While α-actin isoforms predominate in adult striated muscle, skeletal muscle-specific knockouts (KOs) of nonmuscle cytoplasmic βcyto - or γcyto -actin each cause a mild, but progressive myopathy effected by an unknown mechanism. Using transmission electron microscopy, we identified morphological abnormalities in both the mitochondria and the sarcoplasmic reticulum (SR) in aged muscle-specific βcyto - and γcyto -actin KO mice. We found βcyto - and γcyto -actin proteins to be enriched in isolated mitochondrial-associated membrane preparations, which represent the interface between mitochondria and sarco-endoplasmic reticulum important in signaling and mitochondrial dynamics. We also measured significantly elongated and interconnected mitochondrial morphologies associated with a significant decrease in mitochondrial fission events in primary mouse embryonic fibroblasts lacking βcyto - and/or γcyto -actin. Interestingly, mitochondrial respiration in muscle was not measurably affected as oxygen consumption was similar in skeletal muscle fibers from 12 month-old muscle-specific βcyto - and γcyto -actin KO mice. Instead, we found that the maximal rate of relaxation after isometric contraction was significantly slowed in muscles of 12-month-old βcyto - and γcyto -actin muscle-specific KO mice. Our data suggest that impaired Ca2+ re-uptake may presage development of the observed SR morphological changes in aged mice while providing a potential pathological mechanism for the observed myopathy.

Nature, it turns out, wields an extraordinary influence on emotions and trading. Traders and investors need to understand how their physiological design directs their behaviour toward money, other traders, the markets, profit strategies and so on. For traders, training like an athlete makes perfect sense – for example, if one observes the increase that takes place in cardiovascular tone of a trader's body following a day of trading in volatile markets. Physically active people also exhibit lower levels of anxiety and depression, partly due to the effects of exercise on norepinephrine, which helps the brain cope more effectively with stress. Weight training also activates the adrenal glands which help to regulate stress. Money might not sleep, but traders and investors should. Not sleeping enough increases cortisol levels, decreases leptin, a hormone that regulates appetite via feelings of satiety, and upregulates ghrelin: this is …

Physical activity is a key component of our environment that is a major contributor to the prevention of many chronic diseases that plague our society. Dr Frank Booth and colleagues have argued on numerous occasions that the human body evolved to expect and respond to high levels of physical activity, whereas currently our society provides us with technical innovations that encourage low levels of physical activity.1 The unfortunate result is that people are less physically active than ever and there are epidemic elevations in various chronic diseases. Numerous publications have shown that reduced physical activity significantly and unequivocally increases an individual’s risk for developing conditions such as type 2 diabetes, cardiovascular disease, and obesity.2–6 The data are so convincing that the Center for Disease Control lists physical inactivity as a potential cause for a number of chronic diseases.7 Contrary to the viewpoints of some, there …

We recently demonstrated that SOCS-3 is expressed in skeletal muscle. Thus, we examined the effects of exercise training on SOCS-3 expression in skeletal muscle and the potential role of SOCS-3 in muscle. Female Sprague-Dawley Rats (5–8 mos) were treadmill trained everyday for 12 weeks, resulting in a 62% increase in citrate synthase activity in the plantaris muscle. Exercise training increased SOCS-3 mRNA expression by 80% and 154% in the plantaris and soleus muscle, respectively. To mimic the effects of increased SOCS-3 expression, SOCS-3 cDNA was co-transfected with a NF-kappa beta luciferase construct into cultured myotubes. SOCS-3 overexpression increased NF- kappa beta transcriptional activity by 27-fold. The proximal region of the IL-6 gene promoter contains a NF- kappa beta consensus site, which contributes to increased IL-6 expression in various tissues. We attempted to determine if SOCS-3 induced IL-6 transcription though activation of this consensus site. SOCS-3 cDNA was co-transfected into cultured myotubes with either the IL-6 luciferase construct or a mutated NF- kappa beta IL-6 luciferase construct. SOCS-3 overexpression increased IL-6 transcriptional activity by 15-fold, however when the NF- kappa beta site was mutated SOCS-3 failed to increase IL-6 transcriptional activity. We subsequently found that IL-6 mRNA expression was elevated in the plantaris and soleus muscles of the trained animals compared to the sedentary animals. These data suggest that training-induced elevations in SOCS-3 expression in skeletal muscle may contribute to the exercise-induced increase in IL-6 expression through activation of NF- kappa beta. Supported by NIH HL 40306-15 and HL 72790-02 (RLM).

Suppressor of cytokine signalling‐3 (SOCS‐3) has been implicated in the onset of insulin resistance in non‐muscle tissue. Thus, we examined the effects of exercise training on SOCS‐3 expression and the potential role of SOCS‐3 in muscle. Female Sprague‐Dawley rats (5–8 months) were treadmill trained for 12 weeks and the muscles were removed 24 h after the last bout of exercise. Exercise training increased SOCS‐3 mRNA expression by 80% and 154% in the plantaris and soleus muscle, respectively. To mimic the effects of increased SOCS‐3 expression, SOCS‐3 cDNA was cotransfected with a NF‐kappa B (NF‐κB) luciferase construct into cultured C2C12 myotubes. SOCS‐3 overexpression increased NF‐κB transcriptional activity by 27‐fold. The proximal region of the IL‐6 gene promoter contains a NF‐κB consensus site, which contributes to increased IL‐6 expression in various tissues. SOCS‐3 cDNA was cotransfected into cultured C2C12 myotubes with either the IL‐6 luciferase construct or a mutated NF‐κB IL‐6 luciferase construct. SOCS‐3 overexpression increased IL‐6 transcriptional activity by 15‐fold, however, when the NF‐κB site was mutated SOCS‐3 failed to increase IL‐6 transcriptional activity. We subsequently found that IL‐6 mRNA expression was elevated in the plantaris and soleus muscles of the trained animals compared to the sedentary animals. Finally, exercise induced a significant reduction in IκBα and increased phosphorylation of Iκκ suggesting that NF‐κB activation was elevated after exercise training. These data suggest that training‐induced elevations in SOCS‐3 expression in skeletal muscle may contribute to the exercise‐induced increase in IL‐6 expression through alterations in the mechanisms that mediate NF‐κB activity.

Eccentric contractions (EC) are known to result in muscle hypertrophy, potentially through activation of the Akt-mammalian target of rapamycin-p70 S6 kinase (p70S6K) signaling pathway. Previous work has also demonstrated that EC result in the opening of stretch-activated channels (SAC), and inhibition of these channels resulted in an attenuation of EC-induced muscle hypertrophy. The purpose of this study was to test the hypothesis that a known intracellular pathway directly associated with muscle hypertrophy is coupled to the opening of SAC. Specifically, we measured the activation of the Akt, GSK-3beta, p70S6K, and ribosomal protein S6 following a single bout of EC in the rat tibialis anterior (TA) muscle. The TA muscles performed four sets of six repetitions of EC. In vivo blockade of SAC was performed by a continuous oral treatment with streptomycin in the drinking water (4 g/l) or by intravenous infusion of 80 micromol/kg gadolinium (Gd3+). EC increased the degree of Akt and p70S6K phosphorylation in the TA muscle, whereas in animals in which SAC had been inhibited, there was a reduced capacity for EC to induce Akt or p70S6K phosphorylation. Accompanying this reduced activation of Akt and p70S6K was a failure to phosphorylate GSK-3beta or S6 when SAC were inhibited. The results from these data indicate the necessity of functional SAC for the complete activation of Akt and p70S6K pathway in response to EC.

Myoblast differentiation is characterized by a sequence of events that includes an increase in insulin-like growth factor ( IGF)-I and contractile gene expression. The increase in IGF-I expression activates cell signaling mechanisms that participate in the differentiation process. One potential contributor is the SOCS-3 ( suppressor of cytokine signaling-3) gene, which regulates signaling mechanisms and may be sensitive to changes in IGF-I concentrations. For the first time, the role of SOCS-3 is investigated in myoblast differentiation. SOCS-3 mRNA levels and SOCS-3 transcriptional activity increase during myoblast differentiation. SOCS-3 gene expression is induced, at least in part, by activation of the IGF-I receptor during myoblast differentiation. Overexpression of SOCS-3 cDNA significantly increased transcriptional activation of the 2.0-kb skeletal alpha-actin promoter in differentiating C2C12 myoblasts. In addition, overexpression of SOCS-3 specifically increased serum response factor-driven transcriptional activity but had no effect on nuclear-factor of activated T cell-driven transcriptional activity. SOCS-3 overexpression induced skeletal alpha-actin transcription in a myoblast cell line that cannot respond to endogenous IGF-I, indicating that SOCS-3 can contribute to the myoblast differentiation process in the absence of IGF-I. These data suggest that IGF-I induces myoblast differentiation, in part, by increasing SOCS-3 expression.

Various cellular signaling pathways, such as phosphatidylinositol 3-kinase, calcineurin, Janus kinase 2/signal transducer and activator of transcription 3 (STAT3), and mitogen-activated protein kinase (MAPK) have been suggested to play an important role in skeletal muscle growth. Old muscle, compared with young muscle, lacks the ability to completely regrow its muscle mass after an atrophy-induced stimulus. it is hypothesized that defects and/or delays in the activation of specific cell signaling pathways of aged soleus muscle limit the potential for growth. To test this, 42 male Fischer 344 x Brown Norway rats, 30 mo old, were hindlimb immobilized for 10 days, and their muscle samples were compared with muscle samples analyzed from 3- to 4-mo-old rats in a previous report (Childs TE, Spangenburg EE, Vyas DR, and Booth FW. Am J Physiol Cell Physiol: 285: C391-C398, 2003). After 10 days, the immobilization was removed and rats were allowed to ambulate for a series of days. Alterations in the activation or deactivation status of specific signaling pathways were determined by comparing the phosphorylation (phos) and total concentration of specific signaling proteins (pan) through Western blotting with the 10-day immobilization group. Various cell signals and their respective time groups of the old rats were shown to be significantly different compared with the 10-day immobilization group. For example, peak increases during recovery from the immobilization were observed at 1) the third recovery day for calcineurin B-pan and 2) the sixth recovery day for glycogen synthase kinase-3beta-phos, p70 S6 kinase (p70S6k) -phos and -pan, calcineurin A-pan, STAT3-phos and -pan, p44 MAPK-pan, and p42 MAPK-pan. In contrast, Akt-pan, c-Jun NH2-terminal kinase-phos, and p38 MAPK-phos were observed to decrease from 10-day immobilization values to control levels. Also, Aktphos was unchanged among all groups. In a follow-up experiment in which muscle samples from both the present study and a previous study (Childs TE, Spangenburg EE, Vyas DR, and Booth FW. Am J Physiol Cell Physiol: 285: C391-C398, 2003) were reanalyzed together, the recovery-induced increase in p70S6k-phos from immobilization-atrophy was significantly attenuated in soleus muscles of the old group.

This review is divided into two parts, the first dealing with the cell and molecular biology of muscle in terms of growth and wasting and the second being an account of current knowledge of physiological mechanisms involved in the alteration of size of the human muscle mass. Wherever possible, attempts have been made to interrelate the information in each part and to provide the most likely explanation for phenomena that are currently only partially understood. The review should be of interest to cell and molecular biologists who know little of human muscle physiology and to physicians, physiotherapists, and kinesiologists who may be familiar with the gross behavior of human muscle but wish to understand more about the underlying mechanisms of change.

Aged individuals suffer from multiple dysfunctions during skeletal muscle atrophy. The purpose of this study was to determine differential changes in gene expression in atrophied soleus muscle induced by hindlimb immobilization in young (3–4 months) and old (30–31 months) rats. The hypothesis was that differentially expressed mRNAs with age‐atrophy interactions would reveal candidates that induce loss of function responses in aged animals. Each muscle was applied to an independent set of Affymetrix micoarrays, with 385 differentially expressed mRNAs with atrophy and 354 age‐atrophy interactions detected by two‐factor ANOVA (α of 0.05 with a Bonferroni adjustment). Functional trends were observed for 23 and 15 probe sets involved in electron transport and the extracellular matrix, respectively, decreasing more in the young than in the old. Other functional categories with atrophy in both ages included chaperones, glutathione‐S‐transferases, the tricarboxylic acid cycle, reductions in Z‐line‐associated proteins and increases in probe sets for protein degradation. Surprisingly, myosin heavy chain IIb and IIx mRNAs were suppressed in the atrophied soleus muscle of old rats as opposed to the large increases in the young animals (16‐ and 25‐fold, respectively, with microarrays, and 61‐ and 68‐fold, respectively, with real‐time PCR). No significant changes were observed in myosin heavy chain IIb and IIx mRNA with micoarrays in the atrophied soleus muscles of old rats, but they were found to increase six‐ and fivefold, respectively, with real‐time PCR. Therefore, deficiencies in pre‐translational signals that normally upregulate myosin heavy chain IIb and IIx mRNAs during atrophy may exist in the soleus muscle of old animals.

The purpose of the current study was to examine IGFBP-3, -4, and -5 mRNA and protein expression levels as a function of muscle type, age, and regrowth from an immobilization-induced atrophy in Fischer 344 x Brown Norway rats. IGFBP-3 mRNA expression in the 4-mo-old animals was significantly higher in the red and white portions of the gastrocnemius muscle compared with the soleus muscle. However, there were no significant differences in IGFBP-3 mRNA expression among any of the muscle groups in the 30-mo-old animals. There were no significant differences in IGFBP-5 mRNA expression in any of the muscle groups, whereas in the 30-mo-old animals there was significantly less IGFBP-5 mRNA expression in the white gastrocnemius compared with the red gastrocnemius muscles. Although IGFBP-3 and -5 proteins were detected in the type I soleus muscle with Western blot analyses, no detection was observed in the type II red and white portions of the gastrocnemius muscle. Aging from adult (18 mo) to old animals (30 mo) was associated with decreases in IGFBP-3 mRNA and protein and IGFBP-5 protein only in the soleus muscle. After 10 days of recovery from 10 days of hindlimb immobilization, IGFBP-3 mRNA and protein increased in soleus muscles from young (4-mo) rats; however, only IGFBP-3 protein increased in the old (30-mo) rats. Whereas there were no changes in IGFBP-5 mRNA expression during recovery, IGFBP-5 protein in the 10-day-recovery soleus muscle did increase in the young, but not in the old, rats. Because one of the functions of IGFBPs is to modulate IGF-I action on muscle size and phenotype, it is hypothesized that IGFBP-3 and -5 proteins may have potential modulatory roles in type I fiber-dominated muscles, aging, and regrowth from atrophy.

The insulin‐like growth factor I (IGF‐I) stimulates muscle satellite cell proliferation. Chakravarthy et al., (2000, J Biol Chem 275:35942–35952.) previously found that IGF‐I‐stimulated proliferation of primary satellite cells was associated with the activation of phosphatidylinositol 3′‐kinase (PI3K)/Akt and the downregulation of a cell‐cycle inhibitor p27Kip1. To understand mechanisms by which IGF‐I signals the downregulation of p27Kip1 in rat skeletal satellite cells, the role of Forkhead transcription factor FoxO1 in transcriptional activity of p27Kip1 was examined. When primary rat satellite cells were transfected with a p27Kip1 promoter–reporter gene construct, IGF‐I (100 ng/ml) inhibited specific p27Kip1 promoter activity. Addition of LY294002, an inhibitor of PI3K, reversed the IGF‐I‐mediated downregulation of p27Kip1 promoter activity. Co‐transfection of wild type (WT) FoxO1 into satellite cells increased p27Kip1 promoter activity in the absence of IGF‐I supplementation. Addition of IGF‐I reversed the induction of p27Kip1 promoter activity by WT FoxO1. When a mutated FoxO1 (without Thr24, Ser256, and Ser316 Akt phosphorylation sites) was used, IGF‐I was no longer able to reverse the FoxO1 induced stimulation of p27Kip1 promoter activity that had been seen when WT FoxO1 was present. When the satellite cells were treated with IGF‐I, phosphorylation of Akt‐Ser473 and FoxO1‐Ser256 was increased. In addition, when the cells were pre‐incubated with LY294002 before IGF‐I stimulation, the phosphorylation of Akt‐Ser473 and FoxO1‐Ser256 was inhibited, implying that phosphorylation of Akt and FoxO1 was downstream of IGF‐I‐induced PI3K signaling. However, IGF‐I did not induce phosphorylation of FoxO1 on residues Thr24 and Ser316. These results suggested that IGF‐I induced the phosphorylation of Ser256 and inactivated FoxO1 thereby downregulating the activation of the p27Kip1 promoter. Thus, inactivation of FoxO1 by IGF‐I plays a critical role in rat skeletal satellite cell proliferation through regulation of p27Kip1 expression. J. Cell. Physiol. 196: 523–531, 2003. © 2003 Wiley‐Liss, Inc.

IGF-I activates signaling pathways that increase the expression of muscle-specific genes in differentiating myoblasts. Induction of skeletal alpha-actin expression occurs during differentiation through unknown mechanisms. The purpose of this investigation was to examine the mechanisms that IGF-I uses to induce skeletal alpha-actin gene expression in C2C12 myoblasts. IGF-I increased skeletal alpha-actin promoter activity by 107% compared with the control condition. Ni(+) [T-type voltage-gated Ca(2+) channel (VGCC) inhibitor] reduced basal-induced activation of the skeletal alpha-actin promoter by approximately 84%, and nifedipine (L-type VGCC inhibitor) inhibited IGF-I-induced activation of the skeletal alpha-actin promoter by 29-48%. IGF-I failed to increase skeletal alpha-actin promoter activity in differentiating dysgenic (lack functional L-type VGCC) myoblasts; 30 mm K(+) and 30 mm K(+)+IGF-I increased skeletal alpha-actin promoter activity by 162% and 76% compared with non-IGF-I or IGF-I-only conditions, respectively. IGF-I increased calcineurin activity, which was inhibited by cyclosporine A. Further, cyclosporine A inhibited K(+)+IGF-I-induced activation of the skeletal alpha-actin promoter. Constitutively active calcineurin increased skeletal alpha-actin promoter activity by 154% and rescued the nifedipine-induced inhibition of L-type VGCC but failed to rescue the Ni(+)-inhibition of T-type VGCC. IGF-I-induced nuclear factor of activated T-cells transcriptional activity was not inhibited by nifedipine or Ni(+). IGF-I failed to increase serum response factor transcriptional activity; however, serum response factor activity was reduced in the presence of Ni(+). These data suggest that IGF-I-induced activation of the skeletal alpha-actin promoter is regulated by the L-type VGCC and calcineurin but independent of nuclear factor of activated T-cell transcriptional activity as C2C12 myoblasts differentiate into myotubes.

Currently, the repertoire of cellular and molecular pathways that control skeletal muscle atrophy and hypertrophy are not well defined. It is possible that intracellular regulatory signaling pathways are active at different times during the muscle hypertrophy process. The hypothesis of the given experiments was that cellular signals related to protein translation would be active at early time points of skeletal muscle regrowth, whereas transcriptional signals would be active at later time points of skeletal muscle regrowth. The phosphorylation status of p38 MAPK and JNK increased at the end of limb immobilization but returned to control values at recovery day 3. Transient increases in phosphorylation and in protein concentration occurred during recovery of soleus muscle mass. Phosphorylation of Akt, p70S6k, and signal transducer and activator of transcription 3 (STAT3) peaked on recovery day 3 compared with day 0. Glycogen synthase kinase (GSK)-3beta phosphorylation was increased on the sixth and fifteenth recovery day. In addition, transient peaks in seven protein concentrations occurred at different times of recovery: STAT3, calcineurin A (CaNA), CaNB, and beta4E-BP1 protein concentrations peaked on the third recovery day; p70S6k, STAT3, Akt, and GSK3-beta peaked on the sixth recovery day; and GSK3-beta peaked on the fifteenth recovery day. The apexes of STAT3 and GSK3-beta protein concentrations remained elevated for two recovery time points. Thus the time course of increase in molecules of signaling pathways differed as the young rat soleus muscle regrew from an atrophied state.

A hypothesis is presented based on a coalescence of anthropological estimations of Homo sapiens' phenotypes in the Late Paleolithic era 10,000 years ago, with Darwinian natural selection synergized with Neel's idea of the so-called thrifty gene. It is proposed that humans inherited genes that were evolved to support a physically active lifestyle. It is further postulated that physical inactivity in sedentary societies directly contributes to multiple chronic health disorders. Therefore, it is imperative to identify the underlying genetic and cellular/biochemical bases of why sedentary living produces chronic health conditions. This will allow society to improve its ability to effect beneficial lifestyle changes and hence improve the overall quality of living. To win the war against physical inactivity and the myriad of chronic health conditions produced because of physical inactivity, a multifactorial approach is needed, which includes successful preventive medicine, drug development, optimal target selection, and efficacious clinical therapy. All of these approaches require a thorough understanding of fundamental biology and how the dysregulated molecular circuitry caused by physical inactivity produces clinically overt disease. The purpose of this review is to summarize the vast armamentarium at our disposal in the form of the extensive scientific basis underlying how physical inactivity affects at least 20 of the most deadly chronic disorders. We hope that this information will provide readers with a starting point for developing additional strategies of their own in the ongoing war against inactivity-induced chronic health conditions.

It is thought that changes in sarco(endo)plasmic reticulum Ca 2+ -ATPase (SERCA) of skeletal muscle contribute to alterations in skeletal muscle function during congestive heart failure (CHF). It is well established that exercise training can improve muscle function. However, it is unclear whether similar adaptations will result from exercise training in a CHF patient. Therefore, the purpose of this study was to determine whether skeletal muscle during moderate CHF adapts to increased activity, utilizing the functional overload (FO) model. Significant increases in plantaris mass of the CHF-FO and sham-FO groups compared with the CHF and control (sham) groups were observed. Ca 2+ uptake rates were significantly elevated in the CHF group compared with all other groups. No differences were detected in Ca 2+ uptake rates between the CHF-FO, sham, and sham-FO groups. Increases in Ca 2+ uptake rates in moderate-CHF rats were not due to changes in SERCA isoform proportions; however, FO may have attenuated the CHF-induced increases through alterations in SERCA isoform expression. Therefore, changes in skeletal muscle Ca 2+ handling during moderate CHF may be due to alterations in regulatory mechanisms, which exercise may override, by possibly altering SERCA isoform expression.

To determine whether changes in glycogen synthase kinase-3β (GSK-3β) phosphorylation contribute to muscle hypertrophy, we delineated the effects of GSK-3β activity on C2C12 myotube size. We also examined possible insulin-like growth factor I (IGF-I) signaling of NFAT (nuclear factors of activated T cells)-inducible gene activity and possible modulation of NFAT activation by GSK-3β. Application of IGF-I (250 ng/ml) or LiCl (10 mM) alone (i.e., both inhibit GSK-3β activity) increased the area of C2C12 myotubes by 80 and 85%, respectively. The application of IGF-I (250 ng/ml) elevated GSK-3β phosphorylation and reduced GSK-3β kinase activity by ∼800% and ∼25%, respectively. LY-294002 (100 μM) and wortmannin (150 μM), specific inhibitors of phosphatidylinositol 3′-kinase, attenuated IGF-I-induced GSK-3β phosphorylation by 67 and 92%, respectively. IGF-I suppressed the kinase activity of GSK-3β. IGF-I (250 ng/ml), but not LiCl (10 mM), induced an increase in NFAT-activated luciferase reporter activity. Cotransfection of a constitutively active GSK-3β (cGSK-3β) inhibited the induction by IGF-I of NFAT-inducible reporter activity. LiCl, which inhibits GSK-3β, removed the block by cGSK-3β on IGF-I-inducible NFAT-responsive reporter gene activity. These data suggest that the IGF-I-induced increase in skeletal myotube size is signaled, in part, through the inhibition of GSK-3β.

There are many known growth factors/cytokines that induce skeletal muscle satellite cell proliferation. Currently, the signaling mechanisms in which these growth factors/cytokines activate satellite cell proliferation are not completely understood. Here, we sought to determine signaling mechanisms by which leukemia inhibitory factor (LIF) induces satellite cell proliferation in culture. First, we confirmed that LIF induces proliferation of C2C12 immortalized myoblasts and cultured primary rat satellite cells. In addition, we also found that this increase in proliferation can be inhibited by incubation of the cells in tyrphostin AG 490, a specific inhibitor of Janus-activated kinase (JAK) 2 activity. Furthermore, we also found that incubation of the cells at various time points with LIF (10 ng/ml) induces a significant, transient increase in JAK2 phosphorylation, signal transducers and activators of transcription (STAT3) phosphorylation, and STAT3 transcriptional activity. Increases in the STAT3-sensitive endogenous SOC3 protein followed these transient increases in STAT3 activation. In addition, AG 490 inhibited the increase in STAT3 phosphorylation. Finally, LIF did not change the phosphorylation status of extracellular signal-regulated protein kinase (ERK)1/2 or affect the phosphorylation status of Akt/protein kinase B. However, LY-294002, an inhibitor of phosphoinositide 3-kinase, blocked LIF-induced proliferation of satellite cells. These data suggest that LIF induces satellite cell proliferation by activation of the JAK2-STAT3 signaling pathway, suggesting that this may be an important pathway in muscle growth and/or hypertrophy.