Departments of Biomedical Sciences and Physiology, and Dalton Cardiovascular Institute, University of Missouri, Columbia, Missouri 65211
Submitted 15 October 2002 ; accepted in final form 16 April 2003
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() |
---|
skeletal muscle; hypertrophy; rehabilitation; atrophy
Generally, increases in the translation of mixed proteins will occur before increases in specific structural or metabolic mRNAs during exercise-induced muscle growth (see Ref. 8). Consequently, the hypothesis was tested that alterations in molecules of signaling pathways identified in the regulation of protein translation would be present during early time points of skeletal muscle regrowth, whereas signaling events for transcriptional signals would be activated later in the regrowth process. Therefore, we sought to explore the activation of signaling pathways that occurs during intermediate stages of the regrowth process by utilizing a model in which atrophy of the soleus muscle of the rat is induced by immobilization of the hindlimbs. After 10 days of immobilization, the restrictive stimulus was removed and the animals were allowed to freely ambulate around the cage. In previous studies using this model, normal ambulation has been shown to be an adequate stimulus for soleus muscle recovery to preatrophy size (6). At various time points, the soleus muscle was removed and activation of various signaling pathways thought to be important in the hypertrophic process was determined.
![]() |
METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() |
---|
Hindlimb immobilization. The hindlimb immobilization of the rats was performed according to the previously described procedures (7). Rats were lightly anesthetized with an intraperitoneal injection of ketamine (50 mg/kg) and acepromazine (5 mg/kg) for attachment of the casting material. The hindlimbs of the animals were fixed in a shortened position with plaster casts as described previously (5). The animals were then checked daily for damage to casting material, which was subsequently repaired as necessary. After 10 days, the animals were anesthetized again, as described above, and the casting material of the recovery groups was removed completely and the animals were returned to their cages. No casting material was removed from the atrophy-only group until the animals were completely sedated to ensure no weight bearing occurred in this group. The anesthetic dosage per kilogram of body weight for dissection and death was ketamine (75 mg), xylazine (3 mg), and acepromazine (5 mg). At the time of death, the soleus muscle was carefully excised, weighed, and frozen in liquid nitrogen.
Protein isolation and concentration. Muscle tissue was prepared by
glass on glass homogenization on ice in 50 mM HEPES (pH 7.4), 0.1% Triton-X, 4
mM EGTA, 10 mM EDTA, 15 mM Na4P2O7 ·
H2O, 100 mM -glycerophosphate, 25 mM NaF, 50 µg/ml
leupeptin, 50 µg/ml pepstatin, 40.4 µg/ml aprotinin, and 1 mM
Na3VO4. After homogenization, the samples were stored in
aliquots at -80°C. The protein concentration of the samples was determined
in triplicate via the Bradford procedure (Bio-Rad Protein Assay, Hercules,
CA).
SDS-PAGE, Western blotting, and immunodetection. Protein samples
were solubilized at a concentration of 1.25 mg/ml in loading buffer (62.5 mM
Tris · HCl, pH 6.8, 20% glycerol, 2% SDS, 5% -mercaptoethanol,
and 0.025% bromphenol blue) and boiled for 5 min. Total protein was then
loaded (100 µg/lane) onto 10% SDS-PAGE gels with these exceptions [25 µg
protein/lane for glycogen synthase kinase (GSK)-3
and 15% SDS-PAGE gels
for 4E-BP1 and calcineurin B (CaNB)]. Separated proteins were then transferred
onto nitrocellulose membranes (Osmonics, Westborough, MA) at 51 V for 2.5 h at
4°C in transfer buffer (25 mM Tris-base, 192 mM glycine, and 20%
methanol). To verify transfer of proteins and equal loading of lanes, which
occurred in all cases, the membranes were stained with Ponceau S. For
immunodetection, membranes were blocked at room temperature in blocking buffer
[5% nonfat dry milk in Tris-buffered saline (TBS)-T (0.1% Tween 20)] for 1 h,
serially washed in TBS-T, and incubated with primary antibody in dilution
buffer (5% BSA in TBS-T) overnight at 4°C. After another serial wash with
TBS-T, the membranes were incubated with horseradish peroxidase
(HRP)-conjugated secondary antibody and antibiotin antibody in blocking buffer
for 1 h, followed by another serial wash with TBS-T. With the use of enhanced
chemiluminescence reagent (Perkin Elmer Life Sciences, Boston, MA), the HRP
activity was detected with exposure to Kodak-XAR5 autoradiographic film for
the appropriate durations to keep the integrated optical densities (IODs)
within a linear and nonsaturated range for all bands of each gel. The IODs
were quantified using Image-Quant densitometry software (Molecular Dynamics,
renamed Amersham Biosciences, Sunnyvale, CA). To correct the sample IODs for
exposure time differences, the same quantity of control muscle sample was
loaded on every gel.
Antibodies. The primary antibodies phosphoSer473-Akt
(1: 500 dilution), Akt (1:1,000), phospho-Ser9-glycogen
synthase-3 (GSK-3
) (1:1,000),
phospho-Thr202/Tyr204 p44/42 mitogen-activated protein
kinase (p44/42 MAPK) (1:1,000), p44/42 MAPK (1:1,000),
phospho-Thr180/Tyr182-p38 MAPK (1:1,000), p38 MAPK
(1:1,000), phospho-Thr183/Tyr185-stress-activated
protein kinase/Janus kinase (SAPK/JNK) (1:500), SAPK/JNK (1:1,000),
phospho-Thr389-p70 S6 kinase (p70S6k) (1:500),
p70S6k (1:500), 4E-BP1 (1:1,000), phospho-Tyr705-signal
transducer and activator of transcription 3 (STAT3) (1:1,000), STAT3
(1:1,000), and anti-biotin antibody (1:4,000) were purchased through Cell
Signaling Technology (Beverly, MA). The antibody for GSK-3
(1:2,500
dilution) was purchased through Transduction Laboratories (Lexington, KY).
Antibodies calcineurin A (1:10,000) and CaNB (1:2,000) were purchased through
Sigma-Aldrich (St. Louis, MO) and ABR (Golden, CO). Anti-rabbit and anti-mouse
secondary antibodies (1:7,500) were purchased from Amersham Biosciences
(Piscataway, NJ).
Statistical analysis. All muscle mass data were analyzed using a one-way analysis of variance and a Tukey's post hoc test (P < 0.05). Not all protein data showed normality and equal variance, so Western data were analyzed utilizing the nonparametric Kruskal-Wallis test, and post hoc analysis was performed using a Dunn's multiple range test with P < 0.01 designated as statistically significant.
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
Change in phosphorylation levels and total protein expression of various signaling proteins in atrophied and regrowing soleus muscle. Phosphorylation of p38 was significantly reduced 98% in all recovery time points from its value at 10 days of immobilization (Fig. 2A). JNK phosphorylation was significantly reduced by 84 and 99% in the recovery time points of 3 and 15 days, respectively, from the 10-day immobilization group (Fig. 2B).
|
Expression levels of phosphorylated Akt decreased significantly by 55 and 59% after 10 days of immobilization, compared with the control and 3-day recovery time points, respectively (Fig. 3A). There was a significant increase in Akt protein concentration by 37% at the sixth day of recovery, compared with the 10-day immobilization group.
|
The phosphorylation of p70S6k was significantly increased by 12- and 21-fold at day 3 of recovery compared with the control and 10-day immobilized groups, respectively (Fig. 3B). p70S6k protein concentration was significantly higher by 102% at day 6 of recovery compared with the 10-day immobilized group.
4E-BP1 phosphorylation was estimated by determination of the protein
concentration of the ,
, and
forms of 4E-BP1
(Fig. 3C). According
to previous work (23,
29),
4E-BP1 exhibits
the least amount of phosphorylation and therefore migrates the fastest,
whereas the
form is the most phosphorylated and migrates the slowest,
and finally the
form exhibits an intermediate amount of phosphorylation
and therefore migrates between the
and
forms. Significant
increases in the
4E-BP1 form [calculated as (
+
)/
]
were detected in the 3-day recovery compared with the 10-day immobilization
(Fig. 3C). In
addition, significant increases in the
4E-BP1 [calculated as (
+
)/
] were detected in the 10-day immobilization group compared
with the control.
Phosphorylated GSK-3 expression was significantly increased by 318
and 261% on days 6 and 15, respectively, compared with the
10-day-immobilized group (Fig.
4). Also, GSK-3
protein concentration was significantly
increased by 102 and 105% on days 6 and 15, respectively,
compared with the immobilized group.
|
STAT3 phosphorylation was 751 and 259% higher in the 3- and 6-day recovery groups, respectively, compared with the 15-day recovery group (Fig. 5A). Total STAT3 protein expression was significantly increased by 253 and 280% in the 3- and 6-day recovery groups, respectively, compared with the 10-day immobilization group.
|
Calcineurin A (CaNA) protein concentration was significantly elevated in the 3-day recovery group compared with the 10-day-immobilized group by 110% (Fig. 5B). CaNB subunit protein concentration was significantly increased in the 3-day recovery compared with the 10-day-immobilized group by 105% (Fig. 5C).
No changes in p44/42 MAPK (ERK1 or 2) phosphorylation status were detected at any time point in immobilized group or any recovery groups (data not shown).
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
Phosphorylated p38 MAPK and p38 MAPK protein concentration were significantly elevated after the 10 days of immobilization and subsequently returned to control in the 15-day recovery period. Hunter et al. (21) found no change in p38 activity and concluded that cytokine-mediated pathways were not activated in the rat soleus muscle on the seventh day of hindlimb unloading. Possibly limb immobilization, which decreases EMG activity to 515% of control (13), activates cytokine signaling pathways, whereas unloading does not increase p38 activity, because no EMG reductions (2) occur at the 7-day time point. Interestingly, JNK followed a trend similar to that of p38, in that the highest expression levels were found at the end of the immobilization period and the expression levels subsequently returned to control during the recovery process. A JNK1-mediated signal cascade has been shown to contribute to the progression of the disease pathogenesis in dystrophic muscle (24). Because environmental stresses and inflammatory cytokines usually simultaneously activate both p38 MAPK and JNK (25), one interpretation may be that regrowth of the soleus muscle removes a stressor associated with inactivity-induced muscle atrophy. An alternative interpretation is based on p38 MAPK overexpression in a transgenic heart. Constitutive activation of p38 MAPK produced interstitial fibrosis, expression of fetal marker genes characteristic of cardiac failure, no significant hypertrophy at the organ level, and premature death at 79 wk (27). Therefore, the possibility exists that p38 and JNK activation plays a role in skeletal muscle atrophy during hindlimb immobilization.
The increases in Akt and p70S6k phosphorylation and in
4E-BP1 in the soleus muscle at the third day of recovery from 10 days of
immobilization-induced atrophy suggest their potential contribution to
increasing protein translation and muscle mass
(Fig. 3C). This
confirms a portion of our hypothesis that signaling related to translation
would increase during early time points of muscle regrowth after ending
immobilization. Others have reported that during the early phases of increased
muscle loading, there are subsequent increases in insulin-like growth factor-1
(IGF-I) protein expression (1).
Akt, p70s6k, and 4E-BP1 are all known to be activated by increases
in IGF-I (26,
36) concentration, so it is
not unreasonable to suggest that early activation of signaling proteins
associated with protein translation may be at least partially mediated by
increases in IGF-I concentration associated with increased muscle loading.
The phosphospecific antibody employed on the third recovery day showed an increased p70S6k Thr389 phosphorylation that others have found necessary for complete enzymatic activation of p70S6k-specific kinase activity (16). Previously, an increase in p70S6k phosphorylation in skeletal muscles 336 h after exercise correlated tightly with the percent change in muscle mass after 6 wk of eccentric-type training against an unloaded antagonistic muscles (3). However, in the current study, p70S6k phosphorylation and p70S6k protein returned from their higher values to control levels on the sixth and the fifteenth days, respectively, of daily voluntary cage activity after limb immobilization, time points that were not examined in the aforementioned eccentric training study (3). Furthermore, Bodine et al. (4) found that a muscle injection of a constitutively active p70S6k induced significant increases in skeletal muscle fiber size in normal and denervated muscle fibers. Thus increased phosphorylation of p70S6k likely plays some role in initiating the regrowth process, but in the regrowth model p70S6k phosphorylation does not appear to play a role in the continuance of soleus muscle regrowth 615 days after immobilization ended.
Interestingly, previous reports have suggested that p70S6k phosphorylation may be regulated by an upstream kinase, termed protein kinase B or Akt, under specific conditions (36). Here, we found increases in p70S6k Thr389-phosphorylation and Akt Ser473-phosphorylation occurring from recovery days 0 to 3, suggesting that Akt may be signaling p70S6k phosphorylation. The lack of a prolonged activation of Akt differs from the findings of Bodine et al. (4), who found that Akt was in fact phosphorylated in the functional overload model at 3, 7, and 14 days, which could be explainable by the more extreme nature of muscle loading in the functional overload model compared the regrowth model.
Alteration in the phosphorylation status of 4E-BP1 also transiently
occurred at the third recovery day. 4E-BP1 has been shown to be resolved into
multiple electrophoretic forms termed ,
, and
,
representing progressively increased phosphorylated forms
(29). In addition, 4E-BP1 is
thought to be activated via Akt and mTOR
(23). Hypophosphorylated
4E-BP1 binds to eIF4E, sequestering this protein in an inactive 4E-BP1
· eIF4E complex. Full phosphorylation of 4E-BP1, the
form,
regulates translation initiation and protein synthesis by inducing the
dissociation of the 4E-BP1 · eIF4E complex.
4E-BP1 increased 147%
on recovery day 3 compared with the 10-day-immobilized value, which
suggests that stimulation of the translational initiation of protein synthesis
may occur early in the regrowth process. The possible early activation of
protein synthesis in the immobilization-recovery model would be in agreement
with the conclusion made by Kimball et al.
(23), who stated that a
consistent finding, despite markedly different experimental models of
exercise, is that muscle protein synthesis is elevated after acute resistance
exercise with no changes in muscle RNA. Thus signaling via Akt,
p70S6k, and 4E-BP1 seems to be early in the regrowth process and
may be related to a stimulation of mRNA translation.
Although 4E-BP increased 15-fold at the end of the 10 days of
immobilization,
4E-BP was barely detectable in the control condition.
It is notable that the only time point in which
4E-BP was visibly
detectable was after the 10 days of immobilization. As protein synthesis rates
are decreased in atrophying immobilized muscles
(34) and muscles from
immobilized limbs have no alteration in insulin sensitivity for protein
synthesis (9), the increase in
phosphorylated
4E-BP1 seems paradoxical. Potentially, other factors
than the regulation of mRNA translation are rate limiting for protein
synthesis in soleus muscle atrophy at the tenth day of immobilization, such as
eIF2B (23), or, as shown,
p70S6k phosphorylation was not increased at this time
(Fig. 3B).
On the third and sixth recovery days, STAT3 phosphorylation at tyrosine residue 705 and protein concentration were significantly increased compared with the 15-day recovery. STAT3 is a transcription factor phosphorylated by JAK2 in response to elevations in concentration of various growth factors such as growth hormone (GH) (20) or leukemia inhibitory factor (LIF) (32). GH exerts many of its physiological functions by regulating the transcription of genes for a variety of proteins, including IGF-1 (20), which increases in hypertrophying skeletal muscle (37). GH also increases the expression of IGF-I mRNA in C2C12 myoblasts (15), which would amplify the growth effect (1). LIF activates satellite cell proliferation, the time course proliferation of which in other skeletal muscle growth models (30) is coincident with increased STAT3 activation (32). Thus the increase in STAT3 could play a role in soleus muscle regrowth.
Mitchell et al. (28) concluded that published reports provide a controversial role for CaN (also called protein phosphatase 2B) in regulating skeletal muscle growth. CaN is a serine/threonine protein phosphatase and the only known phosphatase activated by Ca2+ and calmodulin (CaM). Its 58- to 64-kDa catalytic subunit (CaNA) contains specific domains with regulatory functions, including an autoinhibitory domain near the carboxy terminus, a CaM-binding domain, and a binding domain of a regulatory subunit, CaNB (31). According to Shibasaki et al. (31), Ca2+ binding to CaNB plays a structural rather than a regulatory role by allosterically activating CaNA through conformational folding. In the current experiment, the protein contents of CaNA and CaNB increased only on recovery day 3. Dunn et al. (12) concluded that unless significantly compromised, the endogenous pool of calcineurin accommodates the signaling requirements related to extremes in functional demand in type II muscle, but they did not examine type I muscle as used in the current study. A fiber type differential in endogenous calcineurin expression exists (28, 33). CaNA is expressed at higher levels in skeletal muscles dominated by fast fiber expression (33), whereas CaNB protein expression is higher in muscles composed mostly of slow fibers (28). The transient increases in both CaNA and CaNB proteins in the regrowing soleus muscle thus suggest an additional regulatory mechanism for CaNA and CaNB expression other than fiber type, which confirms the previous observations of Spangenburg et al. (33). Our current observations confirm Mitchell et al.'s (28) statement that CaN-mediated signaling pathways in skeletal muscle may be regulated not only by inductive signals that increase intracellular Ca2+ but also via a complex mechanism involving regulation of expression of CaN subunits.
The increase in GSK-3 phosphorylation and protein at the sixth and
fifteenth days of recovery is intriguing due to GSK-3
's reported role as
a negative regulator of protein translation and gene expression
(19). Phosphorylation of the
serine 9 residue of GSK-3
inhibits its kinase activity, thereby
maintaining NFAT transcriptional activity in C2C12
myotubes (35). In addition,
activated (unphosphorylated) GSK-3
plays a significant role in the
regulation of protein synthesis, in that it has the ability to inhibit the
activity of the protein translation initiation factor eIF2B
(23). Inhibition of
GSK-3
by hypertrophic stimuli has been proposed to be an important
mechanism contributing to the development of cardiac hypertrophy
(19). Application of
inhibitors of GSK-3
activity (i.e., IGF-I or LiCl) produced significant
increases in myotube size in culture
(35). The associative increase
in GSK-3
phosphorylation status and the continued regrowth of the soleus
muscle on recovery days 6 and 15 identifies a potential
signal for continued muscle mass enlargement, confirming another part of our
hypothesis that a longer-acting signal would affect transcriptional
regulation. Because there was no change in Akt phosphorylation levels on
recovery days 6 and 15 when GSK-3
was phosphorylated,
it is likely that a different upstream kinase is regulating GSK-3
in the
regrowing soleus muscle. GSK-3
activity can be inhibited by at least
eight molecules (Akt, Wnt, ILK, PKA, PKC
, LiCl, p90RSK, and
p70S6k) and activated by tyrosine kinase
(19).
An intriguing observation was that seven protein concentrations
significantly increased from values found in control or 10-day-immobilized
groups with their peak values occurring at different recovery times [four on
the third recovery day (STAT3, CaNA, CaNB, and 4E-BP1), three on the
sixth recovery day (Akt, p70S6k, STAT3, and GSK-3
), and one
on the fifteenth recovery day (GSK-3
) (where the significant increases
of STAT3 and GSK-3
concentrations extended for two recovery time
points)]. Thus alterations in signaling during muscle regrowth not only
involve changes in phosphorylation status as suggested for CaN by Mitchell et
al. (28) but also comprise
increases in the protein concentrations for seven other signaling molecules.
Thus sequential changes not only occur for phosphorylation of signaling
proteins but also for concentration of proteins functioning in the signaling,
suggesting multiple adaptive responses to restore the mass of the regrowing
soleus muscle as it recovers from atrophy. The upstream signals coordinating
the staggered transient increases of protein concentrations remain to be
determined.
The experimental design for the current experiment does have limitations. Although the design allows for measurements of multiple signaling proteins that potentially could play some role in regrowth as atrophied soleus muscle recovers from hindlimb immobilization, the size of the soleus muscle limits selection of assays. To test a broad range of molecules for potential alterations during muscle regrowth, soleus muscle mass was only sufficient to perform Western analysis. In addition, arbitrary decisions were made as to the selection of recovery time points.
These data are the first to demonstrate the activation and deactivation of signaling pathways occurs in a specifically designed manner over an extended time course of muscle growth. Not only were crucial steps in multiple signaling pathways altered by phosphorylation and protein expression, but their transient and staggered nature is intriguing because it suggests that a response to a return of muscle usage after atrophy is orchestrated by sequential waves of increased molecules of signaling pathways as the young rat soleus muscle regrows from atrophy. An important prediction from the current observations is that future mapping of the integration among multiple signaling pathways, the activities of which wax and wane at different times during muscle regrowth from atrophy, could be a daunting task.
![]() |
DISCLOSURES |
---|
![]() |
ACKNOWLEDGMENTS |
---|
![]() |
FOOTNOTES |
---|
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.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() |
---|
2. Alford EK, Roy RR, Hodgson JA, and Edgerton VR. Electromyography of rat soleus, medial gastrocnemius, and tibialis anterior during hind limb suspension. Exp Neurol 96: 635649, 1987.[ISI][Medline]
3. Baar K and
Esser K. Phosphorylation of p70S6k correlates with increased
skeletal muscle mass following resistance exercise. Am J Physiol
Cell Physiol 276:
C120C127, 1999.
4. Bodine SC, Stitt TN, Gonzalez M, Kline WO, Stover GL, Bauerlein R, Zlotchenko E, Scrimgeour A, Lawrence JC, Glass DJ, and Yancopoulos GD. Akt/mTOR pathway is a crucial regulator of skeletal muscle hypertrophy and can prevent muscle atrophy in vivo. Nat Cell Biol 3: 10141019, 2001.[ISI][Medline]
5. Booth FW.
Time course of muscular atrophy during immobilization of hindlimbs in rats.
J Appl Physiol 43:
656661, 1977.
6. Booth FW.
Regrowth of atrophied skeletal muscle in adult rats after ending
immobilization. J Appl Physiol
44: 225230,
1978.
7. Booth FW and
Kelso JR. Production of rat muscle atrophy by cast fixation. J
Appl Physiol 34:
404406, 1973.
8. Booth FW and
Thomason DB. Molecular and cellular adaptations of muscle in response to
exercise: perspectives of various models. Physiol Rev
71: 541585,
1991.
9. Butler DT and Booth FW. Muscle atrophy by limb immobilization is not caused by insulin resistance. Horm Metab Res 16: 172174, 1984.[ISI][Medline]
11. Dunn SE, Burns
JL, and Michel RN. Calcineurin is required for skeletal muscle
hypertrophy. J Biol Chem 274:
2190821912, 1999.
12. Dunn SE, Chin
ER, and Michel RN. Matching of calcineurin activity to upstream effectors
is critical for skeletal muscle fiber growth. J Cell
Biol 151:
663672, 2000.
13. Fischbach GD and Robbins N. Changes in contractile properties of disused soleus muscles. J Physiol 201: 305320, 1969.[ISI][Medline]
14. Fluck M, Carson
JA, Gordan SE, Ziemiecki A, and Booth FW. Focal adhesion proteins FAK and
paxillin increase in hypertrophied skeletal muscle. Am J Physiol
Cell Physiol 277:
C152C162, 1999.
15. Frost RA,
Nystrom GJ, and Lang CH. Regulation of IGF-I mRNA and signal transducers
and activators of transcription-3 and -5 (Stat-3 and -5) by GH in C2C12
myoblasts. Endocrinology 143:
492503, 2002.
16. Gonzalez-Garcia A, Garrido E, Hernandez C, Alvarez B, Jimenez C,
Cantrell DA, Pullen N, and Carrera ACA. new role for the
p85-phosphatidylinositol 3-kinase regulatory subunit linking F.RAP to p70 S6
kinase activation. J Biol Chem
277: 15001508,
2002.
17. Gordon SE,
Davis BS, Carlson CJ, and Booth FW. ANG II is required for optimal
overload-induced skeletal muscle hypertrophy. Am J Physiol
Endocrinol Metab 280:
E150E159, 2001.
18. Gordon SE,
Fluck M, and Booth FW. Selected contribution: skeletal muscle focal
adhesion kinase, paxillin, and serum response factor are loading dependent.
J Appl Physiol 90:
11741183, 2001.
19. Hardt SE and
Sadoshima J. Glycogen synthase kinase-3: a novel regulator of
cardiac hypertrophy and development. Circ Res
90: 10551063,
2002.
20. Herrington J and Carter-Su C. Signaling pathways activated by the growth hormone receptor. Trends Endocrinol Metab 12: 252257, 2001.[ISI][Medline]
21. Hunter RB,
Stevenson E, Koncarevic A, Mitchell-Felton H, Essig DA, and Kandarian SC.
Activation of an alternative NF-B pathway in skeletal muscle during
disuse atrophy. FASEB J 16:
529538, 2002.
23. Kimball SR,
Farrell PA, and Jefferson LS. Invited Review: role of insulin in
translational control of protein synthesis in skeletal muscle by amino acids
or exercise. J Appl Physiol 93:
11681180, 2002.
24. Kolodziejczyk SM, Walsh GS, Balazsi K, Seale P, Sandoz J, Hierlihy AM, Rudnicki MA, Chamberlain JS, Miller FD, and Megeney LA. Activation of JNK1 contributes to dystrophic muscle pathogenesis. Curr Biol 11: 12781282, 2001.[ISI][Medline]
25. Kyriakis JM and
Avruch J. Mammalian mitogen-activated protein kinase signal transduction
pathways activated by stress and inflammation. Physiol
Rev 81:
807869, 2001.
26. Li M, Li C, and Parkhouse WS. Differential effects of des IGF-1 on Erks, AKT-1 and P70 S6K activation in mouse skeletal and cardiac muscle. Mol Cell Biochem 236: 115122, 2002.[ISI][Medline]
27. Liao P,
Georgakopoulos D, Kovacs A, Zheng M, Lerner D, Pu H, Saffitz J, Chien K, Xiao
RP, Kass DA, and Wang Y. The in vivo role of p38 MAP kinases in cardiac
remodeling and restrictive cardiomyopathy. Proc Natl Acad Sci
USA 98:
1228312288, 2001.
28. Mitchell PO,
Mills ST, and Pavlath GK. Calcineurin differentially regulates maintenance
and growth of phenotypically distinct muscles. Am J Physiol Cell
Physiol 282:
C984C992, 2002.
29. Pause A, Belsham GJ, Gingras AC, Donze O, Lin TA, Lawrence JC Jr, and Sonenberg N. Insulin-dependent stimulation of protein synthesis by phosphorylation of a regulator of 5'-cap function. Nature 371: 762767, 1994.[ISI][Medline]
30. Schiaffino S, Bormioli SP, and Aloisi M. Cell proliferation in rat skeletal muscle during early stages of compensatory hypertrophy. Virchows Arch 11: 268273, 1972.
31. Shibasaki F, Hallin U, and Uchino H. Calcineurin as a multifunctional regulator. J Biochem (Tokyo) 131: 115, 2001.[ISI][Medline]
32. Spangenburg EE and Booth FW. Multiple signaling pathways mediate LIF-induced skeletal
muscle satellite cell proliferation. Am J Physiol Cell
Physiol 283:
C204C211, 2002.
33. Spangenburg EE,
Williams JH, Roy RR, and Talmadge RJ. Skeletal muscle calcineurin:
influence of phenotype adaptation and atrophy. Am J Physiol Regul
Integr Comp Physiol 280:
R1256R1260, 2001.
34. Tucker KR,
Seider MJ, and Booth FW. Protein synthesis rates in atrophied
gastrocnemius muscles after limb immobilization. J Appl
Physiol 51:
7377, 1981.
35. Vyas DR,
Spangenburg EE, Abraha TW, Childs TE, and Booth FW. GSK-3 negatively
regulates skeletal myotube hypertrophy. Am J Physiol Cell
Physiol 283:
C545C551, 2002.
36. Whiteman EL, Cho H, and Birnbaum MJ. Role of Akt/protein kinase B in metabolism. Trends Endocrinol Metab 13: 444451, 2002.[ISI][Medline]
37. Yan Z, Biggs RB, and Booth FW. Insulin-like growth factor immunoreactivity increases in muscle after acute eccentric contractions. J Appl Physiol 74: 410414, 1993.[Abstract]