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Human Performance Laboratory, Ball State University, Muncie, Indiana
Submitted 21 October 2004 ; accepted in final form 17 December 2004
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ABSTRACT |
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 TOP ABSTRACT MATERIALS AND METHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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MRF4; Myf5; MyoD; myogenin; FAT/CD36; CPT1; HKII; PDK4
It has been suggested that training adaptations at the cellular level are the result of cumulative effects of transient changes in gene transcription after each acute bout of exercise (7, 21). In human studies, transient changes in mRNA levels of the above myogenic (10, 38) or metabolic (17, 18, 26, 27, 35) genes after an acute bout of exercise have been reported. These studies were limited to time points that did not extend beyond 6 h after exercise. Although this information has provided valuable information regarding the molecular response in the first few hours after exercise, data beyond these time points are warranted for a more complete understanding for gene induction with exercise in human skeletal muscle.
More recently, human and animal studies have examined gene expression beyond 6 h after exercise. These studies found no increase in HKII and PDK4 mRNA beyond 8 h after high-intensity intermittent one-legged knee extension (22) or 3 h of knee extension (29, 36). No change in mRNA levels of MRF4 and MyoD beyond 12 h after an acute bout of resistance exercise has been reported (2, 30, 36, 40). In contrast, Bickel et al. found an increase in myogenin mRNA 24 h after a single (2) or second (2, 3) electromyostimulation-induced resistance exercise bout. Animal studies investigating gene expression of the above myogenic (9) and metabolic (23) genes after an acute bout of exercise have reported that HKII mRNA levels return to baseline within 24 h after run exercise (23). Conversely, myogenin mRNA was elevated or remained elevated at 24 h after electromyostimulation-induced resistance exercise (9). Thus there does not appear to be universal agreement on the timing aspects related to gene induction with exercise in skeletal muscle. Certainly, mode and intensity of exercise as well as the muscle and species studied contribute to these varied results.
To date, no studies have examined the effects of resistance or run exercise on both myogenic and metabolic gene expression in human skeletal muscle. Given that most key myogenic and metabolic genes appear to return to baseline within 24 h, it would be informative to know when these regulators peak after an acute bout of exercise in humans. Such knowledge could aid the timing of biopsies for future human-based exercise studies and provide a greater understanding of the molecular responses to exercise. Therefore, the purpose of this study was to examine the time course of select myogenic and metabolic gene induction after an acute bout of resistance (RE) or run (Run) exercise at different time points over a 24 h-period. The selected myogenic genes were MRF4, Myf5, MyoD, and myogenin, whereas the selected metabolic genes were CD36, CPT1, HKII, and PDK4. To our knowledge, this is the first human study to investigate gene expression in the gastrocnemius muscle after an acute bout of aerobic exercise. The gastrocnemius and vastus lateralis were selected in this study because they were best suited to study for RE and Run, respectively.
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MATERIALS AND METHODS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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For the Run trial, subjects performed 30 min of treadmill running at 75% of maximal O2 uptake (
O2 max). Subjects completed a O2 max test using an incremental treadmill test to voluntary exhaustion on the same treadmill 
10 days before the trial. Oxygen uptake was measured by using indirect calorimetry via an automated open-circuit system that incorporated a dry-gas meter (Rayfield Equipment, Waitsfield, VT), 3-liter mixing chamber, and electronic O2 and CO2 analyzers (Ametek S-3A/II and CD-3A, respectively; Applied Electrochemistry, AEI Technologies, Pittsburgh, PA). Both analyzers were calibrated with standardized gases before each test. Expired air was measured at 30-s intervals throughout the test. Subjects also performed a 14-min treadmill run to validate the speed of the treadmill that corresponded to 75% of their O2 max 1 wk before the trial. During the 30-min treadmill run trial, expired air at the 13–14th min and 28–29th min stages was collected in Douglas bags and analyzed via the automated open-circuit system. The volume of air in each bag was measured with a spirometer.
For both RE and Run, subjects refrained from physical activity for at least 48 h before the preexercise biopsy and rested in a supine position for 30 min before each biopsy except for the immediate postexercise biopsy (Fig. 1). All trials began around 6 to 7 AM. After the first through the 8-h postexercise biopsies, subjects rested quietly in the laboratory. Thereafter, they were allowed normal ambulation and returned to the laboratory for the 12- and 24-h postexercise biopsies. Subjects also fasted with ad libitum water intake at least 8 h before lying down for the preexercise biopsy and through the 8 h postexercise biopsy. Thereafter, they were fed a standardized meal. The 24-h postexercise biopsy was performed after an overnight fast of at least 8 h (Fig. 1).
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Total RNA extraction and RNA quality check. Each muscle sample was thawed and removed from the RNAlater and placed in a mixture of 0.8 ml of RNA isolation reagent, TRI Reagent, and 4 µl of PolyAcryl Carrier (Molecular Research Center, Cincinnati, OH). The tissue mixture was homogenized and total RNA was extracted according the manufacturer's protocol. The RNA pellet was dissolved in 30 µl of 0.1 mM EDTA and stored at –80°C.
One microliter of each total RNA solution (1:3 vol/vol) was analyzed by using the RNA 6000 Nano LabChip kit on an Agilent 2100 Bioanalzyer (Agilent Technologies, Palo Alto, CA). This system reported detailed information about the integrity and purity of the RNA samples. The sample was electrophoretically separated into two peaks of 18S and 28S ribosomal RNA. Data were displayed as a gellike image and/or an electropherogram (Fig. 2). Sample analyses were performed as described by the manufacturer. The high quality of RNA was confirmed by presence of ribosomal peaks with no additional signals (DNA contamination or RNA degradation) below the ribosomal bands and no shifts to lower fragments.
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Real-time PCR. Quantification of mRNA transcription (in duplicates) was performed in a 72-well Roto-Gene 3000 Centrifugal Real-Time Cycler (Corbett Research, Mortlake, NSW, Australia). GAPDH was used as a housekeeping gene (HKG) for internal control as it had been shown that its expression remained stable using the same Run protocol (14). The reaction mix consisted of 2.5 µl of 10x SYBRgreen real-time PCR buffer (Biosource, Camavillo, CA), 0.625 µl of 10 mM dNTPs, 0.4 µl of 5 U/µl platinum Taq DNA polymerase (Invitrogen), 1 µl each of forward and reverse primers in the case of the eight genes of interest (GOI) or 2.5 µl of 10x primer pairs for GAPDH in the case of GAPDH, 2.5 µl of cDNA, and RNase-free water to 25 µl. All primers used in this study were "mRNA specific" and designed for gene-expression analysis (MRF4, MyoD, and myogenin: Ref. 30; Myf5: GenBank NM 005593; CD36: GenBank NM 000072, Primer ID 4557419a2; CPT1: Ref. 28; HKII and PDK4: Ref. 22; Table 2).
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Relative quantification of real-time PCR assay.
The influence of the RE and Run exercises on the human muscle gene expression was evaluated by a relative quantification method. This method is based on the fact that the difference in threshold cycles (
CT) between GOI and HKG is proportional to the relative expression level of the GOI. To ensure the accuracy of the obtained data, the reaction efficiencies of the two assays should be similar or a correction factor must be introduced into the calculation (see Validation of the 2–CT method).
In the present study the change in expression of target gene normalized to GAPDH was monitored over 24 h. Real-time PCR using SYBRgreen detection was performed on the corresponding cDNA (in duplicates) synthesized from each sample. The data were analyzed using the following equation:
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where CT = (CT target – CT reference)time x – (CT mean target –CT mean reference)time 0, time x is any of the time point of interest, and time 0 represents the 1x expression of the target gene normalized to GAPDH (19). The mean CT values for both the target and GAPDH genes were determined at time zero (Pre) and were used in the equation. The fold change in target gene, normalized to GAPDH and relative to the expression at time zero, was calculated for each sample by use of the above equation.
With the use of this analysis, the value of the mean fold change at time zero should be very close to 1 (i.e., because 20 = 1). The verification of the mean fold changes at time zero is a convenient method to check for errors and variation among the samples at time zero (4, 6, 14, 19). A value that is very different from 1 suggests a high degree of experimental variation. In this study the fold change for the target genes normalized to GAPDH at time zero was 1.09 ± 0.005 for RE and Run together.
Validation of the 2–CT method.
Serial dilutions (1, 0.5, 0.250, 0.125, 0.062, 0.031) of cDNA for GAPDH and target genes were amplified by real-time PCR using gene-specific primers. The amplification calculated by Rotor-Gene software was specific and highly efficient (1.007 ± 0.003 and 0.985 ± 0.001 for RE and Run, respectively) for all genes. The R2 = 0.981 ± 0.001 (RE) and R2 = 0.989 ± 0.001 (Run) and intra-assay coefficient of variation values were 8.01 and 8.98% for the RE and Run groups, respectively.
In addition, the validation of the reference gene was performed to ensure that its expression was unaffected by experimental treatment. The relative amount of GAPDH was calculated by using the 2
equation in which CT = CT time x – CT mean of time zero. The mean ± SE fold change for GAPDH across the time points for RE was 1.08 ± 0.01 with a slope of –0.0118 and for Run was 1.06 ± 0.01 with a slope of –0.0131.
Fiber-composition analysis.
Muscle samples were homogenized in 40 volumes of cold homogenizing buffer containing (in mM) 250 sucrose, 100 KCl, 20 imidazole, and 5 EDTA (pH 6.8) and spun for 30 min at 20,000 g and 4°C. The supernatant was then removed and the pellet resuspended in 40 volumes of cold homogenization buffer (34). An equal volume of sample buffer [10% SDS, 6 mg/ml EDTA, 0.06 M Tris (pH 6.8), 2 mg/ml bromphenol blue, 15% glycerol, and 5% 
-mercaptoethanol] was added, and the samples were stored at –20°C until assayed. To determine myosin heavy chain (MHC) composition, homogenized samples were subjected to SDS-PAGE as described previously (37). Briefly, samples were run overnight at 4°C on a Hoefer SE 600 gel electrophoresis unit utilizing a 3.5% (wt/vol) acrylamide stacking gel with a 5% separating gel. After electrophoresis, the gels were silver stained as described by Giulian et al. (8). MHC isoforms were identified according to migration rate and compared with molecular weight standards. The MHC was categorized as MHC I, IIa, or IIx, and percentages were determined by densitometry (ChemImager 4000, Alpha Innotech, San Leandro, CA).
Statistical analysis. Data were checked for normality and equality of variances. Assumptions for parametric analyses were violated and data transformation was not feasible. As such, the changes in mRNA levels with respect to the preexercise levels at the various time points postexercise for each studied gene were analyzed by using the nonparametric Friedman test. Significant differences between postexercise levels and preexercise level for each gene were determined by Dunn post hoc analysis. Significance was set at P < 0.05 (two-tailed). Statistical analyses were performed by using SPSS 10.0 for Windows and Prism 4 for Windows (Graphpad, San Diego, CA) software packages. Data are presented as means ± SE.
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RESULTS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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DISCUSSION |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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Resistance exercise responses. Activation of the myogenic genes in response to resistance exercise occurred as early as 2 h(MRF4) and remained elevated as late as 12 h (myogenin). Peak induction appears to occur 4–8 h after exercise with little response in the early phase immediately after resistance exercise. These data are in agreement with other studies (10, 30, 36, 38, 40), which have reported changes in select myogenic genes 2–6 h after resistive type exercise.
Psilander et al. (30) and Vissing et al. (36) found that MyoD increased immediately postexercise, which may be partly due to the feeding before the exercise (36) or differences in exercise protocol. In the study by Psilander et al. (30), the authors reported a sustained return to baseline levels of myogenic genes within 24 h after an exhaustive resistance exercise session over a 48-h recovery period, which is in agreement with the present investigation. Thus it appears that feedings before exercise can influence the gene induction response in the early phases after exercise, but this is not sustained and returns to baseline within 24 h independent of preexercise feedings. Myogenin has been reported to be elevated two- to threefold at 24 h after a single (2) or second bout (2, 3) of electromyostimulation-induced resistance exercise, which differs from previous studies (10, 30, 36, 38, 40) and the present investigation. Both of these studies (2, 3) utilized an electrical stimulation protocol to induce RE responses, which differs from the dynamic RE protocol used in the present investigation. It is reasonable to assume that the differences in myogenic gene expression in the extended recovery phase after muscle activation may be related to protocol (multiple sessions vs. single session) and the type of muscle activation (stimulation vs. voluntary lifting) used in the investigations.
Acute resistance exercise had a modest impact on the studied metabolic genes investigated in this study. To our knowledge, this is the first study to examine selected metabolic gene induction in response to resistance exercise. Markers of carbohydrate metabolism, HKII and PDK4, were elevated within 2 h of the exercise bout and peaked at 4 and 8 h, respectively. These data are in agreement with aerobic studies showing that HKII and PDK4 have a robust response to exercise (17, 18, 22, 26, 27, 29, 36). In contrast, CD36 and CPT1, which are important regulators of fat metabolism (15), were not altered in the 24 h after resistance exercise, which is in agreement with other aerobic exercise studies (27 Study B, 35). Therefore, it seems that, other than the more anaerobic nature of the RE bout, these genes (CD36 and CPT1) of fat metabolism do not appear to be influenced by acute exercise or that their mRNA levels do not change within 24 h postexercise.
Running responses.
This is the first study in human skeletal muscle to examine myogenic genes in the gastrocnemius muscle with run exercise. Interestingly, 30 minutes of running at 75% of O2 max was sufficient to increase MyoD, a transcription factor that is involved in myogenesis. The other myogenic genes measured in this investigation, Myf5, myogenin, and MRF4, were relatively unchanged in the 24 h after the run exercise. Kadi et al. (16) found that a single bout of endurance exercise (one-legged cycle ergometry) was sufficient to increase myogenin expression in the vastus lateralis. The differences in mode of exercise or muscle phenotype make direct comparisons difficult, but the study by Kadi et al. and the present investigation indicate that endurance exercise can lead to an induction of select myogenic markers. In this regard, the mode of exercise should be considered because the mechanical stress to the muscle most likely differs between running and cycle-type exercise. In particular, running exercise is not purely metabolic because it has an eccentric component, which could trigger the myogenic response due to muscle damage. Indeed, eccentric muscle actions (25) and running (31) have been shown to alter key myogenic genes (MyoD and myogenin) in rats and early response genes in humans, respectively.
Although MyoD was the only myogenic gene induced in response to run exercise, the increases were fairly robust with 5- to 8-fold changes occurring 8–12 h after the run. What these data imply for muscle adaptation to run exercise is unknown. However, chronic run training leads to an increase in MHC I fibers and a decrease in hybrid muscle fibers (12) and atrophy of the slow-twitch muscle fibers (11). With this in mind, it is unlikely that an increase in myogenic precursors with run exercise would be an indicator of muscle hypertrophy. MyoD has been shown to be more prevalent in fast-twitch muscle fibers (13) and may be involved in the regulation of muscle phenotype (5, 20). Because submaximal run exercise is more reliant on oxidative type of muscle fibers, the recruitment of glycolytic muscle fibers during this type of exercise may be enough of a metabolic challenge to induce transient changes in gene expression that would contribute to alterations in muscle phenotype that would eventually result in a more oxidative muscle. Our previous data (11, 12) coupled with the present observation that MyoD mRNA increases with acute run exercise may be an indication of transcriptional regulation of fiber-specific alterations in MHC expression.
As with the resistance exercise bout, the running bout resulted in an increase in HKII and PDK4 mRNA and a return to baseline levels within 24 h, which is in agreement with previous studies (18, 22, 26, 27 Study A, 29, 36). However, these previous studies reported earlier increases in HKII (3–6 h postexercise) and PDK4 (1–4 h postexercise) compared with the present study (8 h postexercise). This could be due to the longer duration (1–4 h) of endurance exercise used (17, 18, 27 Study B, 29) as opposed to our 30 min of treadmill running protocol. High-intensity intervals (22), prior exercise (26), short-term training of 5 consecutive days (27 Study A), and prior feeding of high glycemic meal (36) might also have caused an earlier increase in HKII and PDK4 mRNA with exercise.
The observation that CD36 and CPT1 did not change with the run exercise bout is in agreement with previous studies using aerobic exercise bouts lasting 1 and 4 h in duration (27 Study B, 35). When untrained subjects undergo an aerobic exercise training program (5 days to 6 wk), CD36 and CPT1 mRNA have been shown to increase (27 Study A, 32, 35). It is possible that the 30-min run at 75% of O2 max might not be a great enough stimulus to induce changes in CD36 and CPT1 mRNA levels given that our recreationally active Run subjects were aerobically fit (mean O2 max of 63 ml·kg–1·min–1). However, CD36 and CPT1 mRNA were not increased after an acute bout of aerobic exercise in both the trained (5–9 days of aerobic training) and untrained states (27 Study A, 35). Thus it appears that these markers of fat metabolism are not altered with acute exercise. Or perhaps they are involved in other modes of transcriptional control that fall outside the range of the 24-h period used in the present investigation.
Additional considerations. Given that different subjects, different muscles, and a different mode of exercise were used for each protocol, direct comparisons between the RE and Run protocols are difficult. However, it is interesting to note that both modes of exercise induced a response in select myogenic and metabolic genes to the acute exercise bout. Fiber-type differences between the muscles studied must also be taken into account because it has recently been shown that slow- and fast-twitch fibers differ in their gene-expression profile with exercise (14). With this in mind, the fiber-type profiles of the RE and Run subjects observed in the present investigation (Table 1) most likely contributed to the gene-expression responses of the acute exercise bout.
When these data are put in context with previous studies, it is clear that there are a host of variables that can influence the gene induction response to exercise in humans. We chose to study our subjects in the postprandial state and wait until after the 8-h biopsy before feeding. This is an important consideration because fasting and feeding have both been shown to alter metabolic gene expression (28, 33). Additionally, muscle biopsy procedures (36) and training status among subjects (39) need to be taken into consideration when comparing human studies.
In summary, this is the first study to investigate both myogenic and metabolic gene expression with acute bouts of resistance or endurance exercise. In general, there is an induction of selected myogenic and metabolic genes with both forms of exercise. Induction of these genes occurs as early as 2 h and last up to 12 h postexercise. Peak induction of the responsive myogenic and metabolic genes generally occurs between 4 and 8 h after the exercise bout. These data provide basic timeline information for studies examining aspects of gene regulation with acute exercise in humans.
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GRANTS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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ACKNOWLEDGMENTS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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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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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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