3-Adrenergic agonist
induces a functionally active uncoupling protein in fat and
slow-twitch muscle fibers
Toshihide
Yoshida1,
Tsunekazu
Umekawa1,
Kenzo
Kumamoto4,
Naoki
Sakane1,
Akinori
Kogure1,
Motoharu
Kondo1,
Yasuo
Wakabayashi2,
Teruo
Kawada3,
Itsuro
Nagase5, and
Masayuki
Saito5
1 First Department of Internal
Medicine and 2 Department of
Biochemistry, Kyoto Prefectural University of Medicine, Kyoto 602;
3 Division of Applied Life Sciences, Graduate
School of Agriculture, Kyoto University, Kyoto 606-01;
4 Department of Anatomy, Meiji
College of Oriental Medicine, Hiyoshi, Kyoto 629-03; and
5 Laboratory of Biochemistry,
Department of Biomedical Sciences, Graduate School of Veterinary
Medicine, Hokkaido University, Sapporo 060, Japan
 |
ABSTRACT |
The mitochondrial uncoupling protein (UCP) has
usually been found only in brown adipose tissue. We recently observed
that a chronic administration of the
3-adrenergic agonist CL-316,243 (CL) induced the ectopic expression of UCP in white fat and skeletal muscle in genetic obese yellow KK mice. The aim of the present study
was to examine whether UCP could be induced in nongenetic obese animals
produced by neonatal injections of monosodium
L-glutamate (MSG). The daily
subcutaneous injection of CL (0.1 mg/kg) to MSG-induced obese mice for
2 wk caused significant reductions of body weight (15%) and white fat
pad weight (58%). Northern and Western blot analyses showed that CL
induced significant expressions of UCP in the white fat and muscle, as
well as in brown fat. Immunohistochemical observations revealed that
the UCP stains in white fat were localized on multilocular cells and
that those in muscle were localized on slow-twitch fibers rich in
mitochondria. Immunoelectron microscopy confirmed the mitochondrial
localization of UCP in the myocytes. The guanosine 5'-diphosphate
(GDP) binding to mitochondria in brown fat doubled after the CL
treatment. Moreover, significant GDP binding was detected in the white
fat and muscle of the CL-treated mice, at about one-fourth and
one-thirteenth the activity of brown fat, respectively, suggesting that
ectopically expressed UCP is functionally active. We concluded that the
3-adrenergic agonist CL can
induce functionally active UCP in white fat and slow-twitch muscle
fibers of obese mice.
CL-316,243; guanosine 5'-diphosphate binding; monosodium
L-glutamate-induced obese mice; white fat; gastrocnemius muscle
 |
INTRODUCTION |
THERMOGENESIS in brown adipose tissue (BAT) is
regulated by sympathetic nerves distributed to this tissue principally
through the
-adrenergic action of norepinephrine (12). There are
three isoforms of the
-adrenergic receptor (AR) in brown adipocytes:
1-,
2-, and
3-AR (16). Because the
3-AR is expressed primarily, but not exclusively, in brown and white adipocytes, it has been expected that an agonist to the
3-AR, which would be a
selective stimulant of lipolysis and BAT thermogenesis, might be useful as an antiobesity drug (1, 10, 20). In fact, various types of
3-AR agonists with different
selectivities and potencies have been developed and confirmed to be
effective in stimulating lipolysis in adipocytes in vitro and in
reducing adipocity in vivo in experimental animals (3, 26, 27, 30). It
has also been shown that
3-AR
agonists are capable of inducing mitochondrial uncoupling protein
(UCP), a key molecule in thermogenesis, in rodent (13, 27) and canine
(6) BAT. We (21) recently demonstrated that the chronic treatment of
genetically obese yellow KK mice with CL-316,243 (CL), a highly
selective
3-AR agonist, induces
UCP expression not only in BAT but also in various fat pads usually considered white adipose tissue (WAT), and even in skeletal muscle. These results, in contrast with the well-accepted view of the specific
expression of UCP in BAT (12), suggest that such an ectopic expression
of UCP in tissues other than typical BAT may also contribute to some
degree to the potent antiobese effect of the
3-AR agonists. However, in
human studies, especially, it has previously been shown that
sympathomimetic stimulation results not in BAT thermogenesis but rather
in a significant increase in skeletal muscle thermogenesis (2), and a
significant part of this thermogenic response has subsequently been
shown to be mediated by
3-adrenoceptors (18).
Therefore, it remains to be determined whether or not the ectopically
expressed UCP is an exceptional phenomenon found only in this
genetically obese strain and whether or not the ectopically expressed
UCP is functionally active. In the present study, we investigated
1) the effects of chronic CL
treatment on the UCP expression in various fat tissues and skeletal
muscle in nongenetic hypothalamic obese mice produced by monosodium
L-glutamate (MSG) injection, and
2) the activity of UCP in the fat
pads and skeletal muscle estimated by guanosine 5'-diphosphate
(GDP) binding (14, 15, 22) to mitochondria, a reliable biochemical
index of the thermogenic activity of UCP. We also examined the
immunohistochemical localization of UCP, particularly focusing on UCP
in skeletal muscle.
 |
METHODS |
Chemicals.
CL-316,243, disodium
(R,R)-5{[2-(3-chlorophenyl)-2-hydroxyethyl]-aminopropyl}-1,3-benzodioxole-2,2-dicarboxylate
(3), was provided by American Cyanamid (Pearl River, NY).
Animals.
Immediately after birth and for 5 consecutive days, seventy-two ICR
female mice (Charles River Japan, Tokyo) received daily subcutaneous
injections of MSG (Wako Pure Chemical, Osaka, Japan) dissolved in
physiological saline at a concentration of 2 mg/g (28-30), which
made these mice obese. Another 72 control female animals received
physiological saline in a similar manner (lean control mice). The mice
were kept in a controlled temperature room (22 ± 2°C) with
artificial light for 12 h a day, and they received commercial powdered
chow (Charles River Japan) and tap water ad libitum. At 6 wk after
treatment, the MSG obese group and the lean control group were each
further divided into two groups; some mice were given a daily
subcutaneous injection of CL (0.1 mg/kg) (30) for 2 wk, and the other
mice were given a daily subcutaneous injection of distilled water
(vehicle) for 2 wk. Thus four groups were examined:
1) 36 CL-treated MSG obese mice,
2) 36 distilled water-treated MSG
obese mice, 3) 36 CL-treated lean
control mice, and 4) 36 distilled
water-treated lean control mice. At the end of 2 wk, body weights of
the mice were measured. These mice were then killed by cervical
dislocation, and the skeletal muscle (gastrocnemius), interscapular
BAT, and inguinal and retroperitoneal WATs were rapidly removed in
their entirety. Some of these tissues were frozen in liquid nitrogen
for Western blot and RNA analyses. Other sets of these tissues were
obtained and treated by the methods described below for
immunohistochemistry and electron-microscopic observations, and some
parts of the gastrocnemius muscles, interscapular BATs, and inguinal
and retroperitoneal WATs were prepared for the assay of GDP binding by
the method described below. The animal care and experimental procedures
were approved by the Animal Care and Use Committee of the Kyoto
Prefectural University of Medicine.
Western blot analysis.
Each tissue (6 individual samples in each group) was homogenized in
5-10 volumes of a solution containing 10 mM
tris(hydroxymethyl)aminomethane · HCl and 1 mM EDTA (pH
7.4) for 30 s with a polytron. After centrifugation at 1,500 g for 5 min, the fat cake was
discarded, and the infranant (fat-free extract) was used for the
Western blot analysis of UCP, as described previously (24, 25).
Briefly, the fat-free extract (10-40 µg protein) was
solubilized, subjected to sodium dodecyl sulfate-polyacrylamide gel
electrophoresis, and transferred to a nitrocellulose filter. After the
filter was blocked with 6% nonfat dry milk, it was
incubated with a rabbit antiserum (×1,000 diluted) against rat
UCP purified from interscapular BAT (16). The filter was then incubated
with 125I-labeled protein A (ICN,
Irvine, CA). The dry blot was exposed to an X-ray film for
autoradiography or an imaging plate of the BAS1000 system (Fuji Film,
Tokyo).
RNA analysis.
Total RNA (6 individual samples in each group) was extracted from
0.1-1 g tissue using TRIzol (GIBCO-BRL, Tokyo). For Northern blot
analysis, 20 or 40 µg total RNA were electrophoresed on a 1.0%
agarose-formaldehyde gel and transferred to and fixed on a nylon
membrane. A 488-bp cDNA probe corresponding to the coding region of rat
UCP was prepared by digestion of whole UCP cDNA (a gift from Dr. D. Ricquier, Centre National de la Recherche Scientifique, Meudon, France)
by BamH I and labeling by random priming with
[
-32P]dCTP (ICN).
The blots were hybridized to the labeled probe at 42°C for 20 h in
the presence of 500 µg/ml salmon sperm DNA and were exposed to an
X-ray film.
Immunohistochemistry.
Six mice, each treated with CL or distilled water from both the MSG and
saline control groups, were anesthetized with diethyl ether and
perfused transcardially with 4% paraformaldehyde in phosphate-buffered
saline (PBS), and the fat pads and gastrocnemius muscles were removed
and fixed in Bouin solution. The fixed tissues were dehydrated in
ethanol, paraffin embedded, and cut into 7-mm-thick sections. The
dewaxed sections were incubated in 0.3% hydrogen peroxide in methanol
to inhibit endogenous peroxidase activity, and then with 10% normal
goat serum, the rabbit antiserum against rat UCP (×500 diluted),
goat anti-rabbit immunoglobulin G (IgG, ×400 diluted; Vector,
Burlingame, CA), and finally with avidin-biotin-peroxidase complex
(Vector) according to the conventional avitin-biotin complex method.
The sections were also counterstained with hematoxylin and examined by
a light microscope. To determine whether the increased expression of
UCP in muscle is found in slow-twitch fibers, a monoclonal antibody
(MEDAC Diagnostika, Hamburg, Germany), which specifically recognizes
slow-type myosin heavy chain (8), was used and stained by the methods
described elsewhere (8, 9) with the muscles
(n = 6 each) of mice treated
with CL or distilled water.
For the electron-microscopic observations, specimens of gastrocnemius
muscle fixed in 1% glutaraldehyde-0.1 M phosphate buffer (pH 7.4) from
six mice of each of the four groups were dehydrated in dimethyl
formamide and embedded in Lowicry K4M (Chemische Werke Lowi,
Waldkaiburg, Germany) in gelatin capsules polymerized by ultraviolet
irradiation. Ultrathin sections were mounted on nickel grids, immersed
in 2% normal goat serum in PBS for 1 h, and treated with the rabbit
antiserum against rat UCP (×100 diluted) in PBS containing 1%
bovine serum albumin (BSA) overnight in a moist chamber at 4°C.
After being washed well with PBS, the sections were treated with
colloidal gold-labeled anti-rabbit IgG F(ab')2 (10 nm; Bio Cell,
Cardiff, UK) diluted at 1:15 in PBS containing 1% BSA for 2 h at room
temperature, postfixed with 2.5% glutaraldehyde for 10 min, and
stained with uranyl acetate for 15 min and lead citrate for 1 min. In
the controls, nonimmune rabbit serum was substituted for the primary
antiserum.
GDP binding.
Interscapular BAT, inguinal and retroperitoneal WATs, and gastrocnemius
muscle samples (6 individual samples from each group) were rapidly
removed after cervical dislocation, weighed, and placed in ice-cold
sucrose buffer. For the preparation of mitochondria, samples were
homogenized in an ice-cold medium (pH 7.2) containing 250 mM sucrose
and 5 mM N-tris
(hydroxy-methyl)-2-aminoethanesulfonic acid (TES). The mitochondria in
the sample were isolated by differential centrifugation according to
the procedure described by Cannon and Lindberg (5). The mitochondrial
protein content was estimated by the method of Lowry and Lindberg (19).
Mitochondrial GDP binding was determined by the method of Nicholls
(22). The mitochondria were incubated at 20°C in 0.5 ml of medium
containing 48,100 Bq of
[3H]GDP, 4,551 Bq of
[14C]sucrose, 100 mM
potassium atractyloside, 20 mM TES (pH 7.1), 10 mM choline chloride,
and 5 mM rotenone. After a 7-min incubation, 0.4 ml aliquots containing
0.26 mg of mitochondrial protein were withdrawn and filtered through a
nitrocellulose membrane filter with a pore size of 0.45 mm (Sartorius,
Gottingen, Germany). The 3H and
14C radioactivities of the filters
were measured by scintillation spectrometry (Packard, Downers Grove,
IL). To calculate the volume of the medium trapped on the filter,
[14C]sucrose was
included.
Data analysis.
Values are expressed as means ± SE. Data were analyzed by one-way or
two-way analysis of variance (ANOVA). After justification of the ANOVA,
the Bonferroni t-test was performed.
Representative results are shown in Figs. 1-6.
 |
RESULTS |
MSG obese mice and lean control mice were given CL (0.1 mg · kg
1 · day
1)
or distilled water once a day. After 2 wk, the MSG obese mice treated
with CL weighed significantly less than those given distilled water and
had smaller white fat pads, as shown in Table
1. The CL treatment also produced a
hypertrophy of the interscapular BAT, with an ~1.3-fold increase in
the mitochondrial protein content (P < 0.05), but had no effects on the tissue weight or protein content
of the gastrocnemius muscle. Similar effects of CL were also observed
in the lean control mice, but to a lesser extent.
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Table 1.
Body and mass weights, mitochondrial protein, and specific GDP binding
in mitochondria of IBAT, WAT, and gastrocnemius muscle of MSG-induced
obese mice and lean controls treated with CL-316,243 for 2 wk
|
|
The presence of UCP and its mRNA was detected by Western and Northern
blot analyses of samples obtained from various regions of the fat pads
and also from skeletal muscle. As shown in Fig. 1, the Northern blot analysis detected no
UCP mRNA in any of the samples except the BAT in the two groups of mice
given distilled water. In contrast, in the MSG obese mice treated with
CL, a clear signal corresponding to UCP mRNA was detected not only in
the BAT but also in the inguinal and retroperitoneal fat pads. In addition, a similar band was also detected in the gastrocnemius muscle.
In the lean control mice given CL, a signal was detected in the
inguinal fat pads in addition to the BAT. The Western blot analysis
also revealed the presence of UCP in the inguinal and retroperitoneal
fat pads and the gastrocnemius muscle of the MSG obese mice treated
with CL (Fig. 2). The UCP protein in muscle was not detected in the lean control mice, regardless of whether they
were given CL.

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Fig. 1.
Northern blot analysis of uncoupling protein (UCP) mRNA of fat pads and
muscles. Obese monosodium
L-glutamate (MSG) and lean
control mice were given CL-316,243 (CL) or distilled water (Dist.) for
2 wk, and the total RNA [20 µg from brown adipose tissues
(BAT), 40 µg from other tissues] was used for Northern blot
analysis. Results are representative of 5 independent samples. WAT,
white adipose tissue.
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Fig. 2.
Western blot analysis of UCP. Mice were treated as described in Fig. 1,
and fat-free extracts containing 10 µg protein of BAT, 20 µg of
WAT, and 40 µg of muscle were used for Western blot analysis. Results
are representative of 3 independent samples.
|
|
The morphological characteristics of the fat pads expressing UCP were
examined immunohistochemically. Macroscopically, the inguinal and
retroperitoneal fat pads of the CL-treated MSG obese mice were less
pale than those of the distilled water-treated mice. Histochemically,
they contained many multilocular cells that were positive for UCP (Fig.
3). Unilocular cells were also present in
the fat pads but were always negative for UCP, and their size was
smaller than the typical unilocular cells found in the distilled
water-treated mice. The fat pads of the distilled water-treated mice
were occupied mostly by unilocular cells, filled with a single large
lipid droplet, which were always negative for UCP. Thus the CL-treated
mice had many typical brown adipocytes in the fat pads usually
considered WAT. The localization of UCP in the gastrocnemius muscle of
the CL-treated MSG obese mice was also examined at both the light- and
electron-microscopic levels. As shown in Fig.
4, the muscle tissue was clearly stained
with anti-UCP serum, the positive regions being in the myocytes. No stains were detected when a preimmune serum or the anti-UCP serum pretreated with purified UCP was used. The muscle tissues from the
distilled water-treated MSG obese mice and lean control mice were
always negative for UCP (data not shown). The mouse gastrocnemius muscle contains both slow-twitch oxidative and fast-twitch glycolytic muscle fibers, whose mitochondrion contents are high and low, respectively. To determine the type of fiber expressing UCP, the muscle
tissue was stained with a monoclonal antibody that specifically recognizes the myosin heavy chain of the slow-twitch fiber. As shown in
Fig. 5, the muscle fibers positive for UCP
were always stained by this monoclonal antibody, suggesting that UCP is
localized in the mitochondria in slow-twitch fibers. The intracellular
localization of UCP in myocytes was also determined by electron
microscopy by use of a gold-labeled anti-UCP antiserum. UCP-gold
particles were localized predominantly on mitochondria of the myocytes
of the CL-treated MSG obese mice (Fig. 6),
whereas no gold particles were found in the other mice (data not
shown).

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Fig. 3.
Immunohistochemical detection of UCP in white fat pads. Sections of an
inguinal fat pad of an obese MSG mouse treated with CL
(A) or distilled water
(B) were stained by a rabbit
antiserum against rat UCP (×80). Results are representative of 3 mice. Arrows show multilocular cells.
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Fig. 4.
Immunohistochemical detection of UCP in skeletal muscle. Gastrocnemius
muscle of an obese MSG mouse treated with CL was stained by a rabbit
antiserum against rat UCP (A).
Sections of gastrocnemius muscle were also stained with hematoxylin
(B). Arrows show typical UCP signal
in myocytes (×80). Results are representative of 3 mice.
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Fig. 5.
Detection of slow-twitch fibers in gastrocnemius muscle of an obese MSG
mouse treated with CL using anti-myosin heavy chain. Results are
representative of 3 mice. A: anti-UCP
stain; B: anti-myosin heavy chain.
Bar, 20 µm.
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Fig. 6.
Immunoelectron microscopy of UCP in skeletal muscle. An ultrathin
section of gastrocnemius muscle of an obese MSG mouse treated with CL
was stained by a rabbit antiserum against rat UCP and colloidal
gold-labeled anti-rabbit immunoglobulin G F(ab')2 (10 nm).
Results are representative of 3 mice. M, mitochondria; MF, myofibril;
Z, Z line; G, glycogen particles. Bar, 0.1 mm.
|
|
The activity of UCP in the fat pads and muscle of the MSG obese mice
was estimated from the specific GDP binding to the mitochondrial fractions prepared from these tissues. As shown in Table 1, the GDP
binding to the mitochondrial fraction of the interscapular BAT was
114.5 ± 8.9 pmol/mg mitoprotein in the distilled water-treated MSG
obese mice; it increased by 2.3-fold in the CL-treated obese mice
(266.3 ± 10.3 pmol/mg). The mitochondrial preparations from the
inguinal and retroperitoneal fat pads and those from the gastrocnemius muscle showed undetectable GDP binding (<10 pmol/mg mitoprotein) in
the distilled water-treated mice, but in the CL-treated MSG obese mice,
significant binding values of 66.9 ± 7.1, 55.0 ± 9.3, and 20.4 ± 3.8 pmol/mg were observed in the inguinal and retroperitoneal WATs and
gastrocnemius muscle, respectively, and in the CL-treated lean mice at
50.8 ± 7.6, 32.7 ± 7.5, and 12.1 ± 3.1, respectively (Table 1).
 |
DISCUSSION |
The present results showed that the chronic treatment of nongenetic
hypothalamic MSG obese mice with a selective
3-AR agonist, CL, resulted in
the significant expression of UCP in the inguinal and retroperitoneal
fat pads (which are conventionally considered white adipose tissue) and
also in skeletal muscle. These results are similar to those obtained
with genetic obese yellow KK mice (21), confirming the ectopic
expression of UCP after chronic treatment with a
3-AR agonist, regardless of the
type of obesity. A similar occurrence of UCP in various fat pads other
than typical BAT was reported in rats exposed to cold (7) or treated
with CL (13, 27), and also in dogs treated with another
3-AR agonist, ICI-D7114 (6).
The present immunohistochemical examinations showed that the
UCP-expressing fat pads contained numerous multilocular cells
indistinguishable from typical brown adipocytes. The fat pads also
contained many typical unilocular "white" fat cells, but they
were always negative for UCP signals. Thus the
3-AR agonist treatment resulted
in the appearance of "brown" adipocytes expressing UCP in adipose
deposits usually considered white fat.
As in yellow KK mice (21), the CL-induced ectopic expression of UCP was
more obvious in the MSG obese mice than in the lean controls. The
effect of the
3-AR agonist on
body adiposity was also more remarkable in the obese animals. Although
the reason for these differences is not clear at present, it may be
related to the different sympathetic nerve activity: that is, both
MSG-treated (28, 29) and yellow KK obese mice (23) had reduced
sympathetic nerve activity compared with their respective lean
controls. Thus the obese mice could be much more sensitive to
adrenergic stimulation.
Several groups (4, 11) have recently cloned genes that code for the
novel uncoupling proteins UCP-2 and UCP-3, which have 59 and 57% amino
acid identity to the conventional UCP (UCP-1), respectively. UCP-2 is
expressed in a wide variety of tissues, including adipose tissues,
lung, and skeletal muscle, whereas UCP-3 is specific to BAT and
skeletal muscle. Thus it may be possible that the UCP signals detected
in the tissues other than typical BAT are those of UCP-2 and UCP-3
rather than UCP-1. However, this is unlikely, because
1) in the present study the signals
were not detected in any tissues except BAT in the lean control animals without CL treatment, and 2) the
ectopic signals were found only after the CL treatment. The UCP-2
expression was reported not to be influenced by
3-AR agonist treatments (11),
although the relationship between UCP-3 and the
3-AR agonist is unclear. The
relative importance of UCP-1, UCP-2, and UCP-3 in regulatory thermogenesis remains to be clarified in further studies.
The thermogenic activity of UCP can be estimated by measuring the
GDP-binding activity of mitochondrial preparation in vitro, although
the GDP binding is not always parallel to the amount of UCP because of
the "unmasking" phenomenon (12, 22). In the present study, we
found that the GDP binding of the mitochondrial preparation of
interscapular BAT was increased more than twofold by the CL treatment,
in parallel with the increase in immunoreactive UCP. In the control
untreated mice, the mitochondrial fractions prepared from the inguinal
and retroperitoneal fat pads showed negligible GDP-binding activity,
below the limit of detection (10 pmol/mg mitochondrial protein).
However, in the obese mice treated with CL, the mitochondrial
preparations from the fat pads showed the GDP binding at about
one-fourth the activity of BAT. These results suggest that the
mitochondria of the inguinal and retroperitoneal fat pads of CL-treated
mice are also thermogenically active because of the significant
expressions of UCP. A low but significant GDP binding was also detected
in the fat pads of lean control mice treated with CL, although Western
and Northern blot analyses detected no UCP signals in the
retroperitoneal fat pad. A possible reason for this discrepancy may be
the difference in sensitivity of these two methods. Alternatively, part
of the GDP binding estimated in the present study may be due to the GDP
binding by UCP-2 and/or UCP-3. Further studies are necessary to
determine the relative contribution of the individual UCP isoforms to
the total mitochondrial GDP-binding activity.
The immunohistochemical examination of the skeletal muscles of the
present CL-treated mice revealed that UCP signals are localized in
slow-twitch fibers, which are known to be rich in mitochondria (8, 9).
The mitochondrial localization of UCP in the myocytes was also
confirmed by immunoelectron microscopy. These results suggest that the
UCP in muscles, as well as that in adipocytes, is also functionally
active in mitochondrial uncoupling. In fact, a very low but significant
GDP-binding activity was detected in the mitochondrial fractions of the
gastrocnemius muscles of the CL-treated obese mice. It is difficult to
evaluate the contribution of the ectopically expressed UCP in white fat
and skeletal muscle to whole body thermogenesis, but a rough estimation
is possible on the basis of data shown in Table 1: in the CL-treated
obese mice, provided that the total weights of BAT, white fat, and
skeletal muscle are ~0.3, 10, and 15 g, respectively, the total GDP
bindings could be expected to be estimated as ~2,200, 731, and 343 pmol. Such a rough estimation suggests a significant contribution of the ectopically expressed UCP to increased energy expediture and thereby to the anti-obese effect of CL.
In conclusion, the chronic CL treatment of MSG-treated hypothalamic
obese mice induced the increased expression of UCP in typical BAT but
also the ectopic expression of UCP in fat pads usually considered WAT
and also in skeletal muscle, similar to the results obtained in
genetically obese yellow KK mice. Because the ectopically expressed UCP
is functionally active in mitochondrial GDP binding, this UCP, in
addition to the UCP in BAT, may also contribute to the increased
thermogenesis and antiobese effect of the
3-AR agonist CL.
Unlike the findings in rodents, the contribution of BAT has
been considered to be negligible in humans because of the apparent lack
of BAT and UCP in adults. However, the present findings imply that the
expression of the UCP gene can be activated in the white fat and even
in skeletal muscle by
3-adrenergic stimulation.
Although it remains to be examined whether this is also the case in
humans, our results undoubtedly suggest that
3-AR and UCP are important
targets for the treatment of human obesity, as for rodent
obesity. This is supported further by recent findings of the
presence of UCP isoforms UCP-2 (11) and UCP-3 (4) in humans, and also
by those demonstrating a role of
3-AR in human obesity (18).
 |
ACKNOWLEDGEMENTS |
We are grateful to American Cyanamid Co. for providing the
CL-316,243, and to Dr. D. Ricquier (Centre National de la Recherche Scientifique, Meudon, France) for the kind gift of the UCP cDNA.
 |
FOOTNOTES |
This work was supported in part by Grants-in-Aid 09671067 and 07671149 for scientific research from the Ministry of Education, Science,
Culture and Sports of Japan, and by The Smoking Research Foundation,
Japan.
Address for reprint requests: T. Yoshida, First Dept. of Internal
Medicine, Kyoto Prefectural Univ. of Medicine, 465-Kajiicho,
Kawaramachi-Hirokoji, Kamikyo-ku, Kyoto 602, Japan.
Received 25 June 1997; accepted in final form 26 November 1997.
 |
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