1Division of Endocrinology, Metabolism, and Molecular Medicine and 2Research Centers in Minority Institutions, Charles R. Drew University of Medicine and Science, Los Angeles, California 90059
Submitted 7 November 2002 ; accepted in final form 21 April 2003
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ABSTRACT |
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regulation; RU-486
Myostatin, formerly known as growth and differentiation factor 8, a member
of the transforming growth factor- superfamily, is an important negative
regulator of skeletal muscle mass. Disrupted myostatin gene expression, either
by gene targeting in mice or as a consequence of naturally occurring mutations
in cattle, is associated with increased skeletal muscle mass resulting from
muscle fiber hyperplasia as well as hypertrophy
(17,
24,
25,
30,
32). Conversely, increased
serum myostatin concentration is associated with a loss of skeletal muscle
mass in men with the AIDS wasting syndrome
(11). Increases in myostatin
expression have also been reported in experimental animals after exposure to
microgravity during space flight
(22) and after hindlimb
suspension (6,
48). We
(44) and other investigators
(45) have demonstrated that
recombinant myostatin protein inhibits C2C12 muscle cell
proliferation and protein synthesis in vitro. The mechanisms that regulate
myostatin gene expression remain poorly understood.
To investigate the regulation of myostatin gene expression, we recently cloned and characterized the 5'-upstream regulatory region of the human myostatin gene and found that the promoter contains a number of response elements important for muscle growth, including seven putative glucocorticoid response elements (GREs). We also demonstrated that dexamethasone dose-dependently increases endogenous myostatin transcription in C2C12 myocytes through a glucocorticoid receptor-mediated mechanism (1, 26). These findings led us to postulate that the increase in myostatin gene expression by glucocorticoids might contribute to the pathogenesis of glucocorticoid-induced skeletal muscle atrophy. In the present study, we tested this hypothesis in vivo in a rat model. First, we determined the effects of graded doses of dexamethasone on intramuscular myostatin gene expression. Second, we investigated the time course of dexamethasone effect on intramuscular myostatin gene expression. Finally, we examined whether the effects of dexamethasone on myostatin mRNA and protein can be reversed by concomitant administration of a glucocorticoid antagonist, RU-486.
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MATERIALS AND METHODS |
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Study 1. We examined effects of different doses of dexamethasone administered intraperitoneally for 5 days on body weight, skeletal muscle weight, and intramuscular myostatin protein and mRNA expression. Sixty adult male rats were divided into six groups with even initial body weight distribution in each group. Three randomly selected groups, I, II, and III, received intraperitoneal injections of 1 ml of dexamethasone (Sigma) dissolved in saline (0.85% NaCl) at daily doses of 60, 600, and 1,200 µg/kg body wt, respectively. These doses were selected because administration of dexamethasone at 600 µg/kg body wt has been shown previously to cause muscle atrophy (12, 40). For each dose experiment, there was a control group also randomly chosen from the three groups. Thus each control animal, receiving a daily intraperitoneal injection of 1 ml of saline (vehicle, 0.85% NaCl) had a pair mate (matched by body weight) in the corresponding group receiving dexamethasone treatment. All dexamethasone-treated animals were allowed free access to food and water. Dexamethasone treatment has been reported to decrease food intake (40). It is not known whether food intake can affect myostatin expression independently in animals; therefore, each control animal was pair fed the same amount of food as was consumed by its dexamethasone-treated pair mate during the previous day.
Study 2. Forty rats were divided into four groups with equal total body weight distribution in each group. Two groups were randomly selected to receive dexamethasone treatment (600 µg/kg body wt) intraperitoneally daily for either 5 days (group I) or 10 days (group II), respectively. The 600 µg/kg body wt dose was selected because this dose resulted in significant loss of muscle mass in study 1. The remaining animals, used as controls, were randomly assigned to one of the two dexamethasone-treated groups. Each control animal, receiving daily intraperitoneal injection of vehicle (1 ml of 0.85% NaCl) had a pair mate (matched by body weight) in the corresponding dexamethasone-treated group. All dexamethasone-treated animals were allowed free access to food and water. Each control animal was allowed free access to water but pair fed the same amount of food as was consumed by its dexamethasone-treated pair mate during the previous day.
Study 3. We examined whether the glucocorticoid receptor antagonist RU-486 could inhibit the effect of dexamethasone on intramuscular myostatin protein and mRNA expression. In this experiment, 50 rats were divided into five groups with an even distribution of initial body weight in each group. The animals were randomly selected for receiving the following treatments for five consecutive days. Group Dex received dexamethasone (600 µg/kg body wt) daily; group Dex/RU-486 received dexamethasone (600 µg/kg body wt) and RU-486 (1,313 µg/kg body wt or 2x molarity of dexamethasone) daily; group RU-486 received RU-486 (1,313 µg/kg body wt) daily; control group PC was given daily saline injections and pair fed; and control group FC was given daily saline injections and free access to food. Animals in groups Dex and FC were allowed free access to the food and water, and their food intakes were recorded daily. Individuals in groups Dex/RU-486, RU-486, and PC were allowed free access to water but were pair fed with the same amount of food as was consumed by their dexamethasone-treated pair mates. In all studies, each individual's body weight was recorded daily.
Muscle collection and calculation of change of muscle mass. After
each experimental protocol, animals were killed by CO2
asphyxiation. The muscles of the gastrocnemius and flexor digitorum
superficialis (G/FDS) complex from both legs were excised together, weighed on
an analytical balance, and quickly frozen on dry ice and stored at -80°C
for subsequent RNA and protein extractions. Although we tried to group animals
with an even distribution of initial body weight, there were still minor
differences in the averages of initial total body weights among experimental
groups. Because the wet muscle weight can be measured only at the end of each
experiment, we adjusted the average muscle weight of each experimental group
by multiplying its measured average muscle weight (MW) by the ratio of its
initial average body weight (BW) vs. that of the control group, i.e.
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Northern blot analysis. Total RNA was extracted from the G/FDS
muscle complex with TRIzol (GIBCO-BRL), following the instructions provided by
the manufacturer. The extracted RNA pellets were dissolved in diethyl
pyrocarbonate-treated water. After the concentrations were determined by a
spectrometer at 260 nm, all RNA samples were kept at -80°C until use. To
determine the relative abundance of myostatin and the reference gene
glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNAs, 25 µg of
heat-denatured total RNA for each muscle were electrophoresed (4 V/cm for 4 h)
on a 0.8% agarose denaturing gel prepared in MOPS [40 mM
3-(N-morpholino)-2-hydroxypropanesulfonic acid], 10 mM sodium
acetate, 1 mM EDTA, and 6.6% formaldehyde (all from Sigma). The RNA was
transferred to a Hybond-XL nylon membrane (Amersham) by overnight capillary
action in 20x SSC (3 M NaCl + 0.3 M Na3citrate, pH 7.0).
Membranes were treated with ultraviolet cross-link (UV Stratelinker 1800;
Strategene) at 70,000 µJ, as recommended by the manufacturer. Membranes
that carried the RNA samples were first prehybridized in hybridization buffer
(5x Denhardt's, 6x SSC, 2.5% SDS, and 100 µg/ml denatured,
sonicated salmon sperm DNA, 5 ml/100 cm2 filters) in hybridization
bottles at 60°C for 2 h. DNA probes were radiolabeled by random
priming with [
-32P]dCTP (ICN) with a High Prime kit (Roche),
following the manufacturer's instructions. For myostatin mRNA detection, a rat
myostatin cDNA fragment of 748 bp (corresponding to nt 178925) was used
as the probe. For GAPDH mRNA detection, a rat GAPDH cDNA fragment of 1.2 kb
was used as the probe.
A radiolabeled probe was boiled for 5 min before being added to the prehybridized membranes at 60°C. After being incubated with the probe overnight at 60°C, the membranes were washed (in SSC + 0.1% SDS at 60°C) to the desired stringency by reducing the SSC concentration. The membranes were then exposed to Kodak Biomax film for 25 days at -80°C to visualize bands corresponding to myostatin and GAPDH transcripts. Concentrations of myostatin and GAPDH mRNA were quantified by densitometry scanning (Fluor-s Multiimager; Bio-Rad, Hercules, CA). The uniformity of loadings was adjusted by the optical density (OD) of the GAPDH band.
Western blot analysis. Protein was extracted from G/FDS complex by
means of a denaturing-reducing lysis buffer containing 1% SDS, Tris ·
HCl, and a 1:20 dilution of -mercaptoethanol. Samples (40 µg each)
were heat denatured (95°C for 5 min), electrophoretically separated using
12% Trisglycine polyacrylamide gels (ReadyGel; Bio-Rad), and the proteins were
visualized using Coomassie brilliant blue staining. The electrophoretically
separated samples were then transferred to a nitrocellulose membrane
(Hybond-ECL; Amersham) and immunodetected using the previously described
procedure with the myostatin polyclonal antibody "B"
(11). An anti-rabbit-IgG
secondary antibody linked to horse-radish peroxidase (HRP) was used. Blots
were developed with an enhanced chemiluminescent substrate for HRP and exposed
to film (ECL Hyperfilm; Amersham). To adjust the protein sample loading, the
membranes were stripped by incubating them in stripping buffer (100 mM
-mercaptoethanol, 2% SDS, 62.5 mM Tris · HCl, pH 6.7) at 50°C
for 30 min and then reprobing them with a rat monoclonal antibody for GAPDH.
The concentrations of myostatin and GAPDH proteins were quantified by
densitometry scanning (Fluor-s Multiimager). The uniformity of loadings was
adjusted by the OD of the GAPDH band.
Separation and quantitation of MHC isoforms. The MHC isoforms of myofibrils were separated by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) according to the method developed by Talmadge and Roy (43). Each gel was composed of two continuous parts, stacking gel (4% acrylamide, 30% glycerol) and separating gel (8% acrylamide, 30% glycerol). SDS-PAGE was run at a constant voltage of 70 V at 4°C for 24 h. The gel was then stained with Coomasie blue. The concentrations of the MHC isoforms were quantified by densitometry scanning (Fluor-s Multiimager).
Statistical analysis. All data are reported as means ± SE. All statistical tests were performed using the Jandel Sigma-Stat statistical software package. P < 0.05 is taken as the level of statistical significance. For study 1, the key outcome variables were body weight, weights of G/FDS complex, and myostatin protein and mRNA concentrations, corrected by GAPDH protein and mRNA. Treatment effects were analyzed by use of a two-way ANOVA model with factors for dose (60, 600, or 1,200 µg/kg body wt) and treatment (saline or dexamethasone). Comparisons between groups were performed with Tukey's test. For study 2, the data were analyzed by use of a two-way ANOVA model with factors for treatment (saline or dexamethasone) and treatment duration (5 or 10 days). Comparisons between groups were performed with the Tukey test. For study 3, the treatment effects in the five groups were analyzed with a one-way ANOVA model. Comparisons between groups were performed with Tukey's test.
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RESULTS |
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Administration of dexamethasone caused a progressive, dose-dependent loss of body weight in rats (Table 1). The mean body weight reductions from baseline in dexamethasone-treated rats in groups I, II, and III (-4.0 ± 0.4, -13.4 ± 0.3, and -17.2 ± 0.4%, respectively) were significantly greater than those in the corresponding pair-fed controls (0.5 ± 0.4, -1.2 ± 0.3, and -4.5 ± 0.4%, respectively, P < 0.05; Table 1). The loss of body weight in pair-fed controls was apparently due to the restriction of their food intake.
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The mean mass of the G/FDS complex was significantly lower in dexamethasone-treated rats than that in pair-fed controls within each dose group (Table 1). The rats treated with 600 and 1,200 µg/kg body wt daily had significantly lower muscle mass than those treated with the 60 µg/kg body wt (P < 0.05 for each comparison). As shown in Table. 1, the losses of wet weight of G/FDS complex in dexamethasone-treated rats from groups I, II, and III were significantly (6.3 ± 0.4, 15.0 ± 0.3, and 16.6 ± 0.4%, respectively, all P < 0.05) higher than those of their corresponding pair-fed controls. Because the 600 µg/kg body wt dose produced near-maximal loss of muscle mass, we selected this dose in the subsequent time course experiments.
Dexamethasone treatment increases intramuscular myostatin gene expression. To determine whether dexamethasone administration affects myostatin expression in skeletal muscle from rats, we measured myostatin protein and mRNA concentrations in the G/FDS complex. The intramuscular myostatin mRNA concentrations, indicated by the abundance of a 2.9-kb transcript by Northern blot analysis, were significantly higher in dexamethasone-treated animals than in the corresponding pair-fed controls (P < 0.01). The dexamethasone-induced increase in myostatin mRNA expression in rats was dose dependent. In rats that received five consecutive daily injections of dexamethasone at 60, 600, or 1,200 µg/kg body wt, the myostatin mRNA expression in the G/FDS complexes was 66.3 ± 0.60, 450 ± 72.4 (P < 0.01), and 527.6 ± 79.8% (P < 0.01) higher, respectively, compared with their corresponding pair-fed controls (Fig. 1). Similarly, dexamethasone treatment also dose-dependently increased intramuscular myostatin protein expression. The myostatin protein expression, indicated by the abundance of a 30-kDa immunoreactive band by Western blot analysis in dexamethasone-treated rats at 600 or 1,200 µg/kg body wt, was 260.5 ± 22.3% (P < 0.01) or 318.4 ± 44.8% (P < 0.01) higher than that in their corresponding controls (Fig. 1). In contrast, rats treated with 60 µg/kg body wt dexamethasone showed no significant difference in myostatin protein expression compared with controls (Fig. 1). The sizes of the myostatin mRNA transcript and myostatin immunoreactive band detected in this study are consistent with the previously reported findings (6, 11, 48).
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Study 2: time course of dexamethasone's effects on body and muscle weight and intramuscular myostatin gene expression. To investigate whether extension of dexamethasone treatment would affect muscle weights and intramuscular myostatin protein and mRNA expression, we treated the rats with a daily injection of 600 µg/kg body wt dexamethasone for either 5 days (group I) or 10 days (group II). Again, dexamethasone administration induced a greater loss of body weight in both groups compared with their corresponding pair-fed controls. The average body weight changes in dexamethasone-treated rats from groups I and II, compared with their initial body weights, were -13.4 ± 0.6 (P < 0.05) and -15.3 ± 0.9% (P < 0.05), respectively. In contrast, the body weight changes of their corresponding pair-fed controls were -1.21 ± 1.0 and 0.42 ± 0.8%, respectively. However, the dexamethasone-induced changes in body weights between animals treated for 5 days and for 10 days were not significantly different from each other.
Overall, the average wet weights of G/FDS complex in dexamethasone-treated rats were 13.9 ± 0.71 and 16.5 ± 1.2% lower after 5 and 10 days of treatment compared with their pair-fed controls (P < 0.05 for each of the two comparisons). However, the wet weights of the G/FDS complex in rats treated with dexamethasone for 5 days (4.0 ± 0.2 g) were not significantly different from those in rats treated for 10 days (3.9 ± 0.3 g; Table 2). Interestingly, rats treated with dexamethasone for 5 days showed a marked decrease of 42.6% (P < 0.05) in the concentration of MHC II(A, X, B) isoforms in the G/FDS complex compared with the pair-fed controls. However, the concentration of MHC II(A, X, B) isoforms in animals treated with dexamethasone for 10 days was only 14.0% (P < 0.05) lower than that in the pair-fed controls (Fig. 2). No significant change was detected for MHC I in either dexamethasone-treated group.
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As expected, the intramuscular myostatin mRNA expression in rats treated with dexamethasone for 5 days was (densitometry intensity reading 32.3 ± 1.5) significantly (4.50-fold, P < 0.01) higher than that (densitometry intensity reading 5.9 ± 1.4) in their pair-fed controls. The myostatin protein expression in these rats was (densitometry intensity reading 19.2 ± 1.4) also significantly higher (2.6-fold, P < 0.01) than that in their pair-fed controls (densitometry intensity reading 5.3 ± 1.0; Fig. 3). However, after 10 days of treatment, intramuscular myostatin protein concentrations were not significantly different between the control and dexamethasone-treated rats (6.3 ± 1.5 vs. 5.8 ± 1.5, P = 0.82 for mRNA and 6.0 ± 1.1 vs. 5.7 ± 1.0, P = 0.86 for protein; Fig. 3).
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Study 3: RU-486 inhibits dexamethasone's effect on body and muscle weights and myostatin gene expression. RU-486 is known to antagonize the effects of glucocorticoids by binding to the glucocorticoid receptors (20, 21, 28). To determine whether RU-486 antagonizes the effects of dexamethasone on myostatin mRNA and protein expression in vivo, we divided rats into five groups of equal body weight distribution. Three randomly selected groups were treated with dexamethasone (600 µg/kg body wt), dexamethasone (600 µg/kg body wt) plus RU-486 (1,313 µg/kg body wt), or RU-486 (1,313 µg/kg body wt) alone, respectively, for 5 days. To determine whether restriction of food intake could influence myostatin expression, we set up two groups of animals as controls, the pair-fed control (PC) and the free-access-to-food control (FC). Animals from both groups were given daily intraperitoneal injections of the vehicle (0.85% NaCl). The animals were killed after five consecutive daily treatments. Changes in their body weight, G/FDS mass, and myostatin gene expression were analyzed.
As shown in Table 3, animals in the FC group gained on average 7.8 ± 0.2% in body weight (P < 0.05) compared with their initial body weight, whereas the PC group lost 1.2 ± 0.3% (P < 0.05) in body weight. Dexamethasone-treated rats showed a much greater loss in body weight (-13.4 ± 0.3%, P < 0.05) compared with the PC group. Interestingly, Dex/RU-486 animals had significantly fewer losses in body weight (-6.4 ± 0.25%, P < 0.05) than those treated with dexamethasone alone, even though both groups of rats were fed the same amount of food. The change of body weight in the RU-486 group showed no significant difference from that in the PC group (Table 3).
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The changes of body weights in the animals were mirrored by the wet muscle weights of each corresponding group. The average wet weight of G/FDS complex of the dexamethasone-treated rats (3.8 ± 0.1 g) was significantly lower than those of groups FC (4.9 ± 0.1 g, P < 0.05), PC (4.5 ± 0.1 g, P < 0.05), Dex/RU-486 (4.1 ± 0.1 g, P < 0.05), and RU-486 (4.5 ± 0.2, P < 0.05), respectively. There was no significant difference between the weights of wet G/FDS complex in the PC rats and in those treated with RU-486 alone (Table 3).
To determine whether RU-486 could alter the dexamethasone-induced increase
in intramuscular myostatin expression in animals, we examined the mRNA and
protein expression in the G/FDS complex from these animals by using Northern
and Western blot analyses. As displayed in
Fig. 4, we found no significant
difference in the intramuscular myostatin mRNA expression among animals from
Groups FC, PC, and RU-486 (densitometry intensity readings 5.9 ± 0.9,
5.8 ± 0.9, and 5.6 ± 1.0, respectively, P < 0.05).
The intramuscular myostatin mRNA expressions were 4.5-fold higher in
dexamethasone-treated rats compared with their pair-fed controls (densitometry
intensity readings 32.3 ± 1.6 vs. 5.8 ± 02, P <
0.01). The action of dexamethasone treatment on myostatin mRNA expression was
effectively nullified by RU-486 administration, as the intramuscular myostatin
mRNA expression in Dex/RU-486 rats (densitometry intensity reading 6.4
± 1.0, P < 0.05) showed no significant difference compared
with that in groups PC or FC (Fig.
4A).
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Similarly, there was no significant difference in the intramuscular myostatin protein expression among animals from the FC, PC, and RU-486 groups (densitometry intensity readings 5.3 ± 0.3, 5.3 ± 0.2, and 5.4 ± 0.3, respectively, P < 0.05). The intramuscular myostatin protein expression in the dexamethasone-treated rats (densitometry intensity reading 19.2 ± 1.4, P < 0.01) was significantly higher than those in groups FC (5.3 ± 0.2, P < 0.05), PC (5.3 ± 0.2, P < 0.05), and RU-486 (5.2 ± 0.2, P < 0.05). Again, the dexamethasone-induced increase in myostatin protein expression was completely blocked by RU-486, because the myostatin protein expression (densitometry intensity reading 5.4 ± 0.3, P < 0.05) in animals concurrently administered dexamethasone plus RU-486 showed no significant difference from that in the PC group (Fig. 4B).
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DISCUSSION |
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Because loss of body weight during dexamethasone administration is largely attributed to skeletal muscle atrophy (10, 12, 39, 41), it was not surprising that the reduction in total body weight found in our dexamethasone-treated rats was proportionately less than the associated loss of G/FDS mass. The mechanism(s) by which glucocorticoids induce such catabolic and anti-anabolic effects on muscle remains unclear. Previous studies have shown that glucocorticoids promote protein degradation and impair protein synthesis, which result in muscle atrophy (2, 10, 12, 39, 41); however, the precise molecular mechanisms of these actions are yet to be elucidated.
A large body of emerging data suggests that myostatin is an important negative regulator of skeletal muscle mass (11, 17, 30, 32, 36, 37, 44, 45). Disruption of myostatin gene expression is associated with dramatic increases in skeletal muscle mass due to muscle fiber hyperplasia and/or hypertrophy (13, 17, 30, 32). Conversely, increased myostatin levels are associated with loss of skeletal muscle mass in conditions as disparate as AIDS wasting syndrome (11), exposure to microgravity during space flight (22), and hindlimb suspension (6, 48). Glucocorticoid-induced protein degradation resulting in muscle atrophy is known to occur predominantly in fast-twitch muscle fibers (MHC IIB), (9, 39, 47). Interestingly, myostatin gene expression appears to be much higher in muscles (e.g., G/FDS complex and extensor digitorum longus), which are mainly composed of fast-twitch fibers, than in those composed of slow-twitch fibers (e.g., soleus) (6, 18, 48). Our previous in vitro observations that recombinant myostatin protein inhibits muscle cell proliferation and protein synthesis (36, 37, 44, 45) suggest that it may have an antianabolic effect.
Previously, we identified multiple putative GREs in the human myostatin gene (26) and demonstrated that, in vitro, dexamethasone dose-dependently upregulated endogenous myostatin mRNA and protein expression by increasing myostatin gene transcription (1, 26). Our present in vivo observation emulates the previous finding (i.e., dexamethasone treatment resulted in significant dose-dependent increase in myostatin expression and a rapid decline of G/FDS mass), suggesting that dexamethasone-induced muscle atrophy is also associated with the upregulation of myostatin expression.
RU-486 inhibits glucocorticoid receptor binding and thus prevents dexamethasone-induced muscle atrophy (14, 20, 46). In the present study, dexamethasone's effects on skeletal muscle mass and myostatin protein and mRNA concentrations were antagonized by concurrent treatment with RU-486. This suggests that dexamethasone affects myostatin expression predominantly through a glucocorticoid receptor pathway. Although dexamethasone's effects on myostatin protein and mRNA were almost completely blocked by RU-486 (Fig. 4), its effects on body weight and skeletal muscle mass, albeit significantly reduced, were not completely prevented (Table 3). Similar partial attenuation of glucocorticoid action has been reported previously (14, 20, 46).
In a previous study, rats given a single injection of dexamethasone showed increases in myostatin mRNA expression of 60 and 270% 4 and 24 h, respectively, after treatment (23). Our study expands this finding, showing that dexamethasone-induced upregulation of myostatin is not only dose dependent but also time dependent. We found that, although the marked upregulation of myostatin mRNA and protein expression induced by dexamethasone could be seen for 5 days after treatment, this hyperexpression was not sustained by extending the treatment to 10 days (Fig. 2). The changes in myostatin expression coincided with those in muscle weight and MHC II. The majority of muscle loss occurred in the first 5 days of treatment, whereas animals treated for 10 days did not experience significantly greater muscle loss than those treated for 5 days (Table 2). Carlson et al. (6) also reported a significant upregulation of intramuscular myostatin after the first day of hindlimb unloading but a return to basal levels after 37 days of hindlimb suspension. Rats given high doses of corticosterone have significantly increased muscle tyrosine levels, an important indicator of net protein degradation (5, 15, 19, 35), after 25 days of treatment; however, after 5 days, the changes in tyrosine release are not sustained (5). Glucocorticoids are known to upregulate proteolysis and stimulate the expression of components of the ubiquitin-proteolytic pathways in both skeletal muscle and myocytes (8, 15, 27, 31, 34, 46); however, significant changes in ubiquitin are not observed after long-term glucocorticoid excess (36). The molecular adaptations that prevent continued loss of muscle mass and attenuate increases in myostatin expression during long-term exposure to catabolic stimuli remain unknown.
In addition to multiple GREs, the myostatin promoter contains other
elements important for muscle growth. The presence of myocyte enhancer
factor-2 and nuclear factor-B, among others
(26), indicates that myostatin
is probably regulated not only by glucocorticoids but also by other factors.
The role of these putative factors in acute and chronic regulation of
myostatin gene expression during illness and other catabolic states remains to
be elucidated.
In summary, we have demonstrated that dexamethasone dose- and time-dependently induces loss of body and muscle weights, upregulates intramuscular myostatin protein and mRNA concentrations, and is associated with a decline in MHC II expression. These effects can be blocked by the addition of RU-486, suggesting that myostatin expression is mediated via glucocorticoid receptors. We propose that muscle loss associated with dexamethasone administration is mediated, in part, by the upregulation of myostatin expression through a glucocorticoid receptor-mediated pathway. The speculation that dexamethasone induces muscle loss by upregulating myostatin transcription should now be further tested.
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DISCLOSURE |
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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REFERENCES |
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