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Originally published In Press as doi:10.1074/jbc.M408862200 on March 14, 2005

J. Biol. Chem., Vol. 280, Issue 20, 19587-19593, May 20, 2005
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Exercise Stimulates Pgc-1{alpha} Transcription in Skeletal Muscle through Activation of the p38 MAPK Pathway*

Takayuki Akimoto{ddagger}§, Steven C. Pohnert{ddagger}, Ping Li, Mei Zhang, Curtis Gumbs, Paul B. Rosenberg, R. Sanders Williams, and Zhen Yan

From the Division of Cardiology, Department of Medicine, Duke University Medical Center, Durham, North Carolina 27710

Received for publication, August 3, 2004 , and in revised form, March 11, 2005.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
 REFERENCES
 
Peroxisome proliferator-activated receptor {gamma} co-activator 1{alpha} (PGC-1{alpha}) promotes mitochondrial biogenesis and slow fiber formation in skeletal muscle. We hypothesized that activation of the p38 mitogen-activated protein kinase (MAPK) pathway in response to increased muscle activity stimulated Pgc-1{alpha} gene transcription as part of the mechanisms for skeletal muscle adaptation. Here we report that a single bout of voluntary running induced a transient increase of Pgc-1{alpha} mRNA expression in mouse plantaris muscle, concurrent with an activation of the p38 MAPK pathway. Activation of the p38 MAPK pathway in cultured C2C12 myocytes stimulated Pgc-1{alpha} promoter activity, which could be blocked by the specific inhibitors of p38, SB203580 and SB202190, or a dominant negative p38. Furthermore, the p38-mediated increase in Pgc-1{alpha} promoter activity was enhanced by increased expression of the downstream transcription factor ATF2 and completely blocked by ATF2{Delta}N, a dominant negative ATF2. Skeletal muscle-specific expression of a constitutively active activator of p38, MKK6E, in transgenic mice resulted in enhanced Pgc-1{alpha} and cytochrome oxidase IV protein expression in fast-twitch skeletal muscles. These findings suggest that contractile activity-induced activation of the p38 MAPK pathway promotes Pgc-1{alpha} gene expression and skeletal muscle adaptation.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
 REFERENCES
 
Adult skeletal muscle is remarkably plastic (1, 2). Increased contractile activity, such as endurance exercise, elicits multiple signals to activate a large set of genes, leading to phenotypic changes in skeletal muscle, including IIb-to-IIa fiber type switching, enhanced mitochondrial biogenesis, and angiogenesis, to match physiologic capability to functional demand. To date, the signaling pathways that link the neuromuscular activity to the gene regulatory machinery are not fully understood.

PGC-1{alpha},1 a transcriptional co-activator cloned from a differentiated brown fat cell line (3), was recently identified as an important regulator of adaptive thermogenesis, glucose metabolism, mitochondrial biogenesis, and muscle fiber type specialization (4). Several lines of evidence are consistent with the notion that PGC-1{alpha} functions in promoting oxidative capacity in skeletal muscle. Firstly, overexpression of PGC-1{alpha} in cultured myoblasts increases mitochondrial biogenesis and oxidative respiration (5), and muscle-specific overexpression of PGC-1{alpha} in transgenic mice results in enhanced mitochondrial biogenesis and more slow-twitch (type I) fiber formation (6). Secondly, endurance exercise induces PGC-1{alpha} mRNA and protein expression in rats and humans (7-9). Thirdly, more slow-twitch fiber formation evoked by genetic or pharmacologic means is associated with enhanced expression of PGC-1{alpha} mRNA and protein in skeletal muscle (10, 11). Finally, PGC-1{alpha} mRNA and protein are highly expressed in slow-twitch, oxidative (type I), and fast-twitch oxidative fibers (type IIa) compared with the fast glycolytic fibers (6, 11, 12). It is currently unknown whether exercise-induced PGC-1{alpha} expression in skeletal muscle is essential for enhanced mitochondrial biogenesis and/or IIb-to-IIa fiber type switching, the latter being different from the reported function of PGC-1{alpha} in transgenic mice in promoting more type I fiber formation (6).

Exercise-induced expression of PGC-1{alpha} in skeletal muscle is, at least in part, because of increased transcription (9). Human, mouse, and rat PGC-1{alpha} promoters share an overall 80-90% identity, suggesting conserved regulation of the PGC-1{alpha} gene. The functional roles of several sequence elements within the PGC-1{alpha} promoter have been tested in cultured myocytes (13, 14) and in mature skeletal muscle in vivo (15). PGC-1{alpha} itself regulates the promoter activity in a positive autoregulation loop, through myocyte enhancer factor 2 (MEF2) at a binding site (-1539) (14), possibly through its interaction with MEF2 (16). A dominant negative form of the cAMP-response element-binding protein (CREB) or mutation of the CRE site (-222) reduces the activation of PGC-1{alpha} promoter by an activated Ca2+/calmodulin-dependent protein kinase (CaMK) (14) (Fig. 7). We have recently confirmed the importance of the MEF2 and CRE binding sites for contractile activity-induced activation of the Pgc-1{alpha} promoter in living mice (15). These findings reveal the importance of regulatory factors that bind the CRE and MEF2 consensus sites in Pgc-1{alpha} gene regulation.

The p38 MAPK pathway in skeletal muscle has been a focus of recent research. In particular, p38 MAPK activity is necessary for myogenic cell differentiation (17) and plays an important role in glucose metabolism and energy expenditure (18, 19). It has been proposed that the p38 MAPK pathway plays a functional role in skeletal muscle adaptation to changing patterns of contractile work (20-24). Based on several previous findings, we propose that the functional role of the p38 MAPK pathway in skeletal muscle adaptation is mediated through regulation of PGC-1{alpha} expression and activity. For example, p38 MAPK can directly stimulate upstream transcription factors of the Pgc-1{alpha} gene, such as ATF2 and MEF2 (14, 25, 26). p38 MAPK can also de-repress PGC-1{alpha} activity by inhibiting the repressor p160 Myb-binding protein (27) and promote PGC-1{alpha} function (18, 28). However, there has been no direct evidence that p38 activity stimulates Pgc-1{alpha} gene transcription in skeletal muscle, and it is not known whether activation of this pathway is sufficient to induce, and necessary for, skeletal muscle adaptation.

In this study, we subjected mice to voluntary running, a physiological model of exercise that mimics many aspects of endurance exercise-induced adaptation, such as fiber type switching, mitochondrial biogenesis, and angiogenesis (12, 29). We demonstrated that voluntary running activates the p38 MAPK pathway and stimulates Pgc-1{alpha} mRNA expression. A link between an activated p38 MAPK pathway and the Pgc-1{alpha} transcription was established in cultured myogenic cells. We further demonstrated that muscle-specific activation of the p38 MAPK pathway in transgenic mice was sufficient to enhance PGC-1{alpha} expression and promote mitochondrial biogenesis.


   EXPERIMENTAL PROCEDURES
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
RESULTS
 DISCUSSION
 REFERENCES
 
Animals—Adult (8-week-old) male C57BL/6J mice (The Jackson Laboratory) were housed in temperature-controlled (21 °C) quarters with a 12-h light/12-h dark cycle and provided with water and food (Purina Chow) ad libitum. The running cage used was described previously (29, 30). Mice in the running group were subjected to voluntary running for different lengths of time as specified under "Results." Plantaris muscles were harvested under anesthesia (50 mg/kg sodium pentobarbital, intraperitoneal) and processed for further analyses. For long-term (4-week) voluntary running mice, the muscle samples were harvested following a 24-h resting period (with locked wheels) after the last bout of running. Animal protocols were approved by the Duke University Institutional Animal Care and Use Committee.

Generation of Transgenic Mice—To examine the relevance of the p38 MAPK-signaling pathway to adaptation in intact muscles, we generated transgenic mouse lines with overexpression of a constitutively active MKK6, MKK6E, in skeletal muscles by using a standard protocol at the Transgenic Mouse Facility of Duke University Comprehensive Cancer Center. Briefly, a PCR fragment of C-terminal FLAG-tagged MKK6E coding region (31, 32) was inserted into a pTarget expression vector (Promega). A 1.5-kb HindIII-HindIII DNA fragment containing this coding region with an upstream chimeric intron was then inserted into plasmid MCKhGHpolyA between a 4.8-kb proximal region from the mouse muscle creatine kinase (MCK) promoter (33) and the human growth hormone polyadenylation site. The MCK promoter, preferentially active in adult skeletal muscles, drove the expression of MKK6E independent of upstream signals (31, 34). PCR analysis was employed for genotyping and confirming the expression of the transgene in adult skeletal muscles. Skeletal muscle samples were harvested from the F1 generation of the transgenic lines and the wild-type littermates (B6SJLF1/J:C57BL6/J background) at 8 weeks of age.

Indirect Immunofluorescence—For fiber type composition determination, plantaris muscles were harvested and frozen in isopentane cooled in liquid nitrogen. Frozen cross-sections (5 µm) were immunostained using monoclonal antibodies against myosin heavy chains (MHC) I, IIa, and IIb as described previously (12).

Semiquantitative RT-PCR Analysis—Total RNA was extracted from skeletal muscles by using TRIzol (Invitrogen) according to the manufacturer's instructions. Semiquantitative RT-PCR analysis was performed to measure endogenous PGC-1{alpha} mRNA expression in plantaris muscle in response to voluntary running as described previously (12, 35).

Western Immunoblot Analysis—For protein analysis, dissected plantaris muscles were homogenized and analyzed as described previously (12). Antibodies used for immunoblot analysis were PGC-1{alpha} polyclonal antibody (SC-13067, Santa Cruz Biotechnology), {alpha}-tubulin antibody (13-8000, Zymed Laboratories Inc.), and MHC mouse monoclonal antibodies (BF-F8 for MHC I, SC-71 for MHC IIa, and BF-F3 for MHC IIb, German Collection of Microorganisms and Cell Cultures). For phosphoprotein detection, the following antibodies from Cell Signaling Technology were used: P-MKK3/6 (9231), MKK3 (9232), P-p38 (9211), p38 (9212), P-ATF2 (9221), and ATF2 (9222). To detect FLAG-tagged MKK6E transgene expression in cultured C2C12 myocytes, ~15 µg of proteins in total cell lysate for each sample was resolved by SDS-PAGE and probed with rabbit anti-FLAG antibody (F-7425, Sigma) with a dilution of 1:1000 as described above. The intensities of the immunoblot bands were quantified by using ImageQuant software.

Plasmid Constructs, Tissue Culture, Cell Transfection, and Reporter Assay—The Pgc-1{alpha}-luciferase reporter gene (Pgc-1{alpha}L) was kindly supplied by Dr. E. N. Olson (13). Plasmid DNAs containing FLAG-tagged MKK3E, MKK6E, p38 (p38{alpha}, p38{beta}, p38{gamma}, and p38{delta}), and the non-phosphorylatable p38 (p38{beta}AF) under the control of the constitutively active cytomegalovirus (CMV) promoter in pcDNA3 were generously provided by Dr. J. Han. The full-length ATF2 cDNA was cloned by an RT-PCR reaction using a skeletal muscle total RNA with forward primer 5'-CGGATCCAATATGAGTGATGACAAACCCTT-3' and reverse primer 5'-TGGTACCACTTCCTGAGGGCTGTGC-3'. The expected 1.5-kb PCR fragment was inserted into pTarget vector (Promega) to generate plasmid CMV-ATF2, in which ATF2 expression is under the control of the constitutively active CMV promoter. Non-phosphorylatable mutant ATF2, ATF2(T69A/T71A), was generated by site-directed mutagenesis as described previously (36), and the changes in the following sequence are underlined: 5'-GTGGCTGATCAGGCTCCAGCGCCAACAAGATTCC-3'. The dominant negative ATF2{Delta}N, kindly provided by Dr. G. Thiel (37), is a truncated ATF2 with N-terminal deletion from amino acids 1-137. For transient transfection, C2C12 myoblasts were grown and maintained in Dulbecco's modified Eagle's medium containing 20% fetal bovine serum. Myoblasts were plated in 24-well tissue culture plates and transfected using Lipofectamine 2000 (Invitrogen). For each transfection, 50 ng of PGC-1{alpha}L and 60 ng of plasmid DNA containing MKK3E, MKK6E, p38{alpha}, p38{beta}, p38{delta}, p38{gamma}, p38{beta}AF, ATF2, ATF2(T69A/T71A), or ATF2{Delta}N were co-transfected with 50 ng of plasmid DNA SV40-LacZ as a control for the transfection efficiency. Cells were harvested 24 h after transfection in luciferase lysis buffer (Promega). Luciferase and {beta}-galactosidase activities were measured as described previously (38). In the experiments with transfected C2C12 myocytes with MCK-MKK6E, myogenic differentiation was induced in C2C12 cells for 4 days by changing the culture medium to Dulbecco's modified Eagle's medium containing 2% horse serum. All transfection data are presented as means of a minimum of three separate experiments, each performed in duplicate.

Statistics—Data are expressed as mean ± S.E. Statistical significance (p < 0.05) was determined by Student's t test or analysis of variance followed by the Dunnett test for comparisons between two groups or multiple groups, respectively.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
DISCUSSION
 REFERENCES
 
Voluntary Running Induces Skeletal Muscle Adaptation and PGC-1{alpha} Protein Expression—Wild-type mice were subjected to voluntary wheel running for 4 weeks. The running distance increased from 6.7 ± 0.6 kilometers/night (12 h in the dark) on the first day to 13.6 ± 2.0 kilometers/night after 4 weeks of training (p < 0.05, n = 5). Long-term voluntary running resulted in an ~2-fold increase in the percentage of type IIa fibers and a concurrent decrease in type IIb fibers (Fig. 1 and Table I). Western immunoblot analysis showed significant increases of MHC IIa (250%) and PGC-1{alpha} (50%) proteins in plantaris muscles after 4 weeks of running (Fig. 2).


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  		TABLE I
Voluntary running-induced change in fiber type composition in mouse plantaris muscle Values are mean ± S.E., n = 7.

 
A Single Bout of Voluntary Running Stimulates Pgc-1{alpha} mRNA Expression in Plantaris Muscle—We examined Pgc-1{alpha} mRNA expression during and following a single bout (12 h) of running and after 4 weeks of running. Semiquantitative RT-PCR detected a transient increase in Pgc-1{alpha} mRNA in response to an acute bout of exercise but not after 4 weeks of training (Fig. 3). Similarly, a transient increase of Pgc-1{alpha} mRNA in the tibialis anterior muscle following 2 h of motor nerve stimulation at 10 Hz has also been reported recently from our laboratory (39).

Contractile Activity Induces Activation of the p38 MAPK Pathway in Skeletal Muscle in Vivo—To identify the signaling pathways that are potentially involved in skeletal muscle adaptation, we performed Western immunoblot analysis using phospho-specific antibodies against signaling molecules in the MAPK pathways. We observed significant increases in phosphorylation of MAP kinase kinase 3 (MKK3), MKK6, p38, and ATF2 (Fig. 4). Short-term (2 h) nerve stimulation of tibialis anterior muscle resulted in similar activation of the p38 MAPK pathway (not shown).



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  		FIG. 1.
Voluntary running induces type IIb-to-IIa fiber type switching in mouse skeletal muscle. Indirect immunofluorescence staining is shown of type I (red), IIa (blue), IIb (green), and IId/x (no staining) myofibers in mouse plantaris muscles in sedentary control mice and in mice after 4 weeks of running (Trained). An increased percentage of type IIa myofibers with a concurrent decrease in type IIb myofibers is noticeable in trained plantaris muscle.

 
Activation of the p38 MAPK Pathway Stimulates Pgc-1{alpha} Promoter Activity in Myocytes—To determine whether activation of the p38 MAPK pathway is sufficient to stimulate Pgc-1{alpha} transcription, C2C12 cells were transfected with a Pgc-1{alpha}-luciferase reporter gene together with constitutively active p38 activating kinase MKK3E or MKK6E (31, 32, 34), with or without p38{alpha}, -{beta}, -{gamma}, or -{delta}, the different isoforms of p38. Overexpression of MKK3E or wild-type p38 isoforms stimulated Pgc-1{alpha} promoter activity significantly (Fig. 5A). When MKK3E was co-transfected with wild-type p38 isoforms, Pgc-1{alpha} promoter activity was further enhanced. The stimulatory effect of MKK3E was completely blocked by co-transfection with non-phosphorylatable p38{beta}AF (Fig. 5A) and by the specific inhibitors of p38 MAPK, SB203580 and SB202190 (Fig. 5B).

To further examine the stimulatory effects of constitutively active MKKs in differentiated myotubes, we transfected C2C12 myoblasts with MKK6E under the control of the MCK promoter and induced myogenic differentiation by switching from serum-rich growth medium to low serum differentiation medium. We chose MKK6E over MKK3E in this experiment because of preliminary findings that MKK6E is more potent than MKK3E in stimulating Pgc-1{alpha} promoter in myotubes (not shown). FLAG-tagged MKK6E (under the control of MCK promoter) could only be detected in differentiated myotubes, whereas FLAG-tagged MKK6E (under the control of the constitutively active CMV promoter) could be detected in both myoblasts and myotubes (not shown). In myoblasts transfected with MCK-MKK6E, there was no significant increase in Pgc-1{alpha} promoter activity relative to myoblasts transfected with the control empty vector (pCI-neo). There was a 5-fold increase in Pgc-1{alpha} promoter activity in differentiated myotubes in the absence of MKK6E expression, whereas MCK-MKK6E stimulated Pgc-1{alpha} further and resulted in a 30-fold induction in promoter activity in the myotubes (Fig. 5C). The differentiation process was not significantly accelerated by MKK6E expression under the MCK promoter in this short-term transient expression experiment as measured by the expression of MHC IIb protein (Fig. 5C, inset).



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  		FIG. 2.
Voluntary running induces skeletal muscle adaptation. A, Western immunoblot analysis of MHC I, MHC IIa, MHC IIb, and PGC-1{alpha} in plantaris muscle of sedentary control and 4-week trained mice. Soleus muscle (SO) was used as a positive control for MCH I. B, quantification of the proteins in total muscle lysates after normalization by the abundance of {alpha}-tubulin protein. The mean values in the control mice were set as a reference for comparison (n = 7; *, p < 0.05 versus control).

 



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  		FIG. 3.
Transient increase of Pgc-1{alpha} mRNA expression in response to increased contractile activity. A, RT-PCR for Pgc-1{alpha} mRNA in total RNA from plantaris muscle in response to running. Con, R3h, and R12h stand for sedentary control, running for 3 h, and running for 12 h, respectively. P3h, P6h, P12h, and P24h stand for 3, 6, 12, and 24 h after a single bout of running (12 h), respectively. R4w stands for running for 4 weeks. Reaction without reverse transcriptase (-RT) was used as a control for the reaction, and Gapdh mRNA was measured as a control for mRNA quantity and quality. B, quantification of Pgc-1{alpha} mRNA after normalization by the abundance of Gapdh mRNA. The mean values in the control mice were set as references for comparison (n = 5; *, p < 0.05 versus control).

 



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  		FIG. 4.
Voluntary running activates the p38 MAPK pathway in mouse skeletal muscle. A, representative Western blot images of phosphoproteins in plantaris muscles in sedentary control mice and in mice after 3 h of voluntary running. B, quantification of phosphoproteins after normalization by the abundance of total protein. The mean values in the control mice were set as references for comparison (n = 7-11; *, p < 0.05 versus control).

 
ATF2 Is Involved in p38 MAPK-mediated PGC-1{alpha} Gene Regulation—ATF2 is known to bind the CRE element and transactivates promoter activity upon phosphorylation (40). p38 phosphorylates ATF2 at threonine 69 and 71 and activates ATF2 transcriptional activity (25, 41). To examine the role of ATF2 in Pgc-1{alpha} gene regulation, wild-type ATF2, ATF2(T69A/T71A) or ATF2{Delta}N, was expressed in C2C12 cells. Wild-type ATF2, but not ATF2(T69A/T71A) or ATF2{Delta}N, elevates Pgc-1{alpha}-luciferase reporter gene activity significantly (Fig. 5D). When co-transfected with MKK3E, wild-type ATF2, but not ATF2(T69A/T71A), further stimulated Pgc-1{alpha} promoter activity. The stimulatory effect of MKK3E was completely blocked by the dominant negative ATF2{Delta}N.

Transgenic Mice with Muscle-specific Expression of MKK6E Have Enhanced PGC-1{alpha} and COXIV Protein Expression in Fast-twitch Skeletal Muscles—To determine whether activation of the p38 MAPK pathway is sufficient to promote PGC-1{alpha} protein expression, fiber type switching, and mitochondrial biogenesis, we generated transgenic mice with muscle-specific expression of MKK6E. We successfully obtained three F0 transgenic mice (TG171, TG1123, and TG554) with variable levels of MKK6E mRNA expression in skeletal muscle (Fig. 6A). The biological effects of the transgene expression were reflected by an inverse relationship of the transgene expression and the endogenous Mkk6 mRNA expression (Fig. 6A), which is consistent with a recent finding that MKK6 mRNA stability is negatively regulated by p38 activity (42). TG554 (male) showed a high level of MKK6E mRNA expression in skeletal muscle but never gave birth to a live transgenic offspring after four rounds of pregnancy, possibly because of a leaky expression of MKK6E in the heart. A transgenic line was generated by crossing TG1123 with wild-type C57BL6/J mice. TG1123 mice showed significantly higher expression in PGC-1{alpha} and COXIV proteins in white vastus lateralis muscles (Fig. 6, B and C); there was a similar trend of increased PGC-1{alpha} and COXIV protein expression, although not statistically significant, in plantaris muscles.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
REFERENCES
 
Fiber type composition and mitochondrial content of skeletal muscles were determining factors for the efficiency of locomotion and metabolism. Physical inactivity, endemic in the Western lifestyle, contributes to a decrease in the percentage of oxidative fibers and correlates with epidemic emergence of chronic disorders, such as coronary heart disease, obesity, and type 2 diabetes (43). Exercise-induced adaptation in skeletal muscle is an effective means of curbing the increasing health care costs caused by this metabolic syndrome. Understanding the signaling and molecular mechanisms of skeletal muscle adaptation will not only promote a correct and efficient use of exercise as a therapeutic measure but also may facilitate the discovery of new drug targets.

In this study, we subjected wild-type sedentary mice to voluntary running and performed comprehensive phenotypic analysis to determine the effects of long-term exercise on skeletal muscle. We confirmed functional, morphological, and biochemical adaptations in mouse skeletal muscle following 4 weeks of voluntary running. The running distance during the nocturnal activity increased ~2-fold following the training, indicating an increase in overall running capacity. Fiber type analysis with simultaneous staining of three MHC isoforms (I, IIa, and IIb) in plantaris muscles provided morphological evidence of a 2-fold increase in the percentage of type IIa fibers, with a concurrent decrease in type IIb fibers. Finally, quantitative analysis of proteins related to skeletal muscle contractile function and mitochondrial biogenesis showed significantly increased expression of MHC IIa and PGC-1{alpha} proteins in trained plantaris muscles. Taken together, these results provided direct and comprehensive evidence, complementary to previous reports (30, 44), that long-term voluntary running induces significant skeletal muscle adaptation in mice.

Voluntary running has been employed in rodents as a model of exercise to induce skeletal muscle adaptation (30, 45). Although the muscle activity pattern (intermittent bursts of activities) is different from human endurance exercise, such as long-distance running, the observed functional adaptations in skeletal muscle are similar between the animal models and humans in many aspects, which have been characterized in mouse skeletal muscle using various morphological and biochemical analyses (12, 29, 30). A combination of this physiological exercise model with transgenic or targeted mutation approaches provides an excellent opportunity for elucidation of molecular and signaling mechanisms of skeletal muscle adaptation that otherwise would not be feasible in other animal models or in humans.

The molecular mechanisms that regulate skeletal muscle fiber type composition and mitochondrial biogenesis in response to exercise are not completely understood. It has recently been reported that endurance exercise induces PGC-1{alpha} mRNA and protein expression (7-9), suggesting that PGC-1{alpha} protein plays a functional role in exercise-induced skeletal muscle adaptation. Here, we report that a transient increase in Pgc-1{alpha} mRNA in skeletal muscle occurs during and following a single bout of voluntary running or motor nerve stimulation in mice. This transcriptional activation of the Pgc-1{alpha} gene may contribute directly to contractile activity-induced PGC-1{alpha} protein expression that mediates skeletal muscle adaptation.



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  		FIG. 5.
Activation of the p38 MAPK pathway stimulates Pgc-1{alpha} promoter activity in cultured myocytes. A, C2C12 myoblasts were co-transfected with the Pgc-1{alpha}L reporter gene and wild-type p38 isoforms or dominant negative p38{beta}AF with or without MKK3E. Empty plasmid pCI-neo was used as control (Con). Wild-type p38 isoforms are p38{alpha}, p38{beta}, p38{gamma}, or p38{delta} under the control of the constitutively active CMV promoter. Non-phosphorylatable p38{beta}AF is under the control of the CMV promoter. Normalized luciferase activities are presented as -fold changes compared with the control without MKK3E (n = 6-10; *, p < 0.05 versus -MKK3E). B, C2C12 myoblasts were co-transfected with PGC-1{alpha}L and MKK3E (+MKK3E) or pCI-neo (-MKK3E) followed by incubation with 10 µM SB203580, 10 µM SB202190, or the vehicle dimethyl sulfoxide (DMSO). Normalized luciferase activities are presented as -fold changes compared with cells treated with dimethyl sulfoxide (n = 6; *, p < 0.05 versus -MKK3E; +, p < 0.05 versus dimethyl sulfoxide). C, C2C12 myoblasts were co-transfected with PGC-1{alpha}L and MCK-MKK6E or pCI-neo (Con). Luciferase activity was determined in myoblasts (MB) and after induction of myogenic differentiation into myotubes (MT) for 4 days. Normalized luciferase activities are presented as -fold changes compared with myoblasts transfected with pCI-neo (n = 3; *, p < 0.05 versus myoblast; +, p < 0.05 versus control). D, C2C12 myoblasts were co-transfected with PGC-1{alpha}L and MKK3E (+MKK3E) or pCI-neo (-MKK3E) together with pCI-neo control plasmid (Con), ATF2, ATF2(T69A/T71A), or ATF2{Delta}N under the control of the CMV promoter. Normalized luciferase activities are presented as -fold changes compared with the control (n = 6-10; *, p < 0.05 versus -MKK3E).

 



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  		FIG. 6.
Muscle-specific expression of MKK6E enhances PGC-1{alpha} and COXIV protein expression in fast-twitch muscles. A, PCR and RT-PCR analyses in tail genomic DNA and muscle total RNA samples from three F0 transgenic mice (TG171, TG1123, and TG554) for the presence of the FLAG-tagged human MKK6E transgene (hMKK6E transgene) and mRNA (hMKK6E mRNA) and the endogenous Mkk6 mRNA (MKK6 mRNA). B, Western blot analysis for PGC-1{alpha} and COXIV proteins in white vastus lateralis muscles of MCK-MKK6E transgenic mice (line TG1123, TG) and the wild-type (WT) littermates. Actin protein was used as a control for loading. C, quantification of PGC-1{alpha} and COXIV proteins after normalization by the abundance of actin protein. The mean values in the control mice were set as references for comparison (n = 7 and 13 for PGC-1{alpha} and COXIV, respectively; **, p < 0.01 versus WT).

 
Ca2+ has been shown to play an essential role in contractile activity-induced skeletal muscle adaptation (46). Recent studies have linked Ca2+ signaling to PGC-1{alpha} expression in both cultured myocytes (14, 47) and in skeletal muscle of intact animals (11). On one hand, Ca2+ signaling may activate PGC-1{alpha} promoter via the MEF2 site by activating the calcineurin/MEF2 signaling cascade (14). On the other hand, it may also exert a positive regulatory role on PGC-1{alpha} transcription via the CRE site by activating the CaMK pathway (14) (Fig. 7). Using optical bioluminescence imaging analysis, we have shown recently that contractile activity-induced Pgc-1{alpha} promoter activity in skeletal muscle is dependent on the MEF2 (-1539) and the CRE (-222) sequence elements in living mice (39).



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  		FIG. 7.
A model for the exercise-induced transcription of the Pgc-1{alpha} gene. Increased neuromuscular activity elevates intracellular Ca2+, which in turn results in activation of the calcineurin (CnA) and CaMK pathways. Activated CaMK can phosphorylate CREB, which increases Pgc-1{alpha} transcription through transcription factors MEF2 and CREB. In parallel, activated calcineurin can stimulate MEF2 activity and promote Pgc-1{alpha} transcription. Exercise-induced activation of the p38 MAPK pathway can also promote Pgc-1{alpha} transcription by activating transcription factors ATF2 and MEF2 directly and inhibit p160 to de-repress PGC-1{alpha} function, which exerts a positive autofeedback regulation of Pgc-1{alpha} transcription, possibly through interaction with and activation of MEF2.

 
In this study, we used phospho-specific antibodies to characterize activation of signaling molecules that are involved in skeletal muscle adaptation. The p38 MAPK pathway, including MKK3/6, p38, and transcription factor ATF2, was shown to be activated in response to running as shown by the increased phosphorylation of these signaling molecules. Activation of the p38 MAPK pathway was concurrent with a transient increase in Pgc-1{alpha} mRNA following both voluntary running and nerve stimulation, consistent with a functional role of p38 MAPK in skeletal muscle adaptation in vivo (20-23).

To determine whether activation of the p38 MAPK pathway has a role in regulating the Pgc-1{alpha} promoter activity in skeletal muscle, we employed a report gene assay in cultured C2C12 myoblasts by co-transfection of a Pgc-1{alpha}-luciferase reporter gene and the signaling molecules of the p38 MAPK pathway. We first demonstrated that activation of the p38 MAPK pathway is sufficient to stimulate Pgc-1{alpha} promoter activity in myoblasts. Overexpression of a constitutively active form of MKK3 or MKK6, which activates the p38 MAPK pathway by direct phosphorylation of the p38 protein (31, 34), resulted in increased Pgc-1{alpha} promoter activity. The notion that activation of the p38 MAPK pathway is sufficient to stimulate Pgc-1{alpha} transcription is further supported by the finding that overexpression of p38 proteins enhanced the stimulatory effects of MKK3E. This appeared to be mediated by signals transduced through p38, because the inhibitors specific to p38, SB203580 and SB202190, or dominant negative p38{beta}AF significantly reduced MKK3-mediated Pgc-1{alpha} promoter activity. Thus, the stimulatory function of activated MKK on the Pgc-1{alpha} promoter required p38 activity.

Expression of MKK6E under the control of the MCK promoter was used to determine whether activation of the p38 MAPK pathway stimulates Pgc-1{alpha} promoter activity in differentiated myotubes. Because the MCK promoter is inactive in undifferentiated myoblasts, MKK6E was not detectable until the C2C12 cells differentiated into myotubes. When myoblasts transfected with control empty vector were induced to differentiate into myotubes, Pgc-1{alpha}-luciferase reporter gene activity increased by 5-fold, possibly because of increased Pgc-1{alpha} transcription from the endogenous p38 activity (48) as well as increased activity of MEF2 (49). Myotubes with MKK6E had a 30-fold increase in Pgc-1{alpha} promoter activity. This enhanced Pgc-1{alpha}-luciferase reporter gene expression in myotubes induced by MKK6E appears to be a direct result of activated p38 activity. Alternatively, activated p38 promotes myogenic differentiation (48), which in turn stimulates Pgc-1{alpha} transcription. Because we did not observe a significant acceleration of differentiation when MKK6E was expressed from the MCK promoter in the myotubes, we attribute the increased Pgc-1{alpha} promoter activity to a direct stimulatory function of MKK6E.

The Pgc-1{alpha} promoter contains a CRE binding site that can potentially be bound and stimulated by ATF2 (40), which is a downstream target of p38. It is possible that ATF2 activity plays a functional role in linking the activation of the p38 MAPK pathway to enhanced transcription of the Pgc-1{alpha} gene. Supporting this hypothesis is our finding that overexpression of wild-type ATF2, but not a non-phosphorylatable ATF2 or dominant negative ATF2, stimulates Pgc-1{alpha} promoter activity and further enhances the responses of the Pgc-1{alpha} promoter to activated MKK3. This activation can be completely blocked by overexpression of the dominant negative ATF2{Delta}N, in agreement with a recent finding that ATF2 mediates the regulatory function of the p38 MAPK pathway on the Pgc-1{alpha} gene in mouse adipose tissue in response to cold exposure (26). Taken together, we conclude that p38 activity can stimulate Pgc-1{alpha} transcription in skeletal muscle through downstream effectors that include ATF2.

Finally, we tested the hypothesis that activation of the p38 MAPK pathway in adult skeletal muscle in vivo is sufficient to promote Pgc-1{alpha} expression, changes in fiber type composition, and mitochondrial biogenesis. We have obtained evidence of significantly enhanced PGC-1{alpha} and COXIV protein expression in fast-twitch white vastus lateralis muscles in the transgenic mice with skeletal muscle-specific expression of MKK6E. A similar trend of enhanced expression was observed in plantaris muscles of MKK6E transgenic mice, but it was not statistically significant. However, we did not observe significantly enhanced expression of MHC IIa in white vastus lateralis muscles in these mice. Our data suggest that transgenic activation of the p38 MAPK in adult skeletal muscle results in enhanced PGC-1{alpha} expression and mitochondrial biogenesis. Together with the calcineurin and CaMK pathways, the p38 MAPK pathway represents another signaling pathway in the regulation of the Pgc-1{alpha} promoter activity through transcription factors binding to the cis-elements (Fig. 7).

Exercise exerts the most complex and powerful stimuli for functional adaptations in skeletal muscle. Multiple signaling pathways, such as the calcineurin, AMP-activated protein kinase, and protein kinase C pathways, have been reported to be activated in skeletal muscle in response to exercise (44, 50, 51). Each of these pathways has unique temporal and spatial patterns and distinct regulatory targets (6, 52). They collectively form a signaling network in the skeletal muscle that ensures the adaptability as well as the fidelity of the adaptive process, which is likely to be accomplished by coordinated interactions and cross-talks. The present study provides evidence linking activation of the p38 MAPK pathway to skeletal muscle adaptation through its regulatory control of Pgc-1{alpha} gene expression. It remains to be determined whether activation of the p38 MAPK pathway is an obligatory signaling event in skeletal muscle adaptation in response to endurance exercise.


   FOOTNOTES
 
* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. Back

{ddagger} Both authors contributed equally to this work. Back

§ Recipient of a grant-in-aid for overseas research scholars from the Ministry of Education, Science, and Culture of Japan. Back

To whom correspondence should be addressed: Division of Cardiology, Dept. of Medicine, Duke University Medical Center, 4321 Medical Park Dr., Suite 200, Durham, NC 27704. Tel.: 919-479-2373; Fax: 919-477-0632; E-mail: zhen.yan{at}duke.edu.

1 The abbreviations used are: PGC-1{alpha}, peroxisome proliferator-activated receptor {gamma} co-activator 1{alpha}; Pgc-1{alpha}L, Pgc-1{alpha}-luciferase; MEF2, myocyte enhancer factor 2; CRE, cAMP-response element; CREB, CRE-binding protein; CaMK, Ca2+/calmodulin-dependent protein kinase; MAPK, mitogen-activated protein kinase; MCK, muscle creatine kinase; MHC, myosin heavy chain; RT, reverse transcription; CMV, cytomegalovirus; COXIV, cytochrome oxidase IV; MKK, mitogen-activated protein kinase kinase. Back


   ACKNOWLEDGMENTS
 
We thank Drs. E. N. Olson, J. Han, and G. Thiel for kind gifts of plasmid DNA constructs. We appreciate the excellent technical support from C. Ireland.



   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
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