From the
Dana-Farber Cancer Institute and the
Department of Cell Biology, Harvard Medical School, Boston, Massachusetts
02115, the ¶Division of Endocrinology, Department
of Medicine, Beth Israel Deaconess Medical Center and Harvard Medical School,
Boston, Massachusetts 02115, and the ||Sarah W.
Stedman Center for Nutritional Studies, Department of Pharmacology and Cancer
Biology, Duke University Medical Center, Durham, North Carolina 27710
Received for publication, February 21, 2003 , and in revised form, May 1, 2003.
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ABSTRACT |
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INTRODUCTION |
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Many changes in the cellular environment result in modulation of
mitochondrial metabolism. Basal proton leak rate changes in response to
hormonal status and metabolic depression
(1319).
Also, small mammals with high standard metabolic rates have higher proton leak
rates than large mammals with low standard metabolic rates
(20,
21). Furthermore, proton leak
rates differ between phylogenetic groups; it is higher in endotherms than in
ectotherms (22). The fact that
mitochondrial functions can be altered in response to environmental stimuli is
due to the execution of a coordinated program of genes expression. PPAR
coactivator-1
(PGC-1
) has been shown to be a powerful regulator
of multiple aspects of mitochondrial gene expression. Indeed, PGC-1
is
cold-induced in brown fat and muscle where it plays a role in adaptive
thermogenesis by regulating a complex program of increased mitochondrial
biogenesis and uncoupled respiration via coactivation of peroxisome
proliferator-activated receptors (PPARs), nuclear respiratory factor 1, and
perhaps other transcription factors
(23,
24). Furthermore, transgenic
mice expressing physiological levels of PGC-1
protein in skeletal
muscle display an increased content of oxidative type I fibers compared with
their wild-type counterparts
(25).
Homologs of PGC-1 have been discovered, namely PGC-1
(26,
27) and PGC-1-related
coactivator (28). The
expression of PGC-1
and PGC-1-related coactivator is not cold-inducible
in brown fat and skeletal muscle, suggesting that they might have distinct
roles from PGC-1
(26,
28). However, both PGC-1
and PGC-1-related coactivator coactivate nuclear respiratory factor 1,
implying that they may also have a role in mitochondrial metabolism
(26,
28).
To date, there are no bioenergetic mechanisms that might explain how
PGC-1 increases uncoupled respiration, nor any studies examining a role
for its closest homolog PGC-1
in mitochondrial metabolism. In this
report, we measure the respiration and proton leak kinetics of C2C12 muscle
cells expressing either PGC-1
or PGC-1
. We report that
PGC-1
, like PGC-1
, increases mitochondrial metabolism. Although
both PGC-1
and PGC-1
increase proton leak, cells expressing
PGC-1
have a higher proportion of their mitochondrial respiration
linked to proton leak than those expressing PGC-1
.
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EXPERIMENTAL PROCEDURES |
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The cells were isolated by washing with phosphate-buffered saline and trypsinizing for 5 min. Dulbecco's modified Eagle's medium with 2% horse serum was added to stop the reaction, and the cells were resuspended and spin twice at 1000 rpm for 5 min at room temperature. Finally, the cells were resuspended in Dulbecco's phosphate-buffered saline supplemented with 25 mM glucose, 1 mM pyruvate, and 2% bovine serum albumin. Cell viability was determined using trypan blue. In all experiments, viability was approximately 90% or higher.
Proton Leak Kinetics in C2C12 MyotubesProton leak kinetics analyses in cells were carried out as described in Ref. 30. The cells were incubated for 30 min with 0.2 µCi/ml [3H]triphenylmethylphosphonium (TPMP), 0.2 µM TPMP, and 1.5 µM tetraphenylboron as carriers, as well as 2.5 µg of oligomycin/106 cells. Controls were done to ensure that the oligomycin concentration was saturating for each group of cells. After this incubation period, the respiration rates and membrane potentials were determined. The first point of each proton leak curve represents the resting respiration rate and membrane potential of cells in the presence of oligomycin. Increasing amounts of myxothiazol (0.024, 0.048, and 0.072 µM) were then added gradually to decrease membrane potential and respiration rate. Finally, 2.5 µM myxothiazol and 2 µM carbonylcyanide p-trifluoromethoxyphenylhydrazone were added to ascertain nonmitochondrial respiration. Nonmitochondrial respiration was subtracted from the total respiration to obtain mitochondrial respiration. To calculate membrane potential, we used the mitochondrial volume density values measured in the present study (see "Results"), cell volume of 4 µl/106 cells (data not shown using the method described in Ref. 30), resting plasma membrane potential of 46 mV (31), and the following TPMP binding corrections: mitochondrial, 0.4; cytoplasmic, 0.2; and external medium, 0.7 (17, 30).
Contribution of ATP Turnover and Proton Leak to Mitochondrial RespirationThe contribution of ATP turnover and proton leak to mitochondrial respiration was calculated from the results of the proton leak kinetics. ATP turnover represents the fraction of mitochondrial respiration sensitive to oligomycin. Proton leak represents the fraction of mitochondrial respiration that is insensitive to oligomycin.
Electron MicroscopyThe samples were fixed for 1 h in a
mixture of 2.5% glutaraldehyde, 1.25% paraformaldehyde, and 0.03% picric acid
in 0.1 M sodium cacodylate buffer (pH 7.4), washed in 0.1
M cacodylate buffer, postfixed with 1% osmiumtetroxide/1.5%
potassium ferrocyanide for 1 h, washed in water, and stained in 1% aqueous
uranyl acetate for 30 min followed by dehydration in grades of alcohol (5 min
at 70%, 5 min at 90%, and 2 x 5 min at 100%). The samples were then
infiltrated and embedded in TAAB Epon (Marivac Canada Inc., St. Laurent,
Canada). Ultrathin sections (about 60 nm) were cut on a Reichert Ultracut-S
microtome, picked up on to copper grids stained with uranylacetate and lead
citrate, and examined in a JEOL 1200EX. The quantification of mitochondrial
volume density and cristae surface density of cells expressing GFP,
PGC-1, and PGC-1
was carried out as described in Ref.
32.
Isolation of Mitochondria from Transgenic MiceWild-type and
transgenic mice expressing PGC-1 from the muscle creatine kinase
promoter in the muscle tissues (transgenic line 31) were housed on a 12D:12N
photoperiod cycle and fed ad libitum
(25). The mice were
sacrificed, and their whole leg muscle mass was excised and minced with razor
blades. The minced tissue was diluted 1/10 (w/v) in isolation buffer
containing 100 mM KCl, 50 mM Tris-HCl, 2 mM
EGTA, 0.5% bovine serum albumin, pH 7.4, at 4 °C and homogenized in a
mortar with six passes using a medium tight pestle. The homogenate was
centrifuged at 2000 x g at 4 °C for 5 min. The supernatant
was collected and centrifuged at 10,000 x g for 10 min. The
supernatant was discarded, and the pellet was resuspended in isolation medium
and centrifuged again at 10,000 x g for 10 min. The supernatant
was discarded, and the pellet was resuspended in 500 µl of isolation
medium. The protein concentration of the mitochondrial suspensions was
determined using the bicinchoninic acid kit with bovine serum albumin as a
standard.
Proton Leak Kinetics in Isolated MitochondriaThe proton leak kinetics in isolated mitochondria was determined as described in Ref. 30. Mitochondria were incubated (0.3 mg/ml) in assay medium containing 120 mM KCl, 5 mM KH2PO4, 3 mM Hepes, and 1 mM EGTA (pH 7.2 at room temperature) in a chamber thermostatted at 37 °C using a recirculating water bath. The oxygen consumption rates, measured with a Clark-type electrode, and the membrane potential values, determined using a TPMP electrode, were recorded simultaneously. Rotenone (5 µM), oligomycin (1 µg/mg mitochondrial protein), and nigericin (80 ng/ml) were present at the beginning of each run. TPMP was added up to 1.3 µM for calibration. Mitochondria were fed succinate (4 mM), and the inhibitor malonate was gradually added up to 1.3 mM to inhibit mitochondrial respiration and membrane potential. Finally, p-trifluoromethoxyphenylhydrazone (0.15 µM) was added to determine the drift of the TPMP electrode, if any. The oxygen solubility of the medium was considered 406 nmol of oxygen/ml (33), and the TPMP binding correction was considered 0.4 (µl/mg mitochondrial protein)1 (30).
Northern Blot AnalysesTotal RNA was isolated from C2C12 myotubes using Trizol (Invitrogen); 20 µg of RNA was analyzed by electrophoresis and transferred on a membrane. The RNA blots were hybridized with specific cDNA probes.
Statistical AnalysesAll of the statistical analyses were
performed using Sigma Stat 2.0. Comparisons of the contribution of ATP
turnover and proton leak to mitochondrial respiration between cells infected
with GFP and PGC-1 or with GFP and PGC-1
were carried out with
paired Student t test. Comparisons of mitochondrial volume density
and cristae surface density values between GFP-, PGC-1
-, and
PGC-1
-expressing cells were done with one-way analysis of variance and
the a posteriori Tukey test.
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RESULTS |
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We first measured the proton conductance in these cells, as well as cells
expressing a control GFP. Proton conductance can be defined as the flow of
protons (proton leak) across the mitochondrial membrane at a given membrane
potential and represents the fraction of mitochondrial respiration that is not
coupled to ATP production. Respiration, the consumption of oxygen, is used as
a surrogate for proton leak
(30). The highest point of a
proton leak curve represents the respiration rate and membrane potential of
cells/mitochondria in the presence of oligomycin, an inhibitor of the
F1F0-ATP synthase
(30). Then respiration and
membrane potential are inhibited gradually by adding increasing amounts of
myxothiazol, a specific inhibitor of the electron transport chain
(30). As shown in
Fig. 1 (A and
B), the respiration rate of PGC-1- and
PGC-1
-expressing cells at any given membrane potential was substantially
higher than GFP controls. For example, the respiration rate at 190 mV of
PGC-1
cells and the paired GFP control was
7 and 3 nmol of
oxygen/min/106 cells, respectively, an increase of 2.5-fold
(Fig. 1A). The
respiration rate of PGC-1
cells and the paired GFP control at 172 mV was
3 and 0.5 nmol of oxygen/min/106 cells, respectively, a 6-fold
increase (Fig.
1B).
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To compare the proton leak kinetics of the cells expressing PGC-1
and PGC-1
, we expressed the respiration rates and membrane potentials of
these cells as percentages of the initial respiration rate and membrane
potential of paired GFP controls (Fig.
1C). The proton conductance of cells expressing
PGC-1
was approximately three times higher than those expressing
PGC-1
(Fig.
1C). Indeed, the respiration rate of PGC-1
, when
normalized to its paired GFP control, was at least twice that of PGC-1
cells at any given membrane potential (Fig.
1C).
Contribution of ATP Turnover and Proton Leak to PGC-1-related
Mitochondrial RespirationMitochondrial respiration is divided into
two blocks: ATP turnover and proton leak. ATP turnover is the fraction of
mitochondrial respiration coupled to ATP production and is sensitive to
oligomycin. Proton leak is the fraction of mitochondrial respiration not
coupled to ATP production and is insensitive to oligomycin. Metabolic
efficiency represents the balance between the ATP turnover and proton leak
blocks. Therefore, when total mitochondrial respiration is altered, metabolic
efficiency can only be preserved if ATP turnover and proton leak are affected
in a similar way. For example, if mitochondrial respiration doubles, metabolic
efficiency will be preserved if both ATP turnover and proton leak double. We
determined the proportion of mitochondrial respiration devoted to ATP turnover
and proton leak in cells expressing PGC-1, PGC-1
, and GFP. We
used two different approaches to carry out this analysis; because each
approach has its own limitation, together they provide a more reliable
overview of the metabolic organization of these cells.
In the first calculation, we expressed the respiration rate of
PGC-1, PGC-1
, and GFP cells in the presence of oligomycin (from
Fig. 1) as a fraction of their
mitochondrial respiration (data not shown) without taking into account
membrane potential values. The potential problem with this approach is
actually that it does not take into account differing membrane potential
values of the different cells. By this method, PGC-1
-expressing cells
devote 37% of their mitochondrial respiration to proton leak, compared with
23% for paired GFP controls (Fig.
2A). In other words, PGC-1
increases uncoupled
respiration more than coupled respiration. This is in agreement with the
initial study by Wu et al.
(24) using C2C12 myotubes
infected with retroviruses reporting that PGC-1
increases uncoupled
respiration more than coupled respiration. In contrast, cells expressing
PGC-1
display a fraction of mitochondrial respiration caused by proton
leak similar to that of the GFP controls
(Fig. 2B). Indeed, the
ATP turnover and proton leak blocks increased coordinately in
PGC-1
-expressing cells so that they are almost as coupled as GFP control
cells (Fig. 2B).
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As a second approach, we took into account the membrane potential values
and calculated the proportion of mitochondrial respiration devoted to proton
leak at the resting membrane potential of PGC-1, PGC-1
, and GFP
cells. This is the membrane potential in the absence of oligomycin (data not
shown). The membrane potential value of these cells was slightly higher in the
resting state than in the presence of oligomycin, which is the highest point
of the proton leak curves presented in Fig.
1. To obtain the predicted proton leak rate at the resting
membrane potential of these cells, we fitted a linear curve through the proton
leak points of PGC-1
, PGC-1
cells, and paired GFP controls from
Fig. 1 (A and
B) (19).
A caveat with this approach is that the proton leak rate value at the resting
membrane potential of these cells must be predicted without being able to
measure it experimentally. Using this method, we determined that cells
expressing GFP, PGC-1
, and PGC-1
have 37, 62, and 44%,
respectively, of their mitochondrial respiration linked to proton leak.
Although these values are higher than the ones determined above in absolute
terms, the general conclusion remains the same: cells expressing PGC-1
are less efficient than GFP- or PGC-1
-expressing cells. Whether
PGC-1
significantly alters the fraction of respiration related to proton
leak will require more detailed experiments.
Ultrastructural Analyses of Muscle Cells Expressing PGC-1
or PGC-1
Modification in mitochondrial volume
density and changes in the intrinsic properties of mitochondria, such as
membrane properties and cristae surface density, are key factors affecting
proton conductance in cells
(21). To determine whether the
increased proton conductance of cells expressing PGC-1
or PGC-1
was associated with elevated mitochondrial volume density and/or cristae
surface density, we carried out ultrastructural analyses of these cells and of
GFP controls via transmission electron microscopy. To calculate mitochondrial
volume density, a grid was laid on randomly selected micrographs, and the
number of points falling onto mitochondria was expressed as a fraction of
those falling onto the cell area
(32). An analogous approach
was used to determine the cristae surface density of individual mitochondrion
(32). Muscle cells expressing
PGC-1
and PGC-1
showed 25 and 75% increases in mitochondrial
volume density, respectively, compared with GFP controls (Figs.
3A and
4A). Concerning the
ultrastructure of individual mitochondrion, we observed that mitochondria from
C2C12 myotubes expressing PGC-1
displayed a 10% increase in cristae
surface density compared with GFP controls, which was statistically
significant (Figs. 3B
and 4B).
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Proton Leak Kinetics of Mitochondria Isolated from Mice Expressing
PGC-1 TransgenicallyBecause cells expressing
PGC-1
and PGC-1
displayed increased proton leak rates, we wanted
to determine whether this effect was present in isolated mitochondria.
However, performing this type of experiment has been very difficult with
cultured cells because of the problems associated with large scale
purification. Hence, we took advantage of the availability of mice expressing
PGC-1
transgenically in skeletal muscle and compared the proton leak
kinetics of mitochondria isolated from their leg muscle with that of wild-type
mice. Importantly, these mice do not grossly overexpress this coactivator;
they express PGC-1
in type II muscle fibers at the level ordinarily
seen in type I fibers (25).
Generally, the concentration of mitochondrial proteins from a given muscle
mass was higher in transgenic mice than in wild-type animals (data not shown),
suggesting that the ectopic expression of PGC-1
in vivo also
leads to mitochondrial biogenesis. The highest point of a proton leak curve
measured in isolated mitochondria represents the state 4 respiration rate and
membrane potential. As shown in Fig.
5, mitochondria from transgenic mice displayed a slightly higher
respiration rate than those from wild-type mice at any given membrane
potential. In addition, the state 4 respiration rate was 50% higher in
mitochondria from transgenic than wild-type mice, suggesting an increased
substrate oxidation capacity (Fig.
5). Together, these results are consistent with the measurements
performed in cells and demonstrate that PGC-1
can affect the intrinsic
functional properties of mitochondria, notably increasing proton leak
rate.
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Mitochondrial Gene Expression in Cells Expressing PGC-1
or PGC-1
To gain possible insights into the
molecular basis for the changes observed in respiration and proton leak, we
examined the expression of several key mitochondrial components of the
electron transport system (cytochrome c and ATP-synthase), the
metabolism of ROS (manganese-superoxide dismutase and glutathione
peroxidase-1), as well as uncoupling proteins (UCP2 and UCP3). The expression
of cytosolic enzymes involved in the metabolism of ROS (glutathione
peroxidase-1,
-glutamylcysteine synthetase light subunit, and
-glutamylcysteine synthetase heavy subunit) was also examined because
the increased mitochondrial volume density and respiration of PGC-1
and
PGC-1
cells could lead to elevated cellular levels of ROS. As shown in
Fig. 6A, cells
infected with these viruses expressed similar levels of PGC-1
or
PGC-1
mRNA, and both coactivators induced mRNAs for the well studied
PGC-1
target genes cytochrome c and UCP2 to the same extent.
Specifically, the mRNA expression level of cytochrome c, ATP
synthase, UCP2 and UCP3 (Fig.
6A), as well as manganese-superoxide dismutase and
glutathione peroxidase-1 (Fig.
6B) were all higher in cells expressing PGC-1
and
PGC-1
than in control cells (Fig.
6). The quantitative induction in the expression level of mRNA for
certain markers, like ATP synthase and UCP3, in PGC-1
and
PGC-1
-expressing cells matched the 1.25- and 1.75-fold inductions in
their mitochondrial volume density, respectively (Figs.
4 and
6). Others, like cytochrome
c, UCP2, manganese-superoxide dismutase, and glutathione
peroxidase-1, displayed higher inductions of at least 5-fold
(Fig. 6). The fact that
expression of these mRNAs displays a higher induction than the mitochondrial
volume density suggests that each mitochondrion might have an elevated content
of these proteins. Together, these results support the observation that
myotubes expressing PGC-1
and PGC-1
ectopically have an increased
mitochondrial volume density and also suggest that these cells have
mitochondria with different intrinsic properties. Interestingly, PGC-1
cells displayed a higher expression of two cytosolic enzymes involved in the
metabolism of ROS,
-glutamylcysteine synthetase light subunit, and
-glutamylcysteine synthetase heavy subunit, compared with PGC-1
and GFP cells (Fig.
6B).
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DISCUSSION |
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One basic question addressed here is whether PGC-1 has the capacity
to induce mitochondrial biogenesis and increase respiration. As shown in Figs.
1B,
3A, and
4A, PGC-1
powerfully stimulates mitochondrial biogenesis and respiration. Indeed, it is
more potent in this regard than PGC-1
at similar levels of mRNA, even
though several genes are induced by both coactivators to a similar extent.
Another key issue addressed in this study is the fraction of mitochondrial
respiration coupled or uncoupled in cells expressing PGC-1 or
PGC-1
. There are several possible explanations for the increased
fraction of uncoupled respiration observed in the presence of PGC-1
versus PGC-1
. First, it is necessary to examine how coupled and
uncoupled respiration in particular can be regulated. A key factor that needs
consideration with respect to respiration rate relates to the elevated
mitochondrial volume density of these cells. We can account for the
differences in mitochondrial volume density between cells expressing
PGC-1
or PGC-1
and control cells simply by dividing the
respiration rates of these cells by their respective mitochondrial volume
density value. This calculation gives an indication of the respiration rate of
mitochondria inside the cells. After correcting for differences in
mitochondrial volume density, the total respiration rate of cells expressing
PGC-1
or PGC-1
is similar to control cells. However, the proton
leak rate (highest point of the proton leak curves in
Fig. 1) of cells expressing
PGC-1
remains higher than that of cells expressing PGC-1
or
control cells, both of which have similar proton leak rates after correction.
These data suggest the proton leak rate of individual mitochondria is
higher in the presence of PGC-1
than PGC-1
and that mitochondrial
volume density is not the only explanation for the higher fraction of
leak-related respiration in cells expressing PGC-1
.
Modifications in the intrinsic properties of mitochondria could explain
higher proton leak rates in cells expressing PGC-1 versus
PGC-1
. Cells expressing PGC-1
, but not PGC-1
, displayed a
statistically significant increase of 10% in cristae surface density (Figs.
3B and
4B), which would be
expected to augment their proton leak by increasing the area of membrane
across which protons can re-enter the matrix compartment. In support of the
idea that elevated PGC-1
levels increase the proton leak of individual
mitochondria, a separate experiment using mitochondria isolated from the
skeletal muscle of mice expressing PGC-1
at the level found in type I
muscle fibers revealed that these mitochondria have higher resting proton leak
rate (highest point of proton the leak curves in
Fig. 5) and proton conductance
than those from wild-type mice. Therefore, modifications in the intrinsic
properties of mitochondria play an important role in the difference between
PGC-1
and PGC-1
for the proportion of mitochondrial respiration
linked to proton leak.
Another possible difference between cells expressing PGC-1 and
PGC-1
could relate to the function of the uncoupling proteins, that is
the expression of UCP2 and/or UCP3, combined with altered levels of
superoxide, activators of the UCPs. Cells expressing PGC-1
and
PGC-1
both had similarly elevated expression levels of mRNA encoding
UCP2 and UCP3 (Fig. 6).
Intriguingly, only cells expressing PGC-1
had increased levels of two
cytosolic genes, namely glutamylcysteine synthetase light subunit and
glutamylcysteine synthetase heavy subunit, both involved in the
removal/degradation of ROS (Fig.
6B). It is conceivable, or even likely, that both
PGC-1
and PGC-1
action result in the generation of ROS via the
activation of mitochondrial metabolism, thereby activating UCP2 and/or UCP3.
However, it is also possible that PGC-1
is better at the removal of
these ROS because mRNAs for two cytoplasmic enzymes playing a role in
metabolism of ROS are induced by this coactivator but not by PGC-1
.
Whether regulation of ROS in the cytoplasm can influence superoxide on the
matrix side of the inner mitochondrial membrane and regulate UCP2 or UCP3 is
an open question (35). The
relative ability of PGC-1
and PGC-1
to produce ROS and activate
UCPs via this mechanism remains to be determined.
Another important aspect to consider in this analysis concerns ATP
turnover, the fraction of mitochondrial respiration providing ATP for the ATP
consumers. There are important differences regarding ATP turnover between
cells expressing PGC-1 and PGC-1
. Indeed, 75% of the increase in
mitochondrial respiration in cells expressing PGC-1
was due to an
increase in ATP turnover, and 25% was explained by an increase in proton leak.
For cells expressing PGC-1
, the situation was completely reversed; only
25% of the increase in mitochondrial respiration in these cells was due to an
increase in ATP turnover, and 75% was explained by an increase in proton leak.
Together, these data indicate that the activity of the ATP consuming reactions
was increased in cells expressing PGC-1
, providing another potential
influence related to the higher fraction of coupled respiration in these
cells.
Overall, the data presented here indicate that the nature of the
respiration induced by PGC-1 or PGC-1
is different. Indeed, cells
expressing PGC-1
have a less efficient mitochondrial respiration than
those expressing PGC-1
or GFP. This is particularly interesting in light
of the fact that PGC-1
is induced in the cold in both brown fat and
skeletal muscle, two key thermogenic tissues
(23). In contrast,
PGC-1
, which does not lead to a particularly inefficient mitochondrial
metabolism (present study), is not cold-inducible in these tissues
(26). However, it is important
to appreciate that PGC-1
is also induced in muscle by exercise and is
expressed in type 1 muscle, muscle associated with resistance to
fatigue. Also, PGC-1
can induce an increase in type 1 fibers in muscle
composed mainly of type 2 fibers
(25). Furthermore,
PGC-1
is highly expressed in the heart, a tissue very dependent on ATP
production. Certainly, mitochondrial uncoupling in these situations would be
predicted to cause a reduced exercise tolerance in skeletal muscle
and heart failure. These observations can be reconciled by an integrated view
of the relative dependence of ATP turnover and proton leak on mitochondrial
membrane potential (Fig. 7). In
a state of physical rest, such as present in nonexercising muscle or
cold-induced muscle or brown fat, a relatively low ATP demand will cause an
increased mitochondrial membrane potential, which will in turn increase proton
leak. In stark contrast, high ATP demand, expected upon the performance of
muscular work, will slightly decrease membrane potentials resulting in a
tighter coupling of metabolism because proton leak decreases rapidly as
membrane potential is lowered. Said another way, PGC-1
, by setting up a
relatively uncoupled state at rest, probably provides a system where
rapid ATP turnover, such as occurs during the performance of exercise, will
effectively slow down excessive proton leak. PGC-1
, in contrast, drives
relatively less proton leak at rest, so it will provide more ATP when this is
needed but cannot otherwise provide metabolic flexibility. Because both
PGC-1
and PGC-1
are present in many tissues, the relative
expression levels of these coactivators likely will play an important role in
setting metabolic efficiency.
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FOOTNOTES |
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Merck Fellow of the Jane Coffin Childs Memorial Fund for Medical
Research.
** To whom correspondence should be addressed: Dana-Farber Cancer Inst., One Jimmy Fund Way, SM958, Boston, MA 02115. E-mail: bruce_spiegelman{at}dfci.harvard.edu.
1 The abbreviations used are: UCP, uncoupling protein; ROS, reactive oxygen
species; PGC, peroxisome proliferator-activated receptor coactivator;
GFP, green fluorescent protein; TPMP,
[3H]triphenylmethylphosphonium; PPAR, peroxisome
proliferator-activated receptor.
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ACKNOWLEDGMENTS |
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REFERENCES |
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