Content uploaded by Paul Martin Coen
Author content
All content in this area was uploaded by Paul Martin Coen
Content may be subject to copyright.
Available via license: CC BY-NC-ND 3.0
Content may be subject to copyright.
Skeletal Muscle Triglycerides, Diacylglycerols, and
Ceramides in Insulin Resistance
Another Paradox in Endurance-Trained Athletes?
Francesca Amati,
1,2
John J. Dubé,
1
Elvis Alvarez-Carnero,
1
Martin M. Edreira,
3
Peter Chomentowski,
1
Paul M. Coen,
1
Galen E. Switzer,
4,5,6
Perry E. Bickel,
7,8
Maja Stefanovic-Racic,
1
Frederico G.S. Toledo,
1
and Bret H. Goodpaster
1
OBJECTIVE—Chronic exercise and obesity both increase intra-
myocellular triglycerides (IMTGs) despite having opposing effects
on insulin sensitivity. We hypothesized that chronically exercise-
trained muscle would be characterized by lower skeletal muscle
diacylglycerols (DAGs) and ceramides despite higher IMTGs and
would account for its higher insulin sensitivity. We also hypothe-
sized that the expression of key skeletal muscle proteins involved in
lipid droplet hydrolysis, DAG formation, and fatty-acid partitioning
and oxidation would be associated with the lipotoxic phenotype.
RESEARCH DESIGN AND METHODS—A total of 14 normal-
weight, endurance-trained athletes (NWA group) and 7 normal-
weight sedentary (NWS group) and 21 obese sedentary (OBS
group) volunteers were studied. Insulin sensitivity was assessed
by glucose clamps. IMTGs, DAGs, ceramides, and protein expres-
sion were measured in muscle biopsies.
RESULTS—DAG content in the NWA group was approximately
twofold higher than in the OBS group and ~50% higher than in the
NWS group, corresponding to higher insulin sensitivity. While
certain DAG moieties clearly were associated with better insulin
sensitivity, other species were not. Ceramide content was higher
in insulin-resistant obese muscle. The expression of OXPAT/
perilipin-5, adipose triglyceride lipase, and stearoyl-CoA desatur-
ase protein was higher in the NWA group, corresponding to a
higher mitochondrial content, proportion of type 1 myocytes,
IMTGs, DAGs, and insulin sensitivity.
CONCLUSIONS—Total myocellular DAGs were markedly higher
in highly trained athletes, corresponding with higher insulin sen-
sitivity, and suggest a more complex role for DAGs in insulin action.
Our data also provide additional evidence in humans linking
ceramides to insulin resistance. Finally, this study provides novel
evidence supporting a role for specific skeletal muscle proteins
involved in intramyocellular lipids, mitochondrial oxidative capac-
ity, and insulin resistance. Diabetes 60:2588–2597, 2011
Skeletal muscle insulin resistance (IR) is associ-
ated with obesity and physical inactivity and is
crucial for the development of type 2 diabetes (1).
Unfortunately, the causes of IR within muscle are
not known. Concerted efforts have been made over the past
several years to understand the potential role of intra-
myocellular lipid (IMCL) accumulation in the development
of IR (2). Studies in both animal models (3) and humans (4)
provided early evidence that IMCLs, such as triglycerides,
were associated with skeletal muscle IR. However, we sub-
sequently reported on the athletes paradox in which chron-
ically exercised humans were markedly insulin-sensitive
despite having high intramyocellular triglycerides (IMTGs)
(5), a phenomenon corroborated by others (6). These
observations gave pause to the widely held view that
IMCLs cause IR within the muscle and lent support to the
concept that other potentially damaging IMCLs may play
a role in the development of IR.
Diacylglycerols (DAGs) and ceramides are lipid inter-
mediates widely believed to be the true lipotoxic culprits
underlying the reported associations between muscle tri-
glycerides and IR, thereby explaining the athletes paradox
and the root cause of muscle IR. Indeed, several lines of
evidence from cell systems and animal models indicate
that elevated DAGs (7) or ceramides (8) are associated
with impaired insulin signaling and IR. Previous studies
examining skeletal muscle DAG and ceramide content re-
lated to human IR, however, are limited and inconsistent
(9,10). Because it has become clear that chronic exercise
training increases IMTGs (5,11), recent studies have been
conducted to examine whether both DAGs and ceramides
are reduced with exercise (12–14). Another prevalent no-
tion is that higher mitochondria content and capacity for
fatty acid oxidation caused by exercise training are re-
sponsible for lower DAG and ceramide content (15). We
recently have shown that exercise training does indeed
decrease these IMCLs in conjunction with increased oxi-
dative capacity and improved insulin sensitivity (12). The
distinct roles that chronic exercise and obesity may play in
the link between these potentially harmful lipid species and
skeletal muscle IR, however, remain to be elucidated.
Moreover, it is not known whether specific molecular
species of these complex lipids are associated with
skeletal muscle IR according to their fatty acid chain
length or degree of saturation. Therefore, we used mass
spectrometry to quantify the content and molecular spe-
cies profile of both DAGs and sphingolipids within skel-
etal muscle biopsies in human subjects widely disparate
From the
1
Department of Medicine, Division of Endocrinology and Metabo-
lism, School of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania;
the
2
Department of Physiology, School of Biology and Medicine, University
of Lausanne, Lausanne, Switzerland; the
3
Department of Pharmacology and
Chemical Biology, School of Medicine, University of Pittsburgh, Pittsburgh,
Pennsylvania; the
4
Division of General Internal Medicine, University of
Pittsburgh, Pittsburgh, Pennsylvania; the
5
Department of Psychiatry, School
of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania; the
6
Center
for Health Equity Research and Promotion, VA Pittsburgh Healthcare System,
Pittsburgh, Pennsylvania; the
7
Center for Metabolic and Degenerative Dis-
eases, Brown Foundation Institute of Molecular Medicine, University of Texas
Health Science Center at Houston, Houston, Texas; and the
8
Division of En-
docrinology, Diabetes, and Metabolism, Department of Internal Medicine,
University of Texas Health Science Center at Houston, Houston, Texas.
Corresponding author: Bret H. Goodpaster, bgood@pitt.edu.
Received 31 August 2010 and accepted 9 July 2011.
DOI: 10.2337/db10-1221
Ó2011 by the American Diabetes Association. Readers may use this article as
long as the work is properly cited, the use is educational and not for profit,
and the work is not altered. See http://creativecommons.org/licenses/by
-nc-nd/3.0/ for details.
2588 DIABETES, VOL. 60, OCTOBER 2011 diabetes.diabetesjournals.org
ORIGINAL ARTICLE
for physical activity and obesity to test the following
primary hypothesis: endurance-trained athletes, despite
having higher IMTGs, would have the lowest muscular
DAG and ceramide content, the highest insulin sensitivity,
and the highest mitochondrial content compared with both
normal weight and obese sedentary subjects and that these
differences would be more pronounced for saturated DAG
and ceramide species. In addition, we investigated several
proteins associated with lipid droplet esterification and lipol-
ysis: oxidative perilipin adipophilin TIP47 (OXPAT)/perilipin-5
or lipid storage protein 5 because of its reported role in
lipid droplet formation and enhanced fatty acid oxidation
(16,17); the lipogenic enzymes stearoyl-CoA desaturase 1
(SCD1) and diacylglycerol acyltransferase 1 (DGAT1); and
adipose triglyceride lipase (ATGL). These data provide
novel insight into the role of lipid-induced IR within human
skeletal muscle.
RESEARCH DESIGN AND METHODS
A total of 42 older (aged 66.2 60.7 years [range 60–75]) volunteers were recruited
and classified as obese (BMI .30 kg/m
2
) and sedentary (OBS group; n=21),
normal weight (BMI 18–25 kg/m
2
) and sedentar y (NWS group; n= 7), and normal
weight and endurance trained (NWA group; n= 14). Sedentary was defined as
#1 day per week of a structured exercise session of ,20 min. Endurance trained
was defined as five or more structured aerobic exercise sessions per week.
Subjects were in good general health, nonsmokers, and weight stable
(63 kg) for at least 6 months. Fasting lipid profile, electrolytes, plasma glu-
cose, complete blood count, platelet count, HbA
1c
, kidney function, thyroid-
stimulating hormone, and liver function were determined. A standard 75-g oral
glucose tolerance test (OGTT) was performed. Potential subjects were ex-
cluded if they had fasting glycemia $126 or $200 mg/dL after a 2-h OGTT,
hematocrit level ,34%, or a thyroid-stimulating hormone level .8mIU/mL or if
they were taking chronic medication known to affect glucose homeostasis.
The University of Pittsburgh Institutional Review Board approved the pro-
tocol. All volunteers gave written consent.
Body composition. Total body fat mass and lean body mass (LBM) were
measured by dual-emission X-ray absorptiometry (Lunar Prodigy; GE Health-
care, Milwaukee, MI). Visceral (VAT), subcutaneous (SAT), and total (TAT)
abdominal adipose tissue were measured from a single slice (L3–L4 level) by
magnetic resonance imaging (Siemens 3T Trio; Medicals Systems, Erlangen,
Germany) or computed tomography (9800 CT scanner; GE Healthcare) (2,18).
To account for possible differences in the two imaging methods, the following
ratios were computed: VAT-to-TAT, SAT-to-TAT, and VAT-to-SAT.
Physical fitness. VO
2
peak was measured with a graded exercise protocol on
an electronically braked cycle ergometer (Ergoline 800S; Sensormedics, Yorba
Linda, CA) with indirect calorimetry (Moxus, AEI Technologies, Pittsburgh,
PA), as described previously (11).
Insulin sensitivity. Insulin sensitivity was measured as the rate of insulin-
stimulated glucose disposal (R
d
) during a 4-h hyperinsulinemic-euglycemic
clamp (40 mU/m
2
/min) coupled with stable isotope tracer dilution, as de-
scribed previously (5). Subjects were instructed not to perform physical ex-
ercise 48 h before the clamp to avoid the acute effects of exercise on insulin
sensitivity. Indirect calorimetry (Parvomedics TrueOne, Sandy, UT) was used
during the clamp to calculate oxidative and nonoxidative glucose disposal.
Muscle biopsy. Percutaneous muscle biopsies were performed before the
clamp, following the same standardized conditions as described above. Bi-
opsies were obtained from the vastus lateralis as described previously (19).
After trimming of visible adipose tissue with a dissecting microscope (Leica
EZ4; Leica Microsystems, Wetzlar, Germany), two portions of the specimen
(~30 mg each) were flash-frozen in liquid nitrogen and stored at 280°C for
Western blotting and for ceramide and DAG analysis. A third portion was used
for histochemical analysis. A fourth portion (~5 mg) was used for transmission
electron microscopy. All analyses were then performed in a blind manner.
Histochemistry. Histochemical analysis was performed on serial sections
using methods previously described (11,12). IMTG content was determined by
the Oil Red O (ORO). To quantify fiber-specific IMTGs, the sections were
costained with ORO and human myosin heavy-chain MYH7 (type I) and MYH2
(type IIa) (all antibodies from Santa Cruz Biotechnology, Santa Cruz, CA).
Succinate dehydrogenase (SDH) staining was used as a marker for oxidative
capacity. Images were visualized using a Leica microscope (DM4000B; Leica
Microsystems) and digitally captured (Retiga 2000R; Q Imaging, Surrey, BC,
Canada), and semiquantitative image analysis was performed (Northern
Eclipse; Empix Imaging, Cheektowaga, NY). All units for the histology are in
average gray and reported as arbitrary units (AUs). The proportion of type I,
type IIa, and type IIx was computed from ~150 to 300 fibers per subject. The
interassay variability for the fiber type–specific ORO measure assessed on
three subjects was ,6%.
Analysis of DAGs, ceramides, and sphingolipids. Quantification of in-
tramuscular DAGs, ceramides, and sphingolipids was performed using high-
performance liquid chromatography–tandem mass spectrometry, as described
previously (20). Tissue homogenates were fortified with internal standards
and extracted into a one-phase neutral organic solvent system (ethyl acetate/
isopropyl alcohol/water; 60:30:10 vol/vol/vol), evaporated and reconstituted
in methanol, and analyzed by a surveyor/TSQ 7000 LC/MS System (Thermo
Electron Finnigan; Thermo Fisher Scientific, Waltham, MA) (21). Quantitative
analysis was performed in a positive multiple-reaction–monitoring mode, based
on calibration curves generated by adding to an artificial matrix known amounts
of target analytes, synthetic standards, and an equal amount of internal standard.
The DAG and ceramide levels were normalized to total protein levels.
Protein expression. Frozen tissue was homogenized in ice-cold cell lysis
buffer (Cell Signaling Technology, Danvers, MA) with protease inhibitor
cocktail tablets (Roche Diagnostics, Mannheim, Germany). Protein content was
determined in triplicate with a bicinchoninic acid assay (ThermoScientific,
Rockford, IL). The homogenates were mixed with 53Laemmli buffer and
heated for 5 min at 95°C. The samples were loaded in equal amounts of protein
and resolved in a 12% SDS-PAGE followed by transfer onto polyvinylidene
difluoride membranes (Bio-Rad Laboratories, Hercules, CA). After blocking,
membranes were probed overnight at 4°C with the anti-myocardial lipid droplet
protein (MLDP) (OXPAT/perilipin-5) antibody (American Research Products,
Belmont, MA), anti-SCD1 (Alpha Diagnostic International, San Antonio, TX),
anti-DGAT1 (Novus Biologicals, Littleton, CO), and anti-ATGL (Cell Signaling,
Boston, MA). Blots were normalized to either anti–b-actin (Santa Cruz Bio-
technology, Santa Cruz, CA). A prestained molecular-mass marker (10–170
kDa) was use d to determine protein size (Fermentas International, Burlington,
Ontario, Canada). Immunoreactive proteins were detected by chemiluminescence
(Bio-Rad Laboratories) using appropriate horseradish peroxidase–conjugated
secondary antibodies. Specific bands were quantified by densitometry using the
software ImageJ (National Institutes of Health, Bethesda, MD). To reduce bias,
all samples were loaded on the same day, and all gels and membranes were
processed at the exact same time with the same solutions.
Transmission electron microscopy. Transmission electron microscopy anal-
ysis was conducted in batches, as described previously (22). Approximately
18–20 micrographs were obtained from randomly sampled transverse sec-
tions of muscle fibers and were acquired (JEM-1210; Jeol, Tokyo, Japan) at
336,000 magnification using stereological analysis with digital-imaging software
(Metamorph 6.3; Molecular Devices, Sunnyvale, CA) .
Statistical analysis. Data are presented as means 6SEM. After checking
normality with the Shapiro-Wilk test and equality of variance with the Levene
test, one-way ANOVAs were performed to examine group differences. Post
hoc tests were performed with the Tukey-Kramer honestly significant adjust-
ment. If assumptions were not met, comparisons between groups were per-
formed with the Welch adaptation to the ANOVA test. Pairwise correlations
were performed with the Spearman rcorrelation.
To identify patterns of distribution and reduce the dimensionality of the
data, while retaining as much of the variance as possible, a principal component
analysis (PCA) of the individual moieties of DAGs (13 subspecies) and ceramides
(8 subspecies) was performed. PCA was used as exploratory data analysis to
convert a set of possibly correlated variables into a set of data of uncorrelated
variables called principal components or factors, thus revealing the internal
structure of the data in a way that best explains the variance of the data. After
inspecting the correlation matrix, factors were identified on the basis of the
strength ofthe loading; thus the first factor had as high a variance as possible,and
each succeeding factor had the highest variance possible under the constraint
that it be uncorrelated withthe preceding factor.For each factor, Cronbachawas
computed to assess the degree of homogeneity among the indicators. To assess
the physiological relevance of PCA findings, we combined the indictors loading
on each PCA factor by summing the different species scores within each factor
and then performed one-way ANOVAs of these combined scores to compare
groups. The alevel was set a priori to 0.05. Statistical analyses were performed
using JMP 5.0.1.2 and SPSS version 16.0 for the Macintosh.
RESULTS
Subject characteristics, body composition, and physical
fitness. Subject characteristics are presented in Table 1.
OBS subjects had higher BMI and more body fat than
both the NWA and the NWS subjects. VO
2
peak was .50%
higher in NWA subjects compared with both NWS and
OBS subjects.
F. AMATI AND ASSOCIATES
diabetes.diabetesjournals.org DIABETES, VOL. 60, OCTOBER 2011 2589
Insulin sensitivity and metabolic variables. Peripheral
insulin sensitivity was higher in NWA subjects compared
with NWS subjects, who in turn were more insulin-sensitive
than the OBS subjects (Table 2). These differences were
explained by differences in nonoxidative glucose disposal
(P,0.01) but not in oxidative glucose disposal (P=
0.25). These differences remained after adjusting for the
higher VAT in the obese subjects. Hepatic insulin sensi-
tivitywaslowerinOBScomparedwithNWAandNWS
subjects.
OBS subjects had higher fasting and 2-h blood glucose
levels, along with higher fasting insulin levels, reflecting
their overall worse metabolic profile (Table 2). Total and
LDL cholesterols were similar across groups, although it is
important to note that the proportion of subjects who were
taking lipid-lowering medication was significantly different
across the three groups as follows: 0% in the NWA group,
29% in the NWS group, and 38% in the OBS group. HDL
cholesterol and plasma triglycerides, however, were both
more favorable in NWA and NWS subjects.
IMTG and lipid droplet volume density. Total-muscle
IMTG content, measured by ORO staining and electron
microscopy, was higher in NWA and OBS subjects com-
pared with NWS subjects (P= 0.024) (Fig. 1). These dif-
ferences were consistent for type I and type IIa myofibers.
Neither IMTG measured by the ORO staining nor lipid
droplet density was correlated with insulin sensitivity (r=
20.008 and r=20.25, respectively, both P.0.05).
Skeletal muscle DAGs. Total DAG content was ~50%
higher (P,0.01) in the skeletal muscle of NWA compared
with NWS subjects and approximately twofold higher in
NWA compared with OBS subjects (Fig. 2). Similar dif-
ferences were observed for saturated DAGs (P,0.01) and
for DAG species in which one of the fatty acids was un-
saturated (P,0.01). In contrast, OBS subjects had ele-
vated DAG species that contained an unsaturated fatty
acid on both positions compared with both NWS and NWA
subjects (P,0.01). These disparate results also are evi-
dent across the individual DAG species shown in Fig. 2.
PCA identified three factors that combined to explain
89.1% of the total variance in DAGs (Table 3). When the
species belonging to factor 1 were summed and compared
across groups, NWA subjects had a higher (P,0.01)
content (1,774 6118 pmol/mg protein) than both NWS
(1,203 6167) and OBS (807.9 6112.4) subjects. When the
species of factor 2 were summed, OBS subjects had a
higher (P= 0.046) content (17.8 62.3) than NWS (7.6 63.4)
subjects; NWA subjects were not different (12.2 62.4) from
the OBS subjects or from NWS subjects. The sum of the
components of factor 3 was higher in OBS subjects than in
TABLE 1
Subjects’characteristics, body composition, and physical fitness
Athletes Normal-weight subjects Obese subjects
n14 7 21
Sex (male/female) 10/4 3/4 11/10
Age (years) 65.1 61.3 66.5 61.8 66.8 61.0
Weight (kg) 68.1 62.4
B
70.4 63.4
B
93.8 62.0
A
BMI (kg/m
2
) 23.7 60.6
B
24.5 60.9
B
32.9 60.5
A
Waist circumference (cm) 79.5 62.3
B
85.5 63.9
B
109.7 62.2
A
LBM (kg) 51.5 62.5 43.4 63.5 51.1 62.0
Fat mass (kg) 13.9 61.9
A
23.7 62.6
B
38.8 61.5
C
Body fat (%) 20.3 62.4
B
34.6 63.3
A
41.9 61.9
A
TAT (cm
2
) 223.6 632.4
B
359.5 652.3
B
592.3 626.8
A
VAT (cm
2
) 68.3 615.0
B
105.3 624.2
B
204.8 612.4
A
SAT (cm
2
) 155.3 626.9
B
254.2 643.4
B
374.8 622.3
A
VO
2
peak (L/min) 2.81 60.17
A
1.84 62.36
B
1.71 61.36
B
VO
2
peak/LBM (mL/min/kg) 53.8 62.0
A
41.5 62.8
B
33.3 61.6
C
Data are means 6SEM.
A,B,C
Significant differences between groups (one-way ANOVA).
TABLE 2
Metabolic variables
Athletes Normal-weight subjects Obese subjects
n14 7 21
R
d
(mg/min/kg LBM) 12.31 60.55
A
9.74 60.77
B
6.29 60.45
C
HGP (%) 99.99 63.49
A
99.61 64.75
A
85.87 62.74
B
Fasting blood glucose (mmol/L) 4.84 60.17
B
4.56 60.25
B
5.31 60.12
A
HbA
1c
(%) 5.40 60.12
B
5.61 60.18
AB
5.84 60.10
A
Fasting insulin (pmol/L) 17.91 64.70
B
18.68 66.42
B
45.08 63.70
A
Glucose 2-h OGTT (mmol/L) 6.35 60.44
B
6.75 60.64
B
8.26 60.37
A
Total cholesterol (mmol/L) 4.82 60.18 4.67 60.26 4.96 60.15
HDL (mmol/L) 1.67 60.09
A
1.43 60.12
AB
1.33 60.07
B
LDL (mmol/L) 2.83 60.15 2.86 60.21 3.04 60.12
VLDL (mmol/L) 0.33 60.04
B
0.37 60.06
B
0.59 60.04
A
Triglycerides (mmol/L) 0.88 60.12
B
1.03 60.17
B
1.62 60.10
A
Data are means 6SEM. HGP, hepatic insulin sensitivity expressed as the suppression of hepatic glucose production. R
d
, whole-body insulin
sensitivity expressed as the rate of insulin-simulated glucose disposal.
A,B,C
Significant differences between groups (one-way ANOVA).
INTRAMYOCELLULAR LIPIDS AND INSULIN RESISTANCE
2590 DIABETES, VOL. 60, OCTOBER 2011 diabetes.diabetesjournals.org
the NWS and NWA subjects (82.1 615.6, 19.1 623.6, and
32.1 616.7, respectively, P= 0.036). Total DAGs was
positively correlated with insulin sensitivity (r= 0.57, P,
0.05). This positive association was similar for the sum of
the species of factor 1 (r= 0.57, P,0.05). Factors 2 and 3,
however, were not correlated with insulin sensitivity (r=
20.21 and r=20.17, respectively, P.0.05).
Skeletal muscle ceramides and other sphingolipids.
Total ceramide content was higher (P,0.01) in the
skeletal muscle of OBS subjects compared with both NWS
and NWA subjects (Fig. 3). These differences were consis-
tent across both saturated (P= 0.03) and unsaturated (P,
0.01) ceramide species, as well as fatty acid chain length.
Sphingosine-1-phosphate (S1P) also was higher in obesity
(Fig. 3). In contrast, sphingosine was significantly lower in
OBS compared with both NWA and NWS subjects.
PCA identified four significant factors, explaining 87.2%
of the total variance in ceramides (Table 3). When the
molecular species belonging to factor 1 were summed and
compared across groups, OBS subjects were found to have
a higher (P= 0.02) content (159.6 618.4 pmol/mg protein)
than NWS (80.2 627.1) and NWA (82.8 621.0) subjects.
The same pattern was found for factor 2, with a higher
content in OBS (13.7 61.7) than NWS (7.1 62.4) and
NWA (6.2 61.9, P= 0.03) subjects. The component of
factor 3 revealed no differences (P= 0.28) across the
groups, whereas component factor 4 was significantly (P=
0.04) higher in NWA (1.99 60.27) and NWS (1.87 60.35)
compared with OBS (1.10 60.24) subjects. Total ceramide
content was negatively correlated with insulin sensitivity
(r=20.48, P,0.05). Similar associations were observed
for saturated ceramide and unsaturated ceramide (r= 0.44
and r= 0.50, respectively, P,0.05). When the sum of the
species belonging to the PCA factors were correlated to
insulin sensitivity, factors 1 and 2 were significantly neg-
atively correlated with insulin sensitivity (r=20.46 and
r=20.47, respectively, P,0.05), whereas factor 4 was
positively correlated (r= 0.38, P,0.05) and factor 3 was
not significantly correlated (r=20.08, P.0.05) with in-
sulin sensitivity.
Mitochondria, oxidative capacity, and proportion of
skeletal muscle fiber types. Mitochondria volume den-
sity was significantly higher in NWA than in the two sed-
entary groups (P,0.001) (Fig. 4A). SDH content was
higher in NWA than NWS subjects, who in turn had higher
SDH than OBS subjects (P,0.001) (Fig. 4B). Mitochon-
dria volume density and SDH content were positively
correlated with insulin sensitivity (r= 0.51 and r= 0.67,
respectively, all P,0.05).
NWA subjects had a higher (P,0.01) proportion of type 1
myofibers (70.2 64.2%) compared with NWS (34.4 64.0%)
and OBS (43.7 62.1%) subjects and a correspondingly
lower proportion of type 2 fibers. The proportion of type I
fibers was positively correlated with insulin sensitivity (r=
0.56, P.0.05), whereas the proportion of type IIa and IIx
fibers was negatively correlated with insulin sensitivity (r=
20.39 and r=20.59, respectively, P,0.05).
Lipid droplet protein content. NWA subjects had sig-
nificantly more OXPAT/perilipin-5, SCD1, and ATGL than
the two sedentary groups (all P,0.05) (Fig. 5). No dif-
ferences between groups were observed for DGAT1. There
was notably more variability in OXPAT/perilipin-5 ex-
pression, as indicated by the larger error bars, although
this greater variation is not adequately depicted in the
representative bands for this protein. This was likely a re-
sult of the greater biological variability. OXPAT/perilipin-5
content was positively correlated with insulin sensitivity
(r= 0.52, P,0.01) and lipid droplet volume density (r=
0.45, P,0.05). SCD1 was positively correlated with in-
sulin sensitivity (r= 0.58, P,0.01) and with lipid droplet
volume density (r= 0.33, P,0.01). ATGL was positively
correlated with insulin sensitivity (r=20.48, P,0.01) but
not with lipid droplet density (r= 0.28, P= 0.25). DGAT1
was not significantly correlated with BMI, insulin sensi-
tivity, or lipid droplet density.
DISCUSSION
A primary finding of our study was that DAG content
was approximately twofold higher within the highly
FIG. 1. IMCLs and lipid droplet protein content in vastus lateralis.
IMTG measured by the ORO stain (A) and lipid droplet volume density
measured by electron microscopy (B). Bars are mean values and error
bars are SEMs. The letters A and B above the bars denote significant
differences between groups (P<0.05, one-way ANOVA).
F. AMATI AND ASSOCIATES
diabetes.diabetesjournals.org DIABETES, VOL. 60, OCTOBER 2011 2591
insulin-sensitive skeletal muscle compared with obese skel-
etal muscle. This clearly does not support a common con-
temporary view, largely based on evidence from animal
models, that muscular DAGs explain IR of obesity, aging,
and type 2 diabetes (23). To the contrary, our data suggest
that higher DAG content in chronically exercised, insulin-
sensitive muscle represents another athletes paradox (i.e.,
total cellular DAG content is associated with better insulin
sensitivity). These data are particularly significant given
that few studies have examined skeletal muscle DAG con-
tent in association with IR, and still fewer studies have been
performed in human muscle (10,11,13,24). Our data also
are in accord with recent findings that DAGs are not ele-
vated in insulin-resistant muscle after controlling for obe-
sity and physical fitness (24) and higher levels of DAGs in
lean subjects compared with obese volunteers (25), as well
as with an animal study indicating that increased DAG
content in muscle and IR are not necessarily related (26).
FIG. 2. DAGs in vastus lateralis. Total DAG content (A), saturated species (B), unsaturated species (C), and individual species (D). Bars are mean
values, and error bars are SEMs. The letters A, B, and C above the bars denote significant differences between groups (P<0.05, one-way ANOVA).
INTRAMYOCELLULAR LIPIDS AND INSULIN RESISTANCE
2592 DIABETES, VOL. 60, OCTOBER 2011 diabetes.diabetesjournals.org
Our results also differ somewhat from our previous report
demonstrating that moderate exercise reduced muscle DAG
content in previously sedentary obese subjects (12,27). It is
possible that these discrepancies are a result of the amount
and/or intensity of chronic exercise and the model system
used (acute lipid oversupply) or a result of subcellular DAG
location (i.e., membrane-bound or part of neutral lipid
droplets). It also is possible that particular DAG species
may differentially affect insulin action.
Ours is the first study to provide a comprehensive profile
of the distinct molecular species of DAG within muscle of
obese, normal-weight, and athletic human subjects, sug-
gesting that particular DAG species may be associated
with IR in muscle, whereas others clearly are not. This did
not seem to be related to the fatty acid chain length of the
DAG or the total degree of fatty acid saturation because
we found that exercised muscle had higher levels of both
saturated and unsaturated DAG species. It is difficult to
reconcile our results with those in a recent study by
Bergman et al. (28), who found that the degree of satura-
tion of DAGs within muscle in athletes was lower com-
pared with sedentary subjects and was related to higher
insulin sensitivity. One possible explanation is that the
younger athletes in their study had a different diet com-
position. In addition, they measured the total fatty acid
content within the DAG pool, whereas we quantified spe-
cific DAG species. Our methods revealed that DAG species
containing one unsaturated and one saturated fatty acid
are lower in obese muscle but that DAG species containing
unsaturated fatty acids on both positions was higher in
obese muscle. The PCA supports this and suggests that it is
not the overall concentration of DAGs that is deleterious
but that particular DAG moieties may be lipotoxic even in
relatively smaller amounts. Additional studies are needed
to determine whether specific fatty acids stimulate DAG
synthesis and whether this is linked to IR.
Another key finding of our study was that ceramide
content was twofold higher in the obese muscle compared
with normal-weight sedentary and exercise-trained mus-
cles. These results are supported by previous studies in
human subjects reporting higher total ceramide levels in
obese muscle (8) and associations with IR and a lower
oxidative capacity (24,29). This also is in accord with our
previous report that moderate exercise training decreased
intramuscular ceramide levels in previously sedentary obese
subjects (12). Likewise, Bruce et al. (14) found that exer-
cise training tended to reduce both the total ceramide
content and the saturated ceramide species in obese insulin-
resistant subjects without significant weight loss. In agree-
ment with Helge et al. (30), we did not find significant
differences between the athletes and normal-weight sed-
entary subjects with respect to total muscle content of
ceramides, which could be simply attributed to the levels
of ceramide required for normal physiological function.
Our results also are consistent with animal models and cell
systems linking ceramide content in muscle to impaired
insulin signaling and IR (31–33). Higher ceramide levels in
obesity and in association with IR were consistent for both
saturated and unsaturated ceramides, which is consistent
with our recent report (24). The PCA for ceramides sug-
gests, however, that a specific group of ceramides may be
related to better insulin sensitivity. Because ceramides have
several functions in normal biology, including comprising
mitochondria membranes (34), mechanistic investigations
are needed to examine the specific role of these ceramide
species on insulin action.
To our knowledge, this is the first observation of higher
S1P in obese, insulin-resistant muscle. Among the bio-
active sphingolipids, ceramide, sphingosine, and S1P have
been proposed to exert differential effects on cells (35).
Ceramide is converted to sphingosine by the action of
ceramidases, and, subsequently, sphingosine is phosphor-
ylated to generate S1P (29,36). Ceramides and sphingosine
inhibit growth and are involved in apoptosis (37–39), in
contrast to the effects of S1P, which are to promote cellular
proliferation, survival, and inhibition of apoptosis (35).
Furthermore, S1P is implicated in the clearance of ceramide
(40), is a potential regulator of de novo ceramide synthesis
(41,42), and has been shown to inhibit Jun NH
2
-terminal
kinase activation (43). Samad et al. (44) reported a similar
observation in ob/ob mice, which had higher plasma levels
of total ceramide, sphingosine, and S1P compared with their
lean counterparts. Additional inquiry is needed to deter-
mine whether these specific sphingolipids actually affect
insulin action within muscle.
We also placed our novel lipid data within the context
of specific proteins associated with lipid droplet esterifi-
cation, lipolysis, and oxidation. First, the lipid droplet pro-
tein OXPAT/perilipin-5, reported to be highly expressed in
highly oxidative tissues (16,45,46), was more than seven-
fold more abundant in the skeletal muscle of the endurance-
trained athletes, corresponding significantly with their higher
lipid droplet volume and their higher insulin sensitivity.
One previous study in humans demonstrated no correlation
between OXPAT/perilipin-5 protein content in the skele-
tal muscle and insulin sensitivity in obese type 2 diabetic
subjects and BMI-matched control subjects (17). Our data
support a role for OXPAT/perilipin-5 in both higher lipid
droplet content and fatty acid oxidation, and, although our
observed associations with insulin sensitivity are intriguing,
additional studies are needed to determine whether it
directly mediates insulin action. Second, we investigated
two key lipogenic enzymes, SCD1 and DGAT1. SCD1 is
a rate-limiting enzyme that converts saturated fatty acids
to monounsaturated fatty acids and, in animal and cell
culture models, has been previously implicated in the
TABLE 3
Exploratory PCAs
Exploratory PCA on DAG species
Factor 1, eigenvalue 5.7 (43.6%)
Di-C16:0, C18:0/18:1, C16:0/18.1, C16:1/18:0, C16:0/18:0,
Di-C18:0, and Di-C18:1
Factor 2, eigenvalue 3.7 (28.4%)
Di-C16:1, Di-C14:0, and C14:0/18:0
Factor 3, eigenvalue 2.2 (17.1%)
C14:0/18:1, C14:0/16:0, and C16:1/18:1
Exploratory PCA on ceramides species
Factor 1, eigenvalue 3.3 (41.3%)
Lingoceric (C24:0), nervonic (C24:1), stearic (C18:0),
and arachidic (C20:0)
Factor 2, eigenvalue 1.3 (16.6%)
Dihydroceramide 16 and palmitoleic (C18:1)
Factor 3, eigenvalue 1.2 (15.1%)
Palmitic (C16:0)
Factor 4, eigenvalue 1.1 (14.3%)
Myristic (C14:0)
% refers to the percentage of variance of the data explained by each
factor.
F. AMATI AND ASSOCIATES
diabetes.diabetesjournals.org DIABETES, VOL. 60, OCTOBER 2011 2593
development of dysregulated fatty acid metabolism, in-
creased triglyceride storage, and obesity (47,48). In con-
trast, although confirming that it tracks with higher muscle
triglycerides, our data in human muscle clearly demonstrate
that SCD1 is higher in metabolically healthy, more insulin-
sensitive muscle. This is in agreement with observations
of increases in SCD1 content after acute (49) and chronic
aerobic exercise training (27). The lack of differences in
DGAT between the groups is in accord with our previous
report (27). Third, we report elevated levels of ATGL
in endurance-trained athletes compared with sedentary
subjects. These data are supported by previous exercise-
training studies (50,51). ATGL is important as the first
step in hydrolyzing triglycerides to DAG on its way to fur-
ther hydrolysis and ultimate energy generation, which could
explain the higher DAG levels in the muscle of the athletes.
Our interpretation of this finding is that these alterations in
esterification and lipolysis within exercise-trained muscle
FIG. 3. Ceramides and other sphingolipids in vastus lateralis. Total, saturated, and unsaturated ceramide content (A), sphingosine and S1P (B),
and individual species of ceramides (C). Bars are mean rates, and error bars are SEMs. The letters A and B above the bars denote significant
differences between groups (P<0.05, one-way ANOVA).
INTRAMYOCELLULAR LIPIDS AND INSULIN RESISTANCE
2594 DIABETES, VOL. 60, OCTOBER 2011 diabetes.diabetesjournals.org
lead to an elevated pool of neutral DAGs, similar to that
of triglycerides that can be used as a fuel source during
physical activity. Taken together, these data suggest an
enzymatic profile of enhanced neutral lipid storage and
oxidation associated with enhanced insulin sensitivity with
exercise training.
These human studies cannot directly determine mech-
anisms or causes of IR. Although we did not determine
whether IMCLs are associated with specific defects in
insulin signaling, we clearly show that insulin-stimulated
glucose disposal in skeletal muscle differed according to
study groups. Additional studies are needed to examine
whether the specific lipid species or their cellular location,
in particular DAG within muscle, are mechanistically linked
with impaired insulin signaling. It will also be important to
determine whether the fatty acid composition of the diet
affects insulin action within muscle because dietary patterns
have been shown to influence lipid metabolite content and
saturation (3). We did not measure other intracellular lipids
that have been implicated in muscle IR, such as long-chain
fatty acyl-CoA (52,53) because of the limited availability of
tissue within the relatively small biopsy samples. Another
important element to consider is that the amount and
intensity of physical activity performed by these highly
trained athletes do not allow us to extrapolate these
findings in athletes to more moderate physical activity as
emphasized for general health. Thus, the possibility remains
that there is some threshold level or dose-response effect of
physical activity on insulin sensitivity and these lipids.
In summary, high-level exercise training was unex-
pectedly associated with considerably higher total DAG
content within skeletal muscle concomitant with higher
insulin sensitivity, thus extending the athletes paradox to
muscle DAGs and to triglycerides across fiber types.
Therefore, the total cellular DAG content cannot explain
skeletal muscle IR. This suggests instead a role for specific
molecular species of DAGs and possibly their subcellular
location in this metabolic abnormality. Ceramides and S1P,
on the other hand, are consistently elevated in obese and
insulin-resistant muscle. Moreover, formation and metabolism
FIG. 4. Mitochondria and markers of oxidative capacity in vastus lateralis. Mitochondria volume density (A), example of micrographs (B), and SDH
content (C). Bars are mean values, and error bars are SEMs. The letters A, B, and C above the bars denote significant differences between groups
(P<0.05, one-way ANOVA).
F. AMATI AND ASSOCIATES
diabetes.diabetesjournals.org DIABETES, VOL. 60, OCTOBER 2011 2595
of these complex lipids were associated with key pro-
teins expressed in human muscle, which in turn corre-
sponded significantly with IR. These novel translational
data form a basis for more mechanistic studies to be per-
formed in model systems to determine exact causes of lipid-
induced IR.
ACKNOWLEDGMENTS
This study was supported by National Institutes of Health
(NIH) Grants (R01-AG20128 to B.H.G. and R01-DK068046
to P.E.B.), funding from the American College of Sports
Medicine Foundation (to F.A.), University of Pittsburgh
Student and Faculty Grants (to B.H.G. and F.A.), an NIH/
National Center for Research Resources/Clinical and
Translational Science Award (UL1 RR024153), and Univer-
sity of Pittsburgh Obesity and Nutrition Research Center
Grant 1P30DK46204.
No potential conflicts of interest relevant to this article
were reported.
F.A. researched data; contributed to the study concept,
design, analysis, and interpretation of the data; and wrote
the manuscript. J.J.D. researched data and reviewed and
edited the manuscript. E.A.-C., M.M.E., P.C., and M.S.-R. re-
searched data. P.M.C. reviewed and edited the manuscript.
G.E.S. contributed to the statistical analyses and reviewed
and edited the manuscript. P.E.B. contributed to the discus-
sion and reviewed and edited the manuscript. F.G.S.T.
researched data and contributed to the discussion. B.H.G.
contributed to the study concept and design and the analysis
and interpretation of the data and wrote the manuscript.
Parts of this article were presented in abstract form
at the 27th Annual ScientificMeetingofTheObesity
Society, Washington, District of Columbia, 24–28 October
2009.
We appreciate the cooperation of our research volun-
teers, along with the skills of Steve Anthony and the nursing
staff of the Clinical and Translational Research Center of
the University of Pittsburgh.
REFERENCES
1. Saltiel AR. Series introduction: the molecular and physiological basis of
insulin resistance: emerging implications for metabolic and cardiovascular
diseases. J Clin Invest 2000;106:163–164
2. Kelley DE, Goodpaster BH. Skeletal muscle triglyceride: an aspect of re-
gional adiposity and insulin resistance. Diabetes Care 2001;24:933–941
3. Lee JS, Pinnamaneni SK, Eo SJ, et al. Saturated, but not n-6 polyunsaturated,
fatty acids induce insulin resistance: role of intramuscular accumulation of
lipid metabolites. J Appl Physiol 2006;100:1467–1474
4. Pan DA, Lillioja S, Kriketos AD, et al. Skeletal muscle triglyceride levels
are inversely related to insulin action. Diabetes 1997;46:983–988
5. Goodpaster BH, He J, Watkins S, Kelley DE. Skeletal muscle lipid content
and insulin resistance: evidence for a paradox in endurance-trained ath-
letes. J Clin Endocrinol Metab 2001;86:5755–5761
6. Russell AP, Gastaldi G, Bobbioni-Harsch E, et al. Lipid peroxidation in
skeletal muscle of obese as compared to endurance-trained humans:
a case of good vs. bad lipids? FEBS Lett 2003;551:104–106
7. Itani SI, Ruderman NB, Schmieder F, Boden G. Lipid-induced insulin re-
sistance in human muscle is associated with changes in diacylglycerol,
protein kinase C, and IkB-a. Diabetes 2002;51:2005–2011
8. Adams JM 2nd, Pratipanawatr T, Berria R, et al. Ceramide content is in-
creased in skeletal muscle from obese insulin-resistant humans. Diabetes
2004;53:25–31
9. Skovbro M, Baranowski M, Skov-Jensen C, et al. Human skeletal muscle
ceramide content is not a major factor in muscle insulin sensitivity. Dia-
betologia 2008;51:1253–1260
10. Perreault L, Bergman BC, Hunerdosse DM, Eckel RH. Altered intramuscular
lipid metabolism relates to diminished insulin action in men, but not women,
in progression to diabetes. Obesity (Silver Spring) 2010;18:2093–2100
11. Pruchnic R, Katsiaras A, He J, Kelley DE, Winters C, Goodpaster BH.
Exercise training increases intramyocellular lipid and oxidative capacity in
older adults. Am J Physiol Endocrinol Metab 2004;287:E857–E862
12. Dubé JJ, Amati F, Stefanovic-Racic M, Toledo FG, Sauers SE, Goodpaster
BH. Exercise-induced alterations in intramyocellular lipids and insulin
resistance: the athlete’s paradox revisited. Am J Physiol Endocrinol Metab
2008;294:E882–E888
13. Bergman BC, Perreault L, Hunerdosse DM, Koehler MC, Samek AM, Eckel
RH. Intramuscular lipid metabolism in the insulin resistance of smoking.
Diabetes 2009;58:2220–2227
14. Bruce CR, Thrush AB, Mertz VA, et al. Endurance training in obese humans
improves glucose tolerance and mitochondrial fatty acid oxidation and alters
muscle lipid content. Am J Physiol Endocrinol Metab 2006;291:E99–E107
15. Zhang D, Christianson J, Liu ZX, et al. Resistance to high-fat diet-induced
obesity and insulin resistance in mice with very long-chain acyl-CoA de-
hydrogenase deficiency. Cell Metab 2010;11:402–411
FIG. 5. Lipid droplet protein content in vastus lateralis. Perilipin-5 (A), SCD1 (B), ATGL (C), and DGAT1 (D) protein contents measured by
Western blotting. Bars are mean values, and error bars are SEMs. The letters A, B, and C above the bars denote significant differences between
groups (P<0.05, one-way ANOVA). The bands depicted in the Western blots are from one subject of each group.
INTRAMYOCELLULAR LIPIDS AND INSULIN RESISTANCE
2596 DIABETES, VOL. 60, OCTOBER 2011 diabetes.diabetesjournals.org
16. Wolins NE, Quaynor BK, Skinner JR, et al. OXPAT/PAT-1 is a PPAR-
induced lipid droplet protein that promotes fatty acid utilization. Diabetes
2006;55:3418–3428
17. Minnaard R, Schrauwen P, Schaart G, et al. Adipocyte differentiation-
related protein and OXPAT in rat and human skeletal muscle: involvement
in lipid accumulation and type 2 diabetes mellitus. J Clin Endocrinol Metab
2009;94:4077–4085
18. Pham DL, Xu C, Prince JL. Current methods in medical image segmenta-
tion. Annu Rev Biomed Eng 2000;2:315–337
19. Goodpaster BH, Theriault R, Watkins SC, Kelley DE. Intramuscular lipid
content is increased in obesity and decreased by weight loss. Metabolism
2000;49:467–472
20. Bielawski J, Szulc ZM, Hannun YA, Bielawska A. Simultaneous quanti-
tative analysis of bioactive sphingolipids by high-performance liquid
chromatography-tandem mass spectrometry. Methods 2006;39:82–91
21. Shah C, Yang G, Lee I, Bielawski J, Hannun YA, Samad F. Protection from
high fat diet-induced increase in ceramide in mice lacking plasminogen
activator inhibitor 1. J Biol Chem 2008;283:13538–13548
22. Toledo FG, Menshikova EV, Azuma K, et al. Mitochondrial capacity in skel-
etal muscle is not stimulated by weight loss despite increases in insulin action
and decreases in intramyocellular lipid content. Diabetes 2008;57:987–994
23. Erion DM, Shulman GI. Diacylglycerol-mediated insulin resistance. Nat
Med 2010;16:400–402
24. Coen PM, Dubé JJ, Amati F, et al. Insulin resistance is associated with
higher intramyocellular triglycerides in type I but not type II myocytes
concomitant with higher ceramide content. Diabetes 2010;59:80–88
25. Jocken JW, Moro C, Goossens GH, et al. Skeletal muscle lipase content
and activity in obesity and type 2 diabetes. J Clin Endocrinol Metab 2010;
95:5449–5453
26. Levin MC, Monetti M, Watt MJ, et al. Increased lipid accumulation and in-
sulin resistance in transgenic mice expressing DGAT2 in glycolytic (type II)
muscle. Am J Physiol Endocrinol Metab 2007;293:E1772–E1781
27. Dubé JJ, Amati F, Toledo FG, et al. Effects of weight loss and exercise on
insulin resistance, and intramyocellular triacylglycerol, diacylglycerol and
ceramide. Diabetologia 2011;54:1147–1156
28. Bergman BC, Perreault L, Hunerdosse DM, Koehler MC, Samek AM, Eckel RH.
Increased intramuscular lipid synthesis and low saturation relate to insulin
sensitivity in endurance-trained athletes. J Appl Physiol 2010;108:1134–1141
29. Straczkowski M, Kowalska I, Baranowski M, et al. Increased skeletal
muscle ceramide level in men at risk of developing type 2 diabetes. Dia-
betologia 2007;50:2366–2373
30. Helge JW, Dobrzyn A, Saltin B, Gorski J. Exercise and training effects on
ceramide metabolism in human skeletal muscle. Exp Physiol 2004;89:119–127
31. Consitt LA, Bell JA, Houmard JA. Intramuscular lipid metabolism, insulin
action, and obesity. IUBMB Life 2009;61:47–55
32. Holland WL, Scherer PE. PAQRs: a counteracting force to ceramides? Mol
Pharmacol 2009;75:740–743
33. Ussher JR, Koves TR, Cadete VJ, et al. Inhibition of de novo ceramide
synthesis reverses diet-induced insulin resistance and enhances whole
body oxygen consumption. Diabetes 2010;59:2453–2464
34. Lanza IR, Nair KS. Mitochondrial function as a determinant of life span.
Pflugers Arch 2010;459:277–289
35. Spiegel S, Milstien S. Sphingosine-1-phosphate: an enigmatic signalling
lipid. Nat Rev Mol Cell Biol 2003;4:397–407
36. Straczkowski M, Kowalska I. The role of skeletal muscle sphingolipids in
the development of insulin resistance. Rev Diabet Stud 2008;5:13–24
37. Hannun YA. Sphingolipid second messengers: tumor suppressor lipids.
Adv Exp Med Biol 1997;400A:305–312
38. Cuvillier O. Sphingosine in apoptosis signaling. Biochim Biophys Acta
2002;1585:153–162
39. Cuvillier O, Pirianov G, Kleuser B, et al. Suppression of ceramide-mediated
programmed cell death by sphingosine-1-phosphate. Nature 1996;381:800–803
40. van Echten-Deckert G, Klein A, Linke T, Heinemann T, Weisgerber J,
Sandhoff K. Turnover of endogenous ceramide in cultured normal and
Farber fibroblasts. J Lipid Res 1997;38:2569–2579
41. Le Stunff H, Galve-Roperh I, Peterson C, Milstien S, Spiegel S. Sphingosine-
1-phosphate phosphohydrolase in regulation of sphingolipid metabolism
and apoptosis. J Cell Biol 2002;158:1039–1049
42. Taha TA, Kitatani K, El-Alwani M, Bielawski J, Hannun YA, Obeid LM. Loss
of sphingosine kinase-1 activates the intrinsic pathway of programmed cell
death: modulation of sphingolipid levels and the induction of apoptosis.
FASEB J 2006;20:482–484
43. Edsall LC, Cuvillier O, Twitty S, Spiegel S, Milstien S. Sphingosine kinase
expression regulates apoptosis and caspase activation in PC12 cells.
J Neurochem 2001;76:1573–1584
44. Samad F, Hester KD, Yang G, Hannun YA, Bielawski J. Altered adipose and
plasma sphingolipid metabolism in obesity: a potential mechanism for
cardiovascular and metabolic risk. Diabetes 2006;55:2579–2587
45. Yamaguchi T, Matsushita S, Motojima K, Hirose F, Osumi T. MLDP, a novel
PAT family protein localized to lipid droplets and enriched in the heart,
is regulated by peroxisome proliferator-activated receptor alpha. J Biol
Chem 2006;281:14232–14240
46. Dalen KT, Dahl T, Holter E, et al. LSDP5 is a PAT protein specifically expressed
in fatty acid oxidizing tissues. Biochim Biophys Acta 2007;1771:210–227
47. Biddinger SB, Almind K, Miyazaki M, Kokkotou E, Ntambi JM, Kahn CR.
Effects of diet and genetic background on sterol regulatory element-
binding protein-1c, stearoyl-CoA desaturase 1, and the development of the
metabolic syndrome. Diabetes 2005;54:1314–1323
48. Hulver MW, Berggren JR, Carper MJ, et al. Elevated stearoyl-CoA desaturase-1
expression in skeletal muscle contributes to abnormal fatty acid partitioning
in obese humans. Cell Metab 2005;2:251–261
49. Schenk S, Horowitz JF. Acute exercise increases triglyceride synthesis in
skeletal muscle and prevents fatty acid-induced insulin resistance. J Clin
Invest 2007;117:1690–1698
50. Yao-Borengasser A, Varma V, Coker RH, et al. Adipose triglyceride lipase
expression in human adipose tissue and muscle: role in insulin resistance
and response to training and pioglitazone. Metabolism 2010;60:1012–
1020
51. Alsted TJ, Nybo L, Schweiger M, et al. Adipose triglyceride lipase in human
skeletal muscle is upregulated by exercise training. Am J Physiol Endo-
crinol Metab 2009;296:E445–E453
52. Ellis BA, Poynten A, Lowy AJ, et al. Long-chain acyl-CoA esters as in-
dicators of lipid metabolism and insulin sensitivity in rat and human
muscle. Am J Physiol Endocrinol Metab 2000;279:E554–E560
53. Bajaj M, Suraamornkul S, Romanelli A, et al. Effect of a sustained reduction
in plasma free fatty acid concentration on intramuscular long-chain fatty
acyl-CoAs and insulin action in type 2 diabetic patients. Diabetes 2005;54:
3148–3153
F. AMATI AND ASSOCIATES
diabetes.diabetesjournals.org DIABETES, VOL. 60, OCTOBER 2011 2597
