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J. Biol. Chem., Vol. 278, Issue 33, 31385-31390, August 15, 2003
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Coactivator-1
Down-regulates GLUT4 mRNA in Skeletal Muscles*

From the Division of Clinical Nutrition, National Institute of Health and Nutrition, 1-23-1 Toyama, Shinjuku-ku, Tokyo 162-8636, Japan
Received for publication, April 24, 2003
| ABSTRACT |
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coactivator
1
(PGC-1
) by exercise may promote mitochondrial biogenesis and
fatty acid oxidation. To examine whether increased PGC-1
expression was
also responsible for an increase of GLUT4 expression, transgenic mice that
overexpress PGC-1
in skeletal muscles driven by a human
-skeletal actin promoter were made. PGC-1
was overexpresssed in
skeletal muscles including type I and II fiber-rich muscles but not in the
heart. With an increase of PGC-1
mRNA, type II fiber-rich muscles were
redder, and genes of mitochondrial oxidative metabolism were up-regulated in
skeletal muscles, whereas the expression of GLUT4 mRNA was unexpectedly
down-regulated. In parallel with a decrease of GLUT4 mRNA, an impairment of
glycemic control after intraperitoneal insulin administration was observed.
Thus, an increase of PGC-1
plays a role in increasing mitochondrial
biogenesis and fatty acid oxidation but not in increasing GLUT4 mRNA in
skeletal muscles. | INTRODUCTION |
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As for mitochondrial biogenesis, two transcription factors, nuclear
respiratory factor
(NRF)-11 and NRF-2,
were the key transcriptional activators of nuclear genes encoding
mitochondrial enzymes (6,
7). NRFs up-regulate
mitochondrial transcription factor A, which stimulates mitochondrial DNA
transcription and replication
(8). Recently, as a regulator
of NRFs, peroxisome proliferator-activated receptor
coactivator-1
(PGC-1
) was found. PGC-1
is an inducible coactivator of nuclear
receptors cloned from brown fat cell cDNA library because of its interaction
with peroxisome proliferator-activated receptor
(PPAR
)
(9). Overexpression of
PGC-1
increased expression of mitochondrial enzymes in 3T3 adipocytes,
stimulated mitochondrial biogenesis in C2C12 myocytes
(9,
10), and induced the
expression of nuclear and mitochondrial genes involved in multiple
mitochondrial energy production pathways, including PPAR
target genes
encoding the mitochondrial fatty acid
-oxidation in neonatal cardiac
myocytes (11). In
vivo cardiac-specific overexpression of PGC-1
in transgenic mice
resulted in uncontrolled massive proliferation of mitochondria
(11). Also, evidence that
PGC-1
co-activated PPAR
(12) and NRF-1
(10) led to a new hypothesis
that PGC-1
is a key molecule for the entire fatty acid oxidation
system, including intracellular fatty acid transport, fatty acid
-oxidation, tricarboxylic acid cycle, and respiratory chain.
In contrast, although nucleotide sequences of GLUT4 promoter responsible
for exercise training-induced up-regulation of GLUT4 expression have been
elucidated (13,
14), little is known about the
transcription factor(s) by which exercise training stimulates GLUT4 expression
(15). Because overexpression
of PGC-1
in type II (fast-twitch) fibers drives the formation of type I
(slow-twitch) fibers (16) and
type I fibers express more GLUT4 with an increased glucose uptake than type II
fibers (17), PGC-1
may
also up-regulate GLUT4 expression in skeletal muscles directly and indirectly.
Indeed, adenovirus-mediated expression of PGC-1
in cultured muscle cell
lines L6 myotube resulted in a large increase in GLUT4, providing evidence
that PGC-1
can also up-regulate GLUT4 gene expression
(18). However, there has been
some evidence against this hypothesis. Starvation increased the capacity for
mitochondrial energy production via fatty acid
-oxidation with a marked
up-regulation of PGC-1
in the heart
(11), whereas starvation did
not increase GLUT4 in skeletal muscles
(19). A single bout of
exercise increased PGC-1
mRNA and protein
(20), but did not increase
GLUT4 mRNA (21). A significant
GLUT4 expression was observed by prolonged exercise or repeated bouts of
exercise (21). To resolve this
question, we made mice that overexpressed PGC-1
in skeletal muscle and
examined whether PGC-1
increases GLUT4 expression as well as enzymes
related to mitochondrial biogenesis and fatty acid oxidation.
| EXPERIMENTAL PROCEDURES |
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ConstructThe human
-skeletal actin promoter was used to
drive skeletal muscle specific expression of a mouse PGC-1
transgene.
This promoter is well characterized, and the 2.2-kb fragment used (a kind gift
from Drs. E. D. Hardeman and K. Guven at the Children's Medical Research
Institute in Australia) contains all necessary elements for selective
expression in the skeletal muscle but not the heart
(22). Complete cDNA of mouse
PGC-1
was obtained by PCR from first strand cDNA using mouse skeletal
muscle total RNA. First strand cDNA was prepared using an Advantage RT-for-PCR
kit (Clontech) after DNase I-digestion. For PGC-1
(GenBank accession
number AF049330
[GenBank]
), forward and reverse primer sequences were
5'-ATGGCTTGGGACATGTGC-3' and
5'-TTACCTGCGCAAGCTTCTCT-3', respectively. Nucleotide sequences of
PGC-1
cDNA were confirmed by sequencing. The transgene construct
contains from 2000 to +200 of human
-skeletal actin promoter,
2.4 kb of complete mouse PGC-1
cDNA, and polyadenylation signal encoded
by the bovine growth hormone (Fig.
1A).
|
Transgenic MiceThis transgene fragment was used for
microinjection into BDF1 mouse eggs at Japan SLC Inc. (Hamamatsu, Japan). We
examined 40 mouse pups, and six integration-positive mice were obtained. Among
these six lines, three lines, B, D, and E, were studied. The male chimeras
harboring PGC-1
transgene were mated with C57BL/6J female mice to
obtain F1 offspring. The heterozygous F1 male offspring from this breeding
were then backcrossed with purebred C57BL/6J females to obtain F2 offspring;
this process was continued until the F3 generation of mice was obtained. The
mice were maintained at a constant temperature of 22 °C with fixed
artificial light cycle (12-h/12-h light/dark). The male heterozygous
PGC-1
transgenic mice and their littermate wild-type mice were
sacrificed at 1315 weeks of age.
Preparation of cDNA Probe and Northern BlotThe cDNA
fragments for mouse, nuclearly encoded COX subunit IV, medium-chain acyl-CoA
dehydrogenase, and NRF-2 (NRF-2
) were obtained by PCR from first-strand
cDNA using mouse brown adipose tissue total RNA. Complete cDNA of mouse
PGC-1
and myocyte-specific enhancer factor 2C (MEF2C) was obtained by
PCR from first-strand cDNA using mouse skeletal muscle total RNA. First-strand
cDNA was prepared using an Advantage RT-for-PCR kit (Clontech) after DNase I
digestion. The DNA fragment for mouse mitochondrially encoded COX subunit II
was obtained by PCR from mouse mitochondrial DNA. The amplified products were
subcloned into pCR2.1-TOPO vector (Invitrogen) and confirmed by sequencing.
The polymerase chain reaction primers used were as follows: COX II (GenBank
accession number J01420
[GenBank]
): forward, 5'-ATGGCCTACCCATTCCAACT-3';
reverse, 5'-TTTAGTGGAACCATTTCTAG-3'; COX IV (GenBank accession
number M37829
[GenBank]
): forward, 5'-GAGCCTGATTGGCAAGAGAG-3'; reverse,
5'-TCACTTCTTCCACTCATTCT-3'; medium-chain acyl-CoA dehydrogenase
(MCAD) (GenBank accession number U07159
[GenBank]
): forward,
5'-ATCGCAATGGGTGCTTTTGA-3'; reverse,
5'-TACACGAGGGTGATGCATCG-3'; NRF-2
(GenBank accession number
M74515
[GenBank]
): forward, 5'-AACGTCTTCAACCATGACTA-3'; reverse,
5'-TATTTAAGAAAAACCTTAAG-3'; MEF2C (Genebank accession, L13171
[GenBank]
):
forward, 5'-ATGGGGAGAAAAAAGATTCAGATTA-3'; reverse,
5'-TCATGTTGCCCATCCTTCA-3'. The PPAR
and GLUT4 cDNAs were
kindly provided by Dr. T. Osumi (Himeji Institute of Technology, Japan) and by
Dr. D. M. Lane (Johns Hopkins University, Baltimore, MD), respectively.
Lipoprotein lipase (LPL) and acyl-CoA oxidase (ACO) cDNA were prepared as
described in our previous study
(21,
23). These cDNA were used as
probes for Northern blotting. Total RNA was isolated using TRIzol reagent
(Invitrogen) following the manufacturer's protocol. A portion of total RNA (20
µg per lane) was denatured with glyoxal and dimethyl sulfoxide and analyzed
by electrophoresis in 1% agarose gels. After transfer to nylon membranes
(PerkinElmer Life Sciences) and UV cross-linking, RNA blots were stained with
methylene blue to locate 28 S and 18 S rRNAs and to ascertain the amount of
loaded RNAs (24). The blots
were hybridized overnight at 42 °C with cDNAs, which had been labeled with
[32P]dCTP (PerkinElmer Life Sciences) by a random prime labeling
kit (Amersham Biosciences). The filters were washed several times with
1x SSC, 0.1% SDS at 42 °C, washed twice at 50 °C, and then
exposed to x-ray film at 80 °C. The amounts of each mRNA were
quantitated with an image analyzer (BAS 1800; Fuji Film, Tokyo, Japan) and
expressed as the intensity of phosphostimulated luminescence.
Oral Glucose and Insulin Tolerance TestFor oral glucose tolerance test, D-glucose (1 mg/g of body weight, 10% (w/v) glucose solution) was administered after overnight fast by stomach tube. Blood samples were obtained by cutting the tail tip before and 30, 60, and 120 min after glucose administration. For insulin tolerance test, human insulin (Humulin R; Eli Lilly Japan K.K., Kobe, Japan) was injected intraperitoneally (0.75 milliunits/g of body weight) in fed animals. Blood glucose was measured on samples obtained from the tail tip before and 15, 30, 60, 90, and 120 min after insulin injection. Blood glucose concentrations were measured using TIDEX glucose analyzer (Sankyo, Tokyo, Japan).
Statistical AnalysisThe glucose and insulin tolerance curve of each group was compared by repeated measure analysis (Statview 5.0; Abacus Concepts, Inc., Berkeley, CA). When they were significant, each group was compared with the others by Fisher's protected least-significant-difference test (Statview 5.0; Abacus Concepts). Statistical significance is defined as p < 0.05. Values are mean ± S.E.
| RESULTS |
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mRNA Are Expressed in Skeletal
Muscles from Transgenic MiceTransgenic lines were made with a DNA
construct that was composed of 5' flanking skeletal muscles specific
regulatory region and promoter of the human
-skeletal actin gene
(Fig. 1A). Three lines
of transgenic mice with a different copy number were examined; line B (copy
number is 15), line D (copy number is 3), and line E (copy number is 1). This
construct was expected to direct PGC-1
expression to skeletal muscles.
Northern blots probed by 1555 nucleotides of PGC-1
cDNA showed
that all these three PGC-1
transgenic mice expressed one transcript
(2.8 kb) from transgene PGC-1
in skeletal muscles such as quadriceps
and gastrocnemius but not in white adipose tissue, liver, spleen, kidney,
heart, lung, brown adipose tissue, and brain
(Fig. 1B).
Irrespective copy number, highest expression (13-fold increase compared with
6.5-kb PGC-1
mRNA in wild-type mice) of transgene PGC-1
in
gastrocnemius was observed in line E, second highest expression (10-fold) was
in line D, and lowest (1-fold) was line B (Figs.
1B and
4), possibly by a different
integration site in mouse genome. These -fold increases are at physiological
levels, because exercise training increased all three sized transcripts by
1013-fold (data not shown).
|
As described previously, three sizes of endogenous PGC-1
transcripts, 6.5, 5, and 3 kb, possibly caused by utilization of different
polyadenylation signals (25,
26), were observed in skeletal
muscles, kidney, heart, brown adipose tissue, and brain. We did not observe
the smaller 1.5-kb transcript that was described in the previous study
(26). With an increase of
exogenous PGC-1
, all three of these endogenous PGC-1
transcripts
in quadriceps and gastrocnemius were decreased.
To examine fiber type-specific expression of this construct, expression
level of PGC-1
in tibialis anterior (TA), extensor digitorum longus
(EDL), and soleus was measured in three lines of PGC-1
transgenic mice
(Fig. 1C). Transgene
PGC-1
expressed in TA that contains both type I and II muscle, EDL that
is type II fiber-rich muscle (white muscle), and soleus that is type I
fiber-rich muscle (red muscle). This data indicates that the promoter of the
human
-skeletal actin gene drive to both type I and II fibers. As
observed in quadriceps and gastrocnemius in
Fig. 1B, endogenous
PGC-1
mRNA in TA, EDL, and soleus muscles also were decreased.
Gross Changes in PGC-1
Transgenic MiceThe
most striking morphological difference was color of glycolytic muscles
(Fig. 2). As indicated in the
previous study (16), TA, EDL,
gastrocnemius, and quadriceps that were rich in type II fibers were paler in
appearance in non-transgenic littermates but became dark red in PGC-1
transgenic mice; also, soleus that were originally red and rich in type I
fibers became redder in PGC-1
transgenic mice. These color differences
in type II fiber-rich muscles were observed in PGC-1
higher expressed,
line D and E transgenic mice that express a high amount of PGC-1
but
were not observed visually in line B transgenic mice that express a low amount
of PGC-1
.
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PGC-1
Overexpression Decreased GLUT4 Expression in
Skeletal Muscles and Impaired the Insulin-induced Glucose-lowering
EffectBecause PGC-1
overexpression increased energy
metabolism, it was expected that both lipid and glucose oxidation would be
increased. Expression level of GLUT4, a rate-limiting step of glucose
oxidation in skeletal muscles, was also expected to increase. Surprisingly,
however, overexpression of PGC-1
down-regulated GLUT4 mRNA in
gastrocnemius, quadriceps, TA, EDL, and soleus
(Fig. 1, B and
C). GLUT4 mRNA did not decrease in white adipose tissue,
brown adipose tissue, and heart, in which transgene PGC-1
did not
express (Fig. 1B).
To examine whether a decrease of GLUT4 mRNA in skeletal muscles affected
glucose homeostasis in the whole body, oral glucose tolerance test and insulin
tolerance test were made in line D and E mice. In both mice lines, the blood
glucose curve after glucose administration did not differ significantly
between transgenic and wild-type mice (Fig.
3A). However, the insulin tolerance test clearly
demonstrated that the glucose-lowering effects of insulin were lower in
PGC-1
transgenic mice compared with their wild-type littermates
(Fig. 3B). This
unfavorable effect was also observed when the insulin tolerance curve was
expressed as percentage decrease from its initial value
(Fig. 3C).
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Possible Mechanism(s) for Down-regulation of GLUT4 Transcription factors which regulate GLUT4 in vivo are still unknown, but using muscle cell lines, MFE2C emerged as a possible transcription factor that up-regulates GLUT4 mRNA (27). We examined the possibility that PGC-1 overexpression down-regulates these transcription factors (Fig. 4). In gastrocnemius, with a decrease of GLUT4 mRNA, expression level of MEF2C decreased. Thus, the decrease of MEF2C might contribute to a decrease of GLUT4 expression.
PGC-1
Overexpression in Skeletal Muscles Is Accompanied
by Up-regulation of Genes Related to Respiratory Chain Function and Fatty Acid
OxidationTo examine the effect of PGC-1
overexpression on
other genes related to energy production, expression levels of genes related
to mitochondria, and several target genes of PPAR
were measured
(Fig. 5). NRF-2,
mitochondrially encoded COX subunit II, and nuclearly encoded COX subunit IV
increased by 1.1
2.6 -fold in three mice lines, parallel with expression
levels of PGC-1
. PPAR
is a nuclear receptor that regulates the
expression of enzymes involved in fatty acid oxidation
(28). The mRNAs of PPAR
itself and its target gene MCAD (a maker of mitochondrial fatty acid
oxidation) increased by 1.6
1.8-fold in mice line D and E that expressed a
high amount of PGC-1. However, ACO (a maker of peroxisomal fatty acid
oxidation) and LPL, other PPAR
target genes, did not increase.
ACO and LPL might be regulated by other transcription
factors in skeletal muscles. These data suggest that in skeletal muscles from
PGC-1
transgenic mice, up-regulation of PGC-1
and its target
genes may lead to mitochondrial fatty acid
-oxidation.
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| DISCUSSION |
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resulted in a large increase in
GLUT4 (18), overexpression of
PGC-1
in skeletal muscles did not up-regulate GLUT4; rather, it
down-regulated GLUT4 mRNA (Figs.
1 and
4). Because PGC-1
transgenic mice showed red color characteristic of oxidative muscle and
up-regulated enzymes related to mitochondrial oxidative phosphorylation and
fatty acid oxidation, it was proved that PGC-1
protein derived from the
transgene was able to induce mitochondrial biogenesis in skeletal muscles
(Figs. 2 and
5). Any discrepancy of
PGC-1
effect on GLUT4 expression between these two studies might be
related to fundamental differences of GLUT4 expression machinery in
vivo and in vitro. Under basal conditions, compared with
skeletal muscle tissues, muscle cell lines (L6, C2C12, Sol18) express far less
GLUT4 than skeletal muscle tissues, and their insulin responsiveness is
minimal (29).
There are several possibilities for the mechanism(s) for down-regulation of
GLUT4 mRNA by transgenic PGC-1
. First, endogenous PGC-1
may have
at least two forms of PGC-1
protein: a full-length form and a smaller
34-kDa form in skeletal muscles that was identified with the use of a
commercially available antibody
(20). Although it has not been
proven that this small form is derived from PGC-1
mRNA and has some
physiological functions, it might up-regulate GLUT4 mRNA. Thus, a decrease in
the smaller 34-kDa form might result in a GLUT4 decrease. In this study, we
have not measured PGC-1
and GLUT4 proteins in PGC-1
transgenic
mice. Because a marked difference of myofibrillar proteins was observed in
PGC-1
transgenic mice, it was very difficult to compare the amount of
PGC-1
or GLUT4 proteins on a total protein basis between PGC-1
transgenic and their wild-type littermates. Second, although, it is not known
whether MEF2C can up-regulate GLUT4 expression in skeletal muscles as well as
in muscle cell lines (27),
down-regulation of transcription factors, MEF2C might contribute to a decrease
of GLUT4 expression (Fig. 4).
Third, a decrease of GLUT4 expression observed in transgenic mice that
overexpressed PGC-1
was caused by the secondary effects of a marked
rearrangement of myofibrillar proteins
(16). Evidence that transgenic
mice that overexpressed less PGC-1
(line B) did not up-regulate GLUT4
expression in the absence of a marked phenotypic changes (Figs.
1 and
2) suggests that at least a
small PGC-1
overexpression did not up-regulate GLUT4 expression
markedly.
Exercise training induced both mitochondrial biogenesis and GLUT4 mRNA in
skeletal muscles with an increase of PGC-1
(20,
30). However, our data
indicate that PGC-1
overexpression down-regulates GLUT4 mRNA. These
findings suggest that exercise training alters other transcription factors
that up-regulate GLUT4 mRNA to overcome the PGC-1
effect to
down-regulate GLUT4 mRNA (Fig.
6). In liver, PGC-1
plays a key role for gluconeogenesis
(31). Because PGC-1
works as an inducer of diabetes in both skeletal muscle and liver, to increase
PGC-1
protein or its activity in vivo may not be a good
pharmacological intervention for prevention and treatment of diabetes
mellitus.
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| FOOTNOTES |
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To whom correspondence should be addressed. Tel: 81-3-3203-5725; Fax:
81-3-3207-3520; E-mail:
ezaki{at}nih.go.jp.
1 The abbreviations used are: NRF, nuclear respiratory factor; PGC-1
,
peroxisome proliferator-activated receptor
coactivator; PPAR,
peroxisome proliferator-activated receptor; MEF2C, myocyte specific enhancer
factor 2C; COX, cytochrome c oxidase; MCAD, medium-chain acyl-CoA
dehydrogenase; LPL, lipoprotein lipase; ACO, acyl-CoA oxidase; TA, tibialis
anterior; EDL, extensor digitorum longus. ![]()
| ACKNOWLEDGMENTS |
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