Diabetologia (2009) 52:514–523
DOI 10.1007/s00125-008-1252-0
ARTICLE
Failure of dietary quercetin to alter the temporal progression
of insulin resistance among tissues of C57BL/6J mice
during the development of diet-induced obesity
L. K. Stewart & Z. Wang & D. Ribnicky & J. L. Soileau &
W. T. Cefalu & T. W. Gettys
Received: 6 August 2008 / Accepted: 3 December 2008 / Published online: 14 January 2009
# Springer-Verlag 2009
Abstract
Aims/hypotheses High-fat diets produce obesity and glucose intolerance by promoting the development of insulin
resistance in peripheral tissues and liver. The present
studies sought to identify the initial site(s) where insulin
resistance develops using a moderately high-fat diet and to
assess whether the bioflavonoid, quercetin, ameliorates
progression of this sequence.
Methods Four cohorts of male C57BL/6J mice were placed
on diets formulated to be low-fat (10% of energy from fat),
high-fat (45% of energy from fat) or high-fat plus 1.2%
quercetin (wt/wt). After 3 and 8 weeks, cohorts were
evaluated using euglycaemic–hyperinsulinaemic clamps,
Electronic supplementary material The online version of this article
(doi:10.1007/s00125-008-1252-0) contains supplementary material,
which is available to authorised users.
L. K. Stewart : J. L. Soileau : T. W. Gettys (*)
Laboratory of Nutrient Sensing and Adipocyte Signaling,
Pennington Biomedical Research Center,
6400 Perkins Road,
Baton Rouge, LA 70808, USA
e-mail: gettystw@pbrc.edu
Z. Wang : W. T. Cefalu
Center for the Study of Botanicals and Metabolic Syndrome,
Pennington Biomedical Research Center,
Baton Rouge, LA, USA
D. Ribnicky
Biotech Center-Rutgers University,
New Brunswick, NJ, USA
Present address:
L. K. Stewart
Department of Kinesiology, Louisiana State University,
Baton Rouge, LA, USA
metabolomic analysis of fatty acylcarnitines and acute in
vitro assessments of insulin signalling among tissues.
Results After 3 and 8 weeks, the high-fat diet produced
whole-body insulin resistance without altering insulindependent glucose uptake in peripheral tissues. The primary
defect was impaired suppression of hepatic glucose production by insulin at both times. Quercetin initially
exacerbated the effect of high-fat diet by further increasing
hepatic insulin resistance, but by 8 weeks insulin resistance
and hepatic responsiveness to insulin were similarly
compromised in both high-fat groups. The high-fat diet,
irrespective of quercetin, increased short-chain fatty acylcarnitines in liver but not in muscle, while reciprocally
reducing hepatic long-chain fatty acylcarnitines and increasing them in muscle.
Conclusions/interpretation Failure of insulin to suppress
hepatic glucose output is the initial defect that accounts for
the insulin resistance that develops after short-term consumption of a high-fat (45% of energy) diet. Hepatic insulin
resistance is associated with accumulation of short- and
medium-, but not long-chain fatty acylcarnitines. Dietary
quercetin does not ameliorate the progression of this
sequence.
Keywords Adipose tissue . Botanicals . Euglycaemic–
hyperinsulinaemic clamp . Glucose uptake .
Hepatic glucose production . Insulin resistance
Abbreviations
Endo Ra Endogenous hepatic glucose production
GIR
Glucose infusion rate
HF
High-fat
HF+Q
HF+quercetin
LF
Low-fat
PI-3K
Phosphoinositol 3-kinase
Diabetologia (2009) 52:514–523
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Introduction
Methods
Type 2 diabetes occurs when the pancreas cannot compensate fully for insulin resistance in peripheral tissues. Insulin
resistance is strongly linked to development of obesity, but
the complex events that occur in multiple organs and lead
to diabetes remain poorly understood. The fat-sensitive
C57BL/6J mouse has emerged as a key model to study the
developmental pathology of the obese/diabetic syndrome
produced by chronic consumption of high-fat (HF) diets
[1]. It remains unclear whether the initial stages of insulin
resistance are the product of a uniform progression of
insulin resistance across tissues or the result of a sequential
but punctuated progression of insulin resistance among
tissues. This lack of consensus is probably due to a
combination of differences in the amount, source and
saturation of dietary fat, as well as animal age and
duration of exposure prior to evaluation of insulin resistance [2–6].
Quercetin, a bioflavonoid abundant in apples, onions and
tea, is a dietary antioxidant associated with improved
antioxidant status [7], lower incidence of ischaemic heart
disease [8], delayed progression of atherosclerosis in
apolipoprotein E-null mice [9] and increased ratio of
oxidised to reduced glutathione in mice [9]. Quercetin also
protected against oxidative damage in isolated mitochondria [10], macrophages [11] and cardiomyoblasts [12].
Recent reports [13, 14] show that dietary quercetin reduced
circulating markers of inflammation and ameliorated
components of metabolic syndrome in genetic and dietinduced models of obesity. Given the emerging consensus
that mitochondrial dysfunction, inflammation, disordered
lipid metabolism and reactive oxygen species are associated
with the development of insulin resistance and diabetes
[15–18], we sought here to determine whether dietary
quercetin could ameliorate the progression of insulin
resistance in C57BL/6J mice consuming a moderately HF
diet.
Growing evidence indicates that lipid accumulation in
tissues not designed for storage is directly involved in
development of insulin resistance [16, 19, 20]. Incomplete
mitochondrial oxidation of fatty acids may be an underlying mechanism, so we also sought to determine whether
dietary quercetin could ameliorate the progression of dietinduced insulin resistance and modify mitochondrial lipid
catabolism. Using longitudinal studies and state-of-the art
in vivo and in vitro approaches, we show that dietinduced insulin resistance begins in the liver, is associated
with hepatic accumulation of short- to medium-, but not
long-chain fatty acylcarnitines, and is not ameliorated by
dietary quercetin.
Animals and diets Male 6-week-old C57BL/6J mice from
Jackson Laboratory (Bar Harbor, ME, USA) were randomly
assigned to the following diets (Research Diets, New
Brunswick, NJ, USA): (1) LF, (10% of energy from fat;
D12450B); (2) HF (45% of energy from fat; D12451); or
(3) HF + quercetin (HF+Q) (45% of energy from fat + 1.2%
quercetin [wt/wt]; D06081502). All diets contained soyabean oil (25 g/kg diet). The LF diet (16.12 kJ/g) contained
20 g lard/kg diet while both HF diets (19.80 kJ/g) contained
178 g lard/kg diet. Quercetin (≥98%; Sigma, St Louis, MO,
USA) was added to the HF+Q diet by cold processing.
Diets were stored at 4°C in light-protected, airtight containers.
Food was changed every 3 days, with free access to water.
Mice were singly housed in shoebox cages with corncob
bedding at 22°C on a 12 h light–dark cycle. Food consumption was monitored weekly over 48 h. Experiments were
approved by the Pennington Institutional Animal Care and
Use Committee and conducted in facilities and using
procedures that were consistent with the recommendations
of the National Research Council’s Guide for the Care and
Use of Laboratory Animals, the Animal Welfare Act, and
Public Health Service Policy on the Humane Care and Use of
Laboratory Animals.
Studies were conducted with four cohorts of mice to
evaluate in vivo and in vitro responses to diets. In cohort 1,
food consumption and body composition were monitored
for 8 weeks, followed by killing 16 mice per group at the
peak (01:00 hours) and 16 mice per group at the nadir
(13:00 hours) of the metabolic cycle. Liver and muscle were
used to analyse fatty acylcarnitines and tissue triacylglycerol. Blood was obtained from 16 additional mice per group
killed at time-points as above of the metabolic cycle. Insulin
resistance was assessed in mice from cohort 2 with
euglycaemic–hyperinsulinaemic clamps after 3 or 8 weeks
on the diets. Acute signalling responses to insulin in liver and
muscle were evaluated after an 8 h fast in cohort 3. In vitro
responses to insulin were measured in adipocytes isolated
from mice in cohort 4 after 8 weeks of diet.
Materials and reagents Anti-Akt, anti-phosphoinositol 3kinase (PI-3K) and anti-IRS1 were from UBI (Lake Placid,
NY, USA). All other reagents were from Sigma. Kits for
measuring plasma NEFA and triacylglycerol were from
ZenBio (Research Triangle Park, NC, USA). Integrity of
quercetin in the HF+Q diet and circulating quercetin were
measured by mass spectrometry [13].
Metabolomic analysis At 8 weeks, quadriceps muscle and
liver were removed from mice in cohort 1, snap-frozen and
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Diabetologia (2009) 52:514–523
extracts prepared for analysis of 45 fatty acylcarnitines by
tandem mass spectrometry at the Stedman Nutrition and
Metabolism Center at Duke Medical Center [21].
by insulin was assessed by measuring glycerol release as
described [24]. The experiment was repeated three times
using three mice per diet for each replication.
Euglycaemic–hyperinsulinaemic clamp Mice were weaned
on to respective diets at 6 weeks and in vivo insulin
sensitivity was assessed after 3 and 8 weeks by the
Vanderbilt Mouse Metabolic Phenotyping Center (Vanderbilt University School of Medicine, Nashville, TN, USA).
Whole-body insulin sensitivity, tissue-specific glucose
uptake and hepatic glucose production (Endo Ra) were
determined using euglycaemic–hyperinsulinaemic clamps
as described previously [22]. Arterial catheters were placed
5 days before the procedure and mice were fasted for 5 h
before clamps.
Statistical analysis Group means for food consumption,
body composition, tissue acylcarnitines, plasma quercetin
(cohort 1), euglycaemic–hyperinsulinaemic clamp variables
(cohort 2) and variables from insulin signalling experiments
(cohorts 3, 4) were analysed within each time point (3 or
8 weeks) by one-way ANOVA using animal within dietary
treatment group as the error term [5]. Post hoc testing of
group means within each time point was made with the
Bonferroni correction using the pooled error term to calculate
standard errors. In cohort 4, dose–response curves for
isoproterenol-induced activation of lipolysis were compared
using nonlinear least squares analysis [5]. For all contrasts,
protection against type 1 errors was set at 5% (α=0.05).
Insulin signalling in muscle and liver After 8 weeks on
diets, mice in cohort 3 were fasted for 8 h and killed at
baseline or 15 and 30 min post insulin injection (5 U/kg
body weight). Liver and muscle samples were removed and
snap-frozen. In whole-cell extracts from each tissue, PI-3K,
Akt-1, Akt-2 and Akt-P (S-473) were analysed by western
blot and IRS-1-associated PI-3K activity was measured in
immunoprecipitates as described previously [23].
Acute insulin signalling in adipocytes After 8 weeks, white
adipocytes were isolated from mice in cohort 4 and
preincubated with vehicle or 1 nmol/l insulin for 5 min at
37°C before addition of half log increments of isoproterenol
from 1 to 1,000 nmol/l. After 60 min, inhibition of lipolysis
Results
Food intake, body weight and body composition (cohort 1)
Energy intake did not differ among groups during the 1st
week, but was 14% and 8% higher in the HF than the LF
group at 3 and 8 weeks, respectively (p<0.05) (Table 1).
The HF diet also affected nutrient partitioning, as evidenced
by higher adiposity at 3 and 8 weeks in this group than in
the LF group (p<0.05) (Table 1). Inclusion of quercetin did
not affect energy intake, body weight or accretion of fat and
protein at either time point (Table 1).
Table 1 Physical and laboratory variables in mice from cohorts 1 and 2, respectively
Variable
Week 0 Week 3
LF
Body weight (g)
Food consumption (kJ per day)
Per cent fat mass (g fat [g body
weight]−1 ×100)
Blood glucose (mmol/l)
Plasma insulin (pmol/l)
Plasma TG (mmol/l)
Plasma NEFA (mmol/l)
Muscle TG (μmol/g tissue)
Liver TG (μmol/g tissue)
19.6±
0.11
–
10.0±
0.11
–
–
–
–
–
–
Week 8
HF
HF+Q
LF
HF
HF+Q
22.3±0.21
22.8±0.28
22.7±0.22
26.8±0.54a
28.9±0.62b
27.9±0.68ab
10.2±0.21a
9.9±0.45a
11.6±0.20b
11.9±0.43b
11.0±0.21a,b
12.0±0.53b
10.7±0.28a
14.5±0.91a
11.5±0.36b
20.9±1.0b
11.7±0.36b
20.7±1.4b
8.1±0.6
8.1±0.6
8.5±0.4
241.4±34.5
189.7±17.2
224.1±34.5
5.73±0.84a
8.37±0.92a
9.19±1.14a
a
a
0.338±0.024 0.396±0.031 0.413±0.031a
4.28±0.49a
4.71±0.32a
6.23±0.66a
5.74±1.15a
6.68±0.79a
6.44±2.26a
7.4±0.6
8.4±0.6
7.9±0.4
172.4±17.2a 344.8±103.4b 137.9±17.24a
5.76±1.61a
8.90±0.81a
6.4±1.35a
a
b
0.318±0.028 0.514±0.058 0.402±0.054a,b
2.39±0.29a
4.99±0.55b
4.86±0.70b
12.0±2.1a
16.4±2.3a
11.0±1.3a
Values are means±SEM
Measures were obtained from eight to ten mice per treatment group in each cohort
Fasting insulin and glucose concentrations (cohort 2) were measured at baseline, prior to the clamp procedure, after 3 or 8 weeks on LF, HF or
HF+Q diets
For each variable, values with different superscripts letters within age (3 weeks or 8 weeks) were significantly different (p<0.05 by ANOVA)
TG, triacylglycerol
Diabetologia (2009) 52:514–523
Euglycaemic–hyperinsulinaemic clamp (cohort 2) In blood
samples taken after a 5 h fast, blood glucose and plasma
insulin were similar among the groups after 3 weeks of diet
(Table 1). Blood glucose did not differ after 8 weeks, but
fasting insulin was higher (p<0.05) in the HF than the LF
group (Table 1). In contrast, fasting insulin in the HF+Q
group was similar to the LF group and lower (p<0.05) than
the HF group at 8 weeks (Table 1). At 8, but not 3 weeks,
plasma NEFA was higher in the HF than the LF group (p<
0.05), while the HF+Q group was intermediate to both
(Table 1). Plasma triacylglycerol levels were similarly
ranked but did not differ among the groups at 3 or 8 weeks
(Table 1). After 3 weeks, the glucose infusion rate (GIR)
required to maintain euglycaemia was 24% lower in mice
on the HF (0.211±0.011 mmol kg−1 min−1) vs LF diet
(0.262±0.011 mmol kg−1 min−1) (Fig. 1a). At 8 weeks,
another reduction of GIR in the HF group to 0.182±
0.019 mmol kg−1 min−1 was observed, consistent with further
loss of insulin sensitivity. In contrast, GIR in the LF group
was relatively unchanged (0.276±0.017 mmol kg−1 min−1)
between 3 and 8 weeks. The addition of quercetin to the
517
HF diet did not ameliorate the reduction in GIR, but did
exacerbate the insulin resistance produced by the HF diet at
3 weeks, such that the GIR was significantly lower (p<0.05)
than the HF group (Fig. 1a). However, between 3 and
8 weeks, the GIR in the HF+Q group improved slightly, no
longer differing from the HF group (Fig. 1b).
Tissue-specific uptake of 2-deoxy[14C]glucose was highest
in heart, followed by diaphragm, soleus, gastrocnemius,
vastus lateralis muscle and white adipose tissue (Table 2).
Interestingly, 2-deoxy[14C]glucose uptake did not differ in
any of the tissues at 3 or 8 weeks (Table 2). Basal
endogenous hepatic glucose production (Endo Ra) was
similar between LF and HF groups at 3 and 8 weeks, but
insulin suppression of Endo Ra was impaired in the latter at
both time points (Fig. 2a, b). Insulin did not suppress Endo
Ra in the HF+Q group at 3 weeks (Fig. 2a), consistent with
the reduction in GIR compared with the HF group (Fig. 1a).
Insulin’s ability to suppress Endo Ra was modestly restored
at 8 weeks in the HF+Q group (Fig. 2b), although basal
Endo Ra was also reduced in this group at 8 weeks
(p<0.05).
Quercetin diet composition, absorption and degradation Given
the changes in insulin resistance in the HF+Q group, it was
important to determine the stability of quercetin in the diet
and examine circulating quercetin levels in the animals. The
HF+Q diet was formulated to contain 1.2% quercetin (wt/wt),
but analysis after manufacture revealed 0.8% (wt/wt).
The concentration was unchanged after 8 weeks of storage
(~1% decrease). We therefore examined in vivo mechanisms and because food intake varies diurnally, plasma
quercetin was measured in samples taken when food consumption is highest (01:00 hours) and lowest (13:00 hours).
At 3 weeks, plasma quercetin varied diurnally between day
(8 μg/ml) and night (12 μg/ml) as expected, with a similar
variation between day (3 μg/ml) and night (7 μg/ml) seen
at 8 weeks. Given no change in food intake or quercetin
stability during this period, the twofold lower plasma
quercetin during the day and 40% lower plasma quercetin
at night between 8 and 3 weeks suggest an in vivo adaptive
response. The data are consistent with a pharmacokinetic
mechanism involving decreased absorption, enhanced degradation, increased excretion or some combination of these.
Fig. 1 GIR (mean±SEM) required to maintain euglycaemia during
hyperinsulinaemic clamp after 3 (a) or 8 (b) weeks on LF (black
circles), HF (white circles) or HF+Q (white squares) diets. The clamp
procedures were conducted with eight to ten mice per diet after 3 and
8 weeks on the respective diets. The steady-state clamp GIRs were
compared by ANOVA as described in the Methods section and were
lower (p<0.05) in mice on HF vs LF diet at 3 (a) and at 8 (b) weeks.
The addition of quercetin to the HF diet further reduced GIR
compared with HF alone at 3 weeks (p<0.05), but had no additional
effect beyond that of HF at 8 weeks (b)
Metabolomic analysis of fatty acylcarnitines and tissue
triacylglycerol To test whether clamp-detected insulin
resistance coincided with specific alterations in fatty acid
catabolism profiles in liver and muscle, 45 acylcarnitines
ranging in size (two to 22 carbons) and saturation were
evaluated [21] after 8 weeks on the respective diets. In
liver, 35 of the 45 acylcarnitines were unaffected by diet
(Electronic supplementary material [ESM] Table 1, with a
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Diabetologia (2009) 52:514–523
Table 2 Mean rates of 2-deoxy[3H]glucose uptake (pmol glucose min−1 [mg tissue] −1) among tissues during hyperinsulinaemic clamp after 3 or
8 weeks on LF, HF or HF+Q diets
Tissue
Soleus
Gastro
Vastus
Adipose
Diaphragm
Heart
Brain
Total Rda
Plasma insulin
3 weeks LF
(n=10)
3 weeks HF
(n=9)
3 weeks HF+Q
(n=10)
8 weeks LF
(n=6)
8 weeks HF
(n=6)
8 weeks HF+Q
(n=7)
Mean
SEM
Mean
SEM
Mean
SEM
Mean
SEM
Mean
SEM
Mean
SEM
0.560
0.110
0.097
0.031
0.920
3.392
0.451
0.292
20.1
0.091
0.012
0.006
0.006
0.079
0.420
0.043
0.018
2.1
0.621
0.110
0.134
0.031
1.030
2.741
0.402
0.268
20.1
0.073
0.012
0.024
0.006
0.128
0.432
0.018
0.024
3.5
0.652
0.097
0.085
0.024
1.097
2.594
0.420
0.261
16.0
0.177
0.012
0.018
0.006
0.238
0.493
0.037
0.031
0.7
0.707
0.134
0.116
0.024
0.822
2.875
0.317
0.361
16.0
0.365
0.049
0.043
0.006
0.177
0.445
0.067
0.029
0.7
0.627
0.110
0.079
0.031
0.804
2.351
0.378
0.269
14.6
0.183
0.031
0.018
0.012
0.116
0.566
0.067
0.014
0.7
0.548
0.110
0.097
0.024
0.865
1.912
0.378
0.262
17.4
0.079
0.012
0.012
0.006
0.134
0.530
0.043
0.013
1.4
The clamp procedures were conducted with eight to ten mice per diet after 3 and 8 weeks on the respective diets.
The mean glucose uptake rates were compared by ANOVA as described in the Methods section
a
The mean total glucose disposal rate (Rd) during the clamps was expressed as mmol glucose kg−1 min−1 ; mean plasma insulin concentration
during the clamps was expressed as pmol/l
Adipose, epididymal white adipose tissue; Soleus, soleus muscle; Gastro, gastrocnemius muscle; Vastus, vastus lateralis muscle
Fig. 2 Rates (mean±SEM) of Endo Ra prior to and during hyperinsulinaemic clamp after 3 (a) or 8 (b) weeks on LF (white bars), HF
(black bars) or HF+Q (hatched bars) diets. The clamp procedures were
conducted with eight to ten mice per diet after 3 and 8 weeks on the
respective diets to test how effectively insulin suppressed Endo Ra.
Basal Endo Ra did not differ among the groups at 3 weeks (a) but was
lower in HF+Q vs LF and HF groups at 8 weeks (b). The HF diet
compromised suppression of Endo Ra by insulin at 3 and 8 weeks.
Quercetin exacerbated the effect of the HF diet at 3 (a) but not at
8 weeks (b). The rates of Endo Ra during the basal and clamp periods
were compared by ANOVA as described in the Methods section.
Within the basal or clamp periods, means denoted by different
superscript letters were significantly different (p<0.05)
Fig. 3 Fatty acylcarnitine profiles were obtained by tandem mass
spectrometry in hepatic extracts from mice after consumption of LF
(n=32; white bars), HF (n=32; black bars) or HF+Q (n=31; hatched
bars) diets for 3 and 8 weeks. The effect of diet on short-chain (a) and
long-chain (b) acylcarnitines was assessed by ANOVA as described in
the Methods section. For each acylcarnitine subtype, means denoted
by different superscripts letters were significantly different (p<0.05).
C4-OH, 3-hydroxy-butyryl carnitine; C4-DC/Ci4-DC, succinyl carnitine or methylmalonyl carnitine; C5:DC, glutaryl carnitine; C8:1-OH/
C6:1-DC, 3-hydroxy-cis-5-octenoyl carnitine or hexenedioyl
carnitine; C10-OH/C8-DC, 3-hydroxy-decanoyl carnitine or hexenedioyl
carnitine; C12:1, dodecenoyl carnitine; C14:1, tetradecadienoyl carnitine;
C16:2, hexadecadienoyl carnitine; C16:1, palmitoleoyl carnitine; C16:1OH, 3-hydroxy-palmitoleoyl carnitine
Diabetologia (2009) 52:514–523
519
Fig. 4 Fatty acylcarnitine profiles were obtained as in Fig. 3 in
quadriceps muscle extracts from mice after consumption of the three
diets for 8 weeks. LF, n=16; HF, n=16; HF+Q, n=16. The effect of
diet on each acylcarnitine was tested by ANOVA as described in the
Methods. Means denoted by different superscript letters were
significantly different (p=0.05). C16-OH/C14-DC, 3-hydroxy-hexadecanoyl or tetradecanedioyl carnitine; C18:2, linoleyl carnitine; C18,
Stearoyl carnitine
a
key to abbreviations in ESM Table 3). Of the remaining ten,
four short-chain metabolites ranging from C4 to C8 were
increased (p<0.05) by the HF relative to the LF diet
(Fig. 3a); quercetin in the HF diet did not modify the
increase in these metabolites. Two medium-chain acylcarnitines were higher (p≤0.05) in HF than LF (Fig. 3b), but
were not increased by HF+Q relative to LF (Fig. 3b). In
contrast, four long-chain metabolites (C14–C16) were
significantly lower in the two HF groups than in the LF
group (Fig. 3b). The acylcarnitine profiles in muscle were
completely different, with only three of the 45 lipid
metabolites altered by diet (ESM Table 2, with a key to
abbreviations in ESM Table 3). The three that changed
were all long-chain species (C16–C18) that were increased
by the HF relative to the LF diet (Fig. 4). The response of
c
Total Akt-1
PI-3K protein
Total Akt-2
PI-3K activity
Akt-P
0
15
Diet
30
0
LF
PI-3K activation
(fold change above basal)
b
15
30
0
HF
15
30
Insulin (min)
a
b
2
b
1
0
LF
HF
HF+Q
e
g
Total Akt-1
PI-3K activity
Akt-P
15
30
0
LF
15
30
0
HF
15
30
HF+Q
PI-3K activation
(fold change above basal)
f
Insulin (min)
Diet
0
30
0
15
30
0
HF
15
30
HF+Q
3
2
1
0
LF
15
30
HF
0
LF
15
HF+Q
30
0
HF
15
30
HF+Q
h
8
6
4
2
0
LF
HF
HF+Q
Fig. 5 Insulin-dependent activation of PI-3K (a, b) and Akt (c, d) in
hepatic extracts from mice after consumption of the three diets for
8 weeks. LF, n=8; HF, n=8; HF+Q, n=8. Insulin-dependent activation
of PI-3K (e, f) and Akt (g, h) in muscle extracts from mice after diet as
above. LF, n=4; HF, n=4; HF+Q, n=4. The mice were fasted for 8 h
prior to killing at 0, 15 and 30 min after injection of insulin (5 U/kg
body weight). PI-3K and Akt activity and expression were measured
Akt phosphorylation
(fold change above basal)
0
Diet
15
LF
d
3
PI-3K protein
Insulin (min)
0
Diet
HF+Q
Akt phosphorylation
(fold change above basal)
Insulin (min)
6
4
2
0
LF
HF
HF+Q
as described in Methods. Representative blots and activity measurements are shown. PI-3K activity and the increase in Akt-P were
expressed as fold activation of PI-3K and Akt-P at 30 min relative to
basal activity. Bar graphs show the means from three mice per time
point per dietary group. The effect of diet on PI-3K activation and the
increase in Akt-P were assessed by ANOVA. Means denoted by
different superscript letters were significantly different (p<0.05)
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Diabetologia (2009) 52:514–523
these three acylcarnitines in the HF+Q group was indistinguishable from the HF group (Fig. 4).
Total hepatic triacylglycerol levels did not differ among
the groups at 3 or 8 weeks (Table 1), but in muscle the HF
and HF+Q groups had similar and higher triacylglycerol
(p<0.05) than the LF group at 8 but not 3 weeks (Table 1).
Collectively, the catabolic profiles suggest differential
responses between the tissues. The simultaneous reduction
of long-chain and accumulation of short-chain acylcarnitines is most consistent with enhanced but incomplete
hepatic fatty acid catabolism in the HF groups. In muscle,
the accumulation of long-chain acylcarnitines in the HF
groups suggests some combination of increased uptake or
decreased catabolism of fatty acids.
Insulin signalling in liver and muscle Hepatic PI-3K level
was unaffected by diet (Fig. 5a), but basal PI-3K activity
was higher (p<0.05) in the HF and HF+Q groups than in
the LF group (Fig. 5a). Insulin increased PI-3K activity
1.5-fold above basal in the HF group but produced no
detectable activation above basal in HF+Q (Fig. 5b). In the
LF group, insulin produced a 1.5- and 2.6-fold activation of
PI-3K at 15 and 30 min (Fig. 5b). Basal Akt phosphorylation was higher in the two HF groups than the LF group
(Fig. 5c), and higher in the HF+Q than in the HF group. In
the two HF groups, insulin produced no additional
activation of Akt; in the LF group it produced a 2.5-fold
activation of Akt (Fig. 5d). The diets had no effect on
hepatic protein content for GLUT2, IRS-1, IRS-2 or insulin
receptor beta (data not shown).
In muscle, insulin produced comparable increases in PI3K activity among the groups (Fig. 5e & f). In contrast,
basal Akt phosphorylation was two- to threefold higher in
the HF group compared with the LF and HF+Q groups.
Despite higher basal Akt in the HF group, insulin produced
comparable additional activation of Akt among the groups
(Fig. 5g, h). As in liver, production of many signalling intermediates did not differ among the groups (IRS-1, IRS-2,
GLUT4, insulin receptor beta, AS160 and LAR).
Insulin signalling in adipocytes To examine acute responsiveness of adipose tissue to insulin, the integrity of insulin
signalling to type 3B phosphodiesterase was assessed by
measuring insulin inhibition of cAMP-dependent lipolysis
in adipocytes isolated from mice after 8 weeks on LF or HF
diet. Figure 6 shows that isoproterenol produced comparable lipolytic responses in adipocytes from mice fed LF
(EC50 =10.5±2.1 nmol/l) vs HF (EC50 =6.9±1.3 nmol/l)
diets and that insulin produced a comparable decrease in
isoproterenol EC50 between the two groups (LF =133±26;
HF =89±21 nmol/l). This shows that inhibition of lipolysis by
insulin was fully functional and not compromised by the HF
diet.
Fig. 6 Inhibition of isoproterenol-dependent activation of lipolysis by
insulin in adipocytes isolated from mice after consumption of LF (a)
or HF (b) diets for 8 weeks. Epididymal white adipocytes were
isolated from three mice per group after a 5 h fast and preincubated
with 1 nmol/l insulin for 5 min prior to addition of half log increments
of isoproterenol and further incubation for 1 h at 37°C in a shaking
water-bath. Glycerol release into the medium was measured and
representative dose–response curves from one of three replicates of the
experiment are shown. Dose–response curves were fitted as described
in the Methods section. Insulin sensitivity was assessed by comparing
the change in EC50 of isoproterenol produced by insulin in adipocytes
from the two dietary groups. White squares and circles, without
insulin; black squares and circles, with insulin; EC50, concentration of
isoproterenol producing half maximal activation of glycerol release
from cells
Discussion
Accumulation of lipid in tissues not designed for storage is
a key step in the initiation and progression of insulin
resistance to diabetes [20, 25, 26]. The defective triacylglycerol storage in adipose tissue that occurs with lipodystrophy results in rapid accumulation of lipid in liver and
muscle. Ectopic fat deposition also occurs after chronic
consumption of HF diets and is temporally related to
periods of rapid weight gain and decreased insulin
sensitivity [1, 3, 27]. An important unresolved question is
whether the initial stage of insulin resistance is the product
of uniform development of insulin resistance across tissues
or the result of a sequential but punctuated progression of
insulin resistance among specific tissues. Recent evidence
suggests that mitochondrial lipid overload and incomplete
fatty acid oxidation may compromise insulin sensitivity in
Diabetologia (2009) 52:514–523
muscle by increasing lipotoxic short-chain lipid metabolites
that impair mitochondrial function [21, 28–30]. The
concept of lipid-induced mitochondrial dysfunction was
developed in muscle, but it is unclear whether a similar
mechanism is associated with development of insulin
resistance in non-muscle sites. The present work addressed
these issues using a combination of in vivo and in vitro
approaches in longitudinal studies of C57BL/6J mice
weaned on to a lard-based HF diet (45% energy from fat),
with quercetin provided as a food admixture. The four
major findings were: (1) whole-body insulin resistance was
detected after 3 and 8 weeks on the HF diet and was
entirely due to an inability of insulin to suppress endogenous glucose production from the liver; (2) muscle, adipose
tissue and all other peripheral tissues retained full insulin
sensitivity; (3) hepatic insulin resistance was associated
with accumulation of short- to medium-chain and depletion
of long-chain fatty acylcarnitines; and (4) dietary quercetin
provided as a 0.8% (wt/wt) food admixture did not
ameliorate diet-induced insulin resistance in this model.
The strong relationship between obesity and diabetes is
well documented, but the mechanisms linking obesity to
insulin resistance are less well understood. This is tied to
the chronic nature of both conditions and the progressive
deterioration in function of multiple, highly integrated
organ systems that work together to maintain glucose
homeostasis. The early recognition that C57BL/6J mice
developed an obese/diabetic syndrome after consumption of
a diet high in saturated fat established the C57BL/6J mouse
as an important model for study of developmental progression of obesity and diabetes [1]. The initial design of
weaning C57BL/6J mice on to a coconut oil-based HF diet
(58% energy from fat) has been modified to use different
amounts (58 vs 45% energy) and sources of fat, and
introduce the diet at different ages. The approach has been
extended to various rat lines that respond similarly and
develop varying degrees of insulin resistance. Although few
of these studies used longitudinal approaches and in vivo
measurements (euglycaemic–hyperinsulinaemic clamps) to
identify where insulin resistance first develops, the consensus is that very-HF diet formulations (55–60% energy from
fat) produce rapid deterioration in whole-body insulin
sensitivity coincident with the appearance of hepatic and
peripheral insulin resistance. The response pattern is
common to rats and mice on very-HF diets, and supports
the view that whole-body insulin resistance is the product
of uniform progression of insulin resistance among peripheral tissues [3, 4, 31, 32]. However, with a slightly lower
percentage of dietary fat (45% of energy), compromised
suppression of hepatic glucose production by insulin may
be the primary initial cause of whole-body insulin resistance, followed by progressive loss of insulin-sensitive
glucose uptake in peripheral tissues. Our findings indicate
521
that mild whole-body insulin resistance after 3 weeks on the
HF diet (45% of energy from fat) was due solely to failure
of insulin to suppress Endo Ra without any change in
insulin-dependent glucose uptake in peripheral tissues. And
although whole-body insulin resistance increased after
8 weeks on the HF diet, it was still entirely accounted for
by a corresponding loss of suppression of hepatic Endo Ra
by insulin. The integrity of the initial steps of insulin
signalling was assessed in mice killed at 0, 15 and 30 min
after acute injection with insulin. These findings paralleled
the in vivo data and show that insulin-dependent activation
of Akt and PI-3K were compromised in liver but not
muscle after 8 weeks on the HF diet. Together, these
findings indicate that the insulin resistance produced by the
HF (45% energy from fat) diet is milder, begins in the liver
and progresses to peripheral tissues more slowly than with
very-HF diets (55–60% of energy from fat). For example,
male Wistar and Sprague Dawley rats consuming a diet
with 59% of energy from fat developed hepatic but not
muscle insulin resistance after 3 days [2, 32]. However,
after 3 weeks, insulin resistance had spread from the liver to
other peripheral tissues [32]. With the more physiologically
relevant level of fat used in the present studies (45% of
energy from fat), insulin resistance did not progress beyond
the liver during the 3–8 week study period.
The initiation of insulin resistance in specific tissues
becomes physiologically important when the pancreas
compensates for whole-body insulin resistance and
increases insulin release. For example, the rapid increase
in fat deposition between 3 and 8 weeks coincides with the
time frame when hyperinsulinaemia became evident in our
HF group [13]. Using independent methods in separate
cohorts, we showed that adipose tissue was fully responsive
to insulin during this period. Given the important lipogenic
and anti-lipolytic roles of insulin in adipose tissue, our
findings suggest that hepatic insulin resistance in conjunction with retention of peripheral insulin sensitivity created
conditions that accentuated fat deposition in adipose tissue.
This response may be a mechanism to prevent or delay
ectopic fat accumulation and may also explain the slower
progression of insulin resistance to peripheral tissues with
the HF (45% of energy from fat) diet [2–4, 32–34].
The precise mechanisms initiating diet-induced insulin
resistance among peripheral tissues remain poorly understood. Although it is generally accepted that ectopic lipid
accumulation is closely tied to insulin resistance [20, 25,
35], the identity of the accumulated lipids and underlying
mechanism(s) that compromise insulin signalling remain
uncertain [29, 30]. It is also not clear whether the same lipid
metabolites are responsible for producing insulin resistance
among all tissues. For example, various lines of evidence
suggest that accumulation of fatty acyl-CoAs compromises
insulin signalling in muscle and liver [36–38], but recent
522
evidence supports a specific causative role for increased
short-chain fatty acylcarnitines in muscle [28, 30]. In our
studies, short-chain acylcarnitines were unaltered in muscle
of the HF groups, with the tissue retaining full responsiveness to insulin despite higher levels of triacylglycerol and
long-chain acylcarnitines. In contrast, the hepatic acylcarnitine profiles suggested enhanced long-chain fatty acid
catabolism in the HF groups and accumulation of short- and
medium-chain fatty acid metabolites. It is unclear whether
the accumulation of short-chain fatty acylcarnitines caused
hepatic insulin resistance through a mechanism similar to
that reported in muscle by Koves et al. [28, 29]. It is also
possible that the altered hepatic acylcarnitine profiles were
merely secondary to the insulin resistance produced in liver
by the HF diets and reflect loss of metabolic flexibility.
Recent studies have shown that systemic glucose homeostasis and endogenous glucose production are rapidly
regulated through hypothalamic lipid sensing mechanisms
[39, 40]. These findings illustrate that other signalling
inputs may be altered by the HF diet and subsequently
modifying insulin’s ability to regulate endogenous glucose
production.
Recent work from our laboratory established that dietary
quercetin provided as a 1% food admixture reduced
circulating markers of inflammation in mice on the HF
(45% of energy from fat) diet [13]. This prompted a more
rigorous evaluation of the response, with special emphasis
on whether the flavonoid would also ameliorate progression
of diet-induced insulin resistance. We hypothesised that
quercetin might have protective effects, if reactive oxygen
species are an underlying mediator of the insulin resistance
produced by ectopic lipid accumulation and the predicted
mitochondrial stress [29]. Our prediction is supported by a
recent report showing significant reductions in insulin
resistance, hyperlipidaemia and inflammation after oral
administration of quercetin to fatty Zucker rats [14].
Previous studies with quercetin [41, 42] reported beneficial
effects from a wide range of doses (50 μg mouse−1 day−1 to
50 mg mouse−1 day−1), so it was particularly surprising to
find that dietary quercetin (0.8% wt/wt) exacerbated dietinduced insulin resistance at 3 weeks in our studies. By
8 weeks, the insulin resistance of the quercetin-supplemented
group was no worse than in animals on HF diet alone. A
composite evaluation of quercetin content and stability in the
diet, intake and serum suggested that quercetin had an
inhibitory effect on insulin signalling at 3 weeks that was
eliminated by an adaptive increase in hepatic metabolism [43]
and/or excretion of the compound between 3 and 8 weeks.
Quercetin is an effective inhibitor of PI-3K and phospholipase A2 with an IC50 of 3.8 μmol/l [44, 45]. Given the
relative permeability of hepatocytes to small molecules and
the effectiveness of quercetin as an inhibitor of PI-3K and
phospholipase A2, it seems likely that the concentration
Diabetologia (2009) 52:514–523
range of serum quercetin at 3 weeks (24–36 μmol/l) was
sufficient to inhibit insulin-dependent activation of PI-3K
and exacerbate insulin resistance. Notwithstanding the
reduction in inflammatory cytokines observed with this high
dose [13], the beneficial effects of significantly lower doses
of quercetin in genetic models of insulin resistance [14] and
atherosclerosis [41] make a compelling case for identifying
lower doses that retain most if not all of these beneficial
effects. It will be important in future studies to determine the
site(s) of action and whether the beneficial effects of
quercetin are mediated through a common mechanism.
Acknowledgements We thank N. Lenard, T. Henagan, A. Adamson,
D. Cason and J. Manuel (Pennington Biomedical Research Center) for
excellent technical support and M. Pellizzon for advice on formulating
the diets (Research Diets). We thank A. Gooch for administrative
support and O. McGuinness and the Vanderbilt Mouse Metabolic
Phenotyping Center (NIH grant U24 DK59637) for conducting the
euglycaemic–hyperinsulinaemic clamps. This work was supported by
the National Center for Complementary and Alternative Medicine and
the Office of Dietary Supplements (P50AT002776-01), and in part by
NIH grant P20-RR021945 from the National Center for Research
Resources, NIH CNRU Center Grant 1P30 DK072476 and NIH
RO1074772 (to T. W. Gettys).
Duality of interest The authors declare that there is no duality of
interest associated with this manuscript.
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