 15 weeks, and their body
weight and composition, and plasma cholesterol,
tri-glyceride and glucose levels were determined during
the course of the experiment At week 5 of the
experi-ment, the average weight of the LDLr)/) mice was
22.28 ± 1.32 g (29.7 ± 4.1% higher than their initial
weight of 17.13 ± 0.65 g, P < 0.05) (Fig 4A) This
increase was comparable to the 27.4 ± 3.8% increase
observed in the ApoE)/)mice (from 16.83 ± 0.24 g to
21.43 ± 0.56 g, P < 0.05), but lower than the 55.4 ±
3.8% increase in the C57BL/6 mice (from
16.26 ± 0.28 g to 25.25 ± 0.50 g, P< 0.005)
(Fig 4A) At week 15 of the experiment, however,
LDLr)/) mice showed an 84.5 ± 8.7% increase in
body weight (with an average final weight of
31.63 ± 2.10 g, P < 0.05) (Fig 4A) This increase
was higher than the 51.4 ± 4.5% increase (P < 0.05)
observed in the weight of the ApoE)/) mice (with an average final weight of 25.46 ± 0.55 g) However, it was still significantly lower than the 119.8 ± 7.6% increase observed in the weight of C57BL/6 mice (with an aver-age final weight of 37.28 ± 0.72 g) fed western-type diet for the same period of time (Fig 4A; P < 0.05) Body composition analysis at the end of the experi-ment revealed that at week 15, LDLr)/) mice had a body fat content of 19.9 ± 1.2%, which was much higher than the body fat content of the ApoE)/) mice (13 ± 1.9%, P < 0.05) but still lower than the body fat content of the C57BL/6 mice (28.06 ± 3.92%,
P < 0.05) fed western-type diet for the same period of time (Fig 4D) The complete body composition analy-sis of the mice fed western-type diet for 15 weeks is summarized in Table 2
Plasma lipid and glucose analysis showed that during the 15-week period, LDLr)/) mice developed severe hypercholesterolemia (1338 ± 135 mgÆdL)1) that was accompanied by moderate hypertriglyceride-mia (242.6 ± 14.9 mgÆdL)1) (Fig 4B,C) Plasma glucose levels were increased moderately (from 71.3 ± 6.7 mgÆdL)1 to 101 ± 4.9 mgÆdL)1, P < 0.05) but were still lower than the levels of C57BL/6 mice (131 ± 5.3 mgÆdL)1) at week 15 of the experiment (Fig 4E; P < 0.005)
The GTT and IST revealed that the LDLr)/) mice fed western-type diet for 15 weeks had similar toler-ance to glucose and sensitivity to insulin as in their starting state (week 0) (Fig 5; P > 0.05) In addition, there was no significant difference in the response to intraperitoneal administration of glucose or insulin
0 15 30 45 60 75
apoE3knock-in C57BL/6 apoE –/–
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Fig 3 Glucose tolerance curves (A, B)
and insulin sensitivity curves (C, D) of
ApoE3knock-in, C57BL/6 and ApoE)/)mice
at weeks 0 and 24 Values indicate the
average plasma glucose levels expressed as
mean ± SEM The statistical significance of
the observed differences among groups at
each time-point is as indicated (*P < 0.05).
Trang 70 5
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Fig 4 Percentage of initial body weight (A), plasma cholesterol levels (B) and plasma tri-glyceride levels (C) of C57BL/6, LDLr)/)and ApoE)/)mice fed western-type diet for a period of 15 weeks (D) Percentage body fat content of ApoE3knock-in, ApoE)/)and C57BL/6 mice at week 15 (E) Fasting plasma glucose levels of ApoE3 knock-in , ApoE)/)and C57BL/6 mice at weeks 0 and 15 Each point on the graphs represents the mean value of the group, and error bars indicate the SEM The statistical significance of the observed differences among groups at each time-point is indicated (*P < 0.05;
**P < 0.005).
Table 2 Body composition of LDLr)/), ApoE)/)and C57BL/6 mice fed western-type diet for 15 weeks Values are in grams expressed as mean ± SEM.
0 15 5 30 45 60 75 90 105 120
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Week 15 Week 15
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C57BL/6 LDLr –/–
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Fig 5 Glucose tolerance curves (A, B) and insulin sensitivity curves (C, D) of LDLr)/) and C57BL/6 mice at weeks 0 and 15 Values indicate the average plasma glucose levels expressed as mean ± SEM The statistical significance of the observed differences among groups at each time-point is as indicated (*P < 0.05).
Trang 8between LDLr)/) and C57BL/6 mice fed western-type
diet for 15 weeks (Fig 5) Interestingly, in our studies,
feeding western-type diet to C57BL/6 for 15 weeks did
not result in insulin resistance or glucose intolerance
(Fig 5B,D)
Taken together, these data indicate that female
LDLr)/) mice fed western-type diet for 15 weeks
appear to be more sensitive than female ApoE)/)mice
but still more resistant than female C57BL/6 mice in
the development of diet-induced obesity and its related
disorders
ApoE promotes diet-induced accumulation of
excess triglycerides in the liver while ApoE or
LDLr deficiency does not
If ApoE and LDLr are involved in the direct delivery
of dietary lipids to tissues, one would expect that
feed-ing ApoE3knock-in and C57BL/6 mice western-type diet
would result in excess accumulation of triglycerides in
the liver, whereas ApoE)/) and LDLr)/) mice would
be resistant to excess hepatic triglyceride accumulation
To test this hypothesis, we isolated liver samples from
mice fed western-type diet for 15 weeks and
deter-mined their triglyceride content (Fig 6A)
Liver samples from ApoE)/) and LDLr)/) mice fed
western-type diet for 15 weeks had similar
triglycer-ide contents of 60.67 ± 4.12 mgÆg)1 and 58.40 ±
5.11 mgÆg)1 of hepatic tissue, respectively (Fig 6A,
P> 0.05) In contrast, ApoE3knock-in and C57BL/6
mice had a much higher hepatic triglyceride content
(215.00 ± 33.56 mgÆg)1 and 213.72 ± 11.89 mgÆg)1
respectively, P < 0.05), confirming that human ApoE3,
murine ApoE, and the LDLr contribute to the
accu-mulation of excess lipids in the liver in response to
western-type diet (Fig 6A)
Effects of ApoE and LDLr deficiency on the direct
delivery of dietary lipids to adipose tissue
As ApoE and LDLr play pivotal roles in the
catabo-lism of chylomicron remnants, we attempted to
evalu-ate the contribution of the direct delivery of
nonhydrolyzed postprandial lipids to the development
of obesity To address this question, groups of
ApoE3-knock-in, ApoE)/), LDLr)/) and C57BL/6 mice that
were maintained on western-type diet for 15 weeks
were gavaged with 0.5 mL of olive oil containing
15 lCi of the nonhydrolyzable [3
H]cholesteryl-hexade-cyl-ether [cholesteryl-1,2-3H(N)] Twenty-four hours
later, mice were killed, and visceral fat and liver
sam-ples were isolated, weighed, and homogenized Then,
the amount of3H radioactivity present in the
homoge-nized tissues was determined using a liquid scintillation counter
ApoE3knock-in mice showed a higher average accu-mulation of dietary [3H]cholesteryl-hexadecyl-ether in adipose tissue (1718 ± 492 c.p.m per gram of tissue) than C57BL/6 mice (1010 ± 202 c.p.m per gram of tissue) (Fig 6B), but this difference between the two groups did not reach statistical significance (P > 0.05) The hepatic accumulation of 3H-label was similar between these two animal groups (3181
± 585 c.p.m per gram of tissue for the ApoE3knock-in mice and 3281 ± 578 c.p.m per gram of tissue for the C57BL/6 mice) (Fig 6C) ApoE)/) and LDLr)/) mice showed lower average accumulation of 3H-label in their fat (618 ± 41 c.p.m per gram of tissue and
664 ± 65 c.p.m per gram of tissue, respectively) and hepatic tissues (1624 ± 209 c.p.m per gram of tissue
0
cpms per gram of hepatic tissue
B A
0
cpms per gram of adipose tissue
0 50 100
2500 2000 1500 1000 500
4000 3500 3000 2500 2000 1500 1000 500
150 200 250
300
apoE3knock-in apoE –/– LDLr –/– C57BL/6
*
*
*
*
C
Fig 6 (A) Hepatic triglyceride content of ApoE3 knock-in , ApoE)/), LDLr)/) and C57BL/6 mice fed western-type diet for 15 weeks The statistical significance of the observed differences is indicated (*P < 0.05; **P < 0.005) (B, C) Total accumulation of tritiated label expressed as counts per minute per gram of adipose (B) or hepatic (C) tissue, 24 h after gavage administration of 0.5 mL of olive oil containing 15 lCi of [ 3 H]hexadecyl-cholesteryl-ether, per mouse.
Trang 9and 1759 ± 752 c.p.m per gram of tissue,
respec-tively) (Fig 6B,C) No measurable radioactivity was
found in the blood of the tested mice at the time of
killing (not shown)
Discussion
ApoE is a major protein involved in the metabolism
of dietary lipids and the removal of atherogenic
lipo-proteins from the circulation Following a lipid-rich
meal, lipids are packaged into chylomicrons which,
following partial lipolysis by lipoprotein lipase (LpL),
are converted into chylomicron remnants and acquire
ApoE [23] Then, lipid-bound ApoE interacts with
LDLr, which mediates the removal of
ApoE-contain-ing lipoproteins from the circulation In the present
study, we show that ApoE3knock-in mice are more
sen-sitive to diet-induced obesity and related metabolic
dysfunctions than wild-type C57BL/6 mice, whereas
ApoE)/) mice are resistant to the development of
these conditions Furthermore, deficiency in LDLr
results in reduced sensitivity to obesity in response to
western-type diet, raising the possibility that the
effects of ApoE may be mediated, at least in part,
via its interactions with LDLr
Interestingly, there were no significant 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
[42,43] Moreover, daily food consumption of the
ApoE3knock-in, C57BL/6 and ApoE)/) mice was similar
among groups, suggesting that different responses to
western-type diet cannot be attributed to differences in
appetite
One possible explanation for the increased sensitivity
of the ApoE3knock-in mice to diet-induced obesity
would be that higher plasma ApoE levels in these mice
than in C57BL/6 mice are responsible for the enhanced
deposition of dietary lipids in adipose tissue To
address this possibility, we compared the ApoE levels
in plasma samples isolated from ApoE3knock-in and
C57BL/6 mice at week 0 of the experiment, using
western blotting This analysis showed that
steady-state plasma ApoE levels in the ApoE3knock-in mice
used in our study are approximately four times lower
than those in wild-type C57BL/6 mice Thus, the
increased sensitivity of ApoE3knock-in mice to
diet-induced obesity is not the result of elevated plasma
ApoE levels in these mice as compared to C57BL/6
mice, and the difference in the ability of human
ApoE3 and murine ApoE to promote obesity in
response to high-fat diet may be due to intrinsic differ-ences between these two peptides The data presented
in Fig 6B raise the possibility that chylomicron and VLDL remnants containing the human ApoE3 isoform are taken up more avidly by adipose tissue than the lipoproteins containing mouse ApoE
In a previous study, Sullivan et al [44] reported that ApoE3knock-in mice and C57BL/6 mice have similar plasma ApoE levels Furthermore, using northern blot analysis, they also showed that ApoE mRNA levels were indistinguishable between ApoE3knock-in mice and C57BL/6 mice, in all tissues tested except for the small intestine, where human ApoE3 mRNA expression was lower than mouse ApoE mRNA expression [44] How-ever, the ApoE3knock-inmice studied by these investiga-tors are different from the ApoE3knock-inmice tested in our experiments, because our ApoE3knock-in mice have been bred for nine generations to the C57BL/6 back-ground It is possible that back-crossing ApoE3knock-in mice to C57BL/6 mice for nine generations resulted in the reduced plasma human ApoE3 levels that we observed
Human ApoE has three natural isoforms in humans: ApoE2, ApoE3 and ApoE4 In vitro receptor binding studies established that lipid-bound ApoE3 and ApoE4 have a similar affinity for LDLr, whereas lipid-bound ApoE2 has a much lower affinity [45,46] In this study,
we focused on ApoE3, mainly because it is the most common ApoE genetic polymorphism in humans [24– 29] If the effects of ApoE3 on obesity are mediated by its lipid-lowering potential via LDLr, then we expect that both ApoE3 and ApoE4 will predispose to a simi-lar extent to diet-induced obesity and insulin resistance
in mice, whereas ApoE2 may have a much lower potential to promote these conditions Further studies are needed in order to address this point, and other mechanisms of ApoE-promoted diet-induced obesity should not be excluded
It is quite interesting that in all our experiments, plasma cholesterol levels correlated inversely with body weight gain and body fat accumulation (Figs 1 and 4)
In the ApoE)/)mice, failure to clear chylomicron rem-nants due to deficiency in ApoE resulted in a steady increase in plasma cholesterol levels and rendered these mice resistant to diet-induced obesity In contrast, in the ApoE3knock-in mice, the efficient catabolism of chylomicron remnants resulted in only slightly elevated plasma cholesterol 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 hypercholesterol-emia but became obese and insulin resistant following western-type diet for 24 weeks
Trang 10Plasma triglyceride levels of the LDLr)/) mice were
moderately elevated at week 0, and remained elevated
during the 15 weeks on high-fat diet, whereas plasma
triglyceride levels of the other animal groups remained
normal for the duration of the experiment This is not
surprising, because in the absence of the LDLr,
reduced clearance of ApoE-containing lipoproteins
from the circulation results in elevated steady-state
plasma ApoE levels Since ApoE is a known inhibitor
of LpL [47], and plasma triglyceride levels correlate
with plasma ApoE levels [48], accumulation of ApoE
in the blood of LDLr)/) mice results in reduced
LpL-mediated lipolysis of plasma triglycerides and
hyper-triglyceridemia
In our studies, LDLr)/) mice became more obese
than ApoE)/) mice but less obese than C57BL/6
mice, raising the possibility that, in addition to
LDLr, other ApoE-recognizing receptors may also
promote the deposition of postprandial lipids in
adi-pose tissue, thus contributing to diet-induced obesity
and related metabolic dysfunctions Indeed, a recent
study showed that adipose-tissue-specific deletion of
the LDLr-related protein makes mice less sensitive to
obesity [49] In the case of the LDLr)/) mice,
LDLr-related protein and possibly other ApoE-recognizing
‘scavenger’ receptors may promote, to some extent,
delivery of ApoE-containing chylomicron remnants to
adipose tissue However, in the case of the ApoE)/)
mice, which lack the endogenous ApoE, all of these
ApoE-mediated receptor processes are blocked, and
ApoE)/) mice are more resistant to body fat gain
than LDLr)/) mice Even though the expression of
LDLr in adipose tissue is much lower than in liver,
our data support a functional role for this receptor
in the ApoE-mediated mechanism of diet-induced
obesity
In a previous study by Schreyer et al [50], it was
suggested that LDLr)/) mice fed a diabetogenic diet
for 16 weeks were more susceptible to diet-induced
obesity and hyperglycemia than C57BL/6 mice,
whereas ApoE)/) mice appeared to be as susceptible
to the development of these conditions as C57BL/6
mice Furthermore, in that study, LDLr)/) mice
developed severe hypertriglyceridemia during the
course of the experiment The diabetogenic diet used
in that study contained a very high fat content of
35.5% (derived mainly from lard) (Bioserve,
French-town, NJ, USA; cat no F1850) In all our
experi-ments, mice were fed the standard western-type diet
containing 21.1% fat (Harlan Teklad; cat no
TD88137) It is possible that the high fat content of
the diabetogenic diet predisposed ApoE)/), LDLr)/)
and C57BL6 mice to the development of diet-induced
obesity, and resulted in saturation of the metabolic pathways that control body fat deposition and plasma lipid and glucose homeostasis Under such conditions, ApoE or LDLr deficiency may not be sufficient to prevent obesity, as other mechanisms contributing to obesity may override the protective effect of ApoE or LDLr deletion that we observed in our experiments Our data on LDLr)/) mice are in agreement with the data of MacDonald et al [51], showing that female LDLr)/) mice do not become excessively obese, and do not develop hyperglycemia and glucose intolerance in response to western-type diet
In our experiments, we studied the role of ApoE in the development of obesity in response to dietary consumption of fat, one of the major causes of human obesity [2,18] Our findings are supported by previous observations by Gao et al [52] and Chiba
et al [53] showing that deficiency in ApoE renders genetically predisposed obese mice less sensitive to spontaneous development of obesity Furthermore,
in vitro studies suggested that ApoE promotes triglyc-eride uptake and deposition in in vitro differentiated adipocytes and in freshly isolated adipose tissue explants [54], whereas VLDL induces adipocyte differ-entiation in an ApoE-dependent manner [53] In the present study, we report for the first time that human ApoE3 increases susceptibility to diet-induced obesity
as compared to mouse ApoE The functional role of ApoE-containing chylomicron and VLDL remnants
in the development of diet-induced obesity is further supported by the observation that ApoE)/) mice remain lean when fed western-type diet for 15 or
24 weeks
One of the hallmarks of obesity-associated insulin resistance is the increase in the circulating levels of insulin [55] Such a change is also evident in our stud-ies, further supporting the hypothesis that the benefi-cial effects of ApoE deletion on weight loss extend to increased insulin sensitivity Furthermore, our data demonstrate a significant increase in plasma leptin levels in ApoE3knock-in and, to a lesser extent, in C57BL/6 mice, an observation that is also consistent with their increased adiposity [56]
ApoE has long been known to be atheroprotective, mainly because of its ability to clear plasma lipids However, our data show that if excess dietary lipids are present in the circulation, this atheroprotective property of ApoE may be counteracted by the enhanced deposition of lipids in adipose tissue Over-all, our findings identify ApoE expression as a key peripheral contributor to the development of obesity and related metabolic dysfunctions in mice