SOCS-3 Induces Myoblast Differentiation

Interleukin-6 Induces Myogenic Differentiation via JAK2-STAT3 Signaling in Mouse C2C12 Myoblast Cell Line and Primary Human Myoblasts

Postnatal muscle growth and exercise- or injury-induced regeneration are facilitated by myoblasts. Myoblasts respond to a variety of proteins such as cytokines that activate various signaling cascades. Cytokines belonging to the interleukin 6 superfamily (IL-6) influence myoblasts' proliferation but their effect on differentiation is still being researched. The Janus kinase (JAK)-signal transducer and activator of transcription (STAT) pathway is one of the key signaling pathways identified to be activated by IL-6. The aim of this study was to investigate myoblast fate as well as activation of JAK-STAT pathway at different physiologically relevant IL-6 concentrations (10 pg/mL; 100 pg/mL; 10 ng/mL) in the C2C12 mouse myoblast cell line and primary human myoblasts, isolated from eight young healthy male volunteers. Myoblasts' cell cycle progression, proliferation and differentiation in vitro were assessed. Low IL-6 concentrations facilitated cell cycle transition from the quiescence/Gap1 (G0/G1) to the synthesis (S-) phases. Low and medium IL-6 concentrations decreased the expression of myoblast determination protein 1 (MyoD) and myogenin and increased proliferating cell nuclear antigen (PCNA) expression. In contrast, high IL-6 concentration shifted a larger proportion of cells to the pro-differentiation G0/G1 phase of the cell cycle, substantiated by significant increases of both MyoD and myogenin expression and decreased PCNA expression. Low IL-6 concentration was responsible for prolonged JAK1 activation and increased suppressor of cytokine signaling 1 (SOCS1) protein expression. JAK-STAT inhibition abrogated IL-6-mediated C2C12 cell proliferation. In contrast, high IL-6 initially increased JAK1 activation but resulted in prolonged JAK2 activation and elevated SOCS3 protein expression. High IL-6 concentration decreased interleukin-6 receptor (IL-6R) expression 24 h after treatment whilst low IL-6 concentration increased IL-6 receptor (IL-6R) expression at the same time point. In conclusion, this study demonstrated that IL-6 has concentration- and time-dependent effects on both C2C12 mouse myoblasts and primary human myoblasts. Low IL-6 concentration induces proliferation whilst high IL-6 concentration induces differentiation. These effects are mediated by specific components of the JAK/STAT/SOCS pathway.

Deletion of suppressor of cytokine signaling 3 (SOCS3) in muscle stem cells does not alter muscle regeneration in mice after injury

Muscles of older animals are more susceptible to injury and regenerate poorly, in part due to a persistent inflammatory response. The janus kinase (Jak)/signal transducer and activator of transcription (Stat) pathway mediates inflammatory signaling and is tightly regulated by the suppressor of cytokine signaling (SOCS) proteins, especially SOCS3. SOCS3 expression is altered in the muscle of aged animals and may contribute to the persistent inflammation and impaired regeneration. To test this hypothesis, we performed myotoxic injuries on mice with a tamoxifen-inducible deletion of SOCS3 specifically within the muscle stem cell compartment. Muscle stem cell-specific SOCS3 deletion reduced muscle mass at 14 days post-injury (-14%, P < 0.01), altered the myogenic transcriptional program, and reduced myogenic fusion based on the number of centrally-located nuclei per muscle fiber. Despite the delay in myogenesis, muscles with a muscle stem cell-specific deletion of SOCS3 were still able to regenerate after a single bout or multiple bouts of myotoxic injury. A reduction in SOCS3 expression in muscle stem cells is unlikely to be responsible for the incomplete muscle repair in aged animals.

Muscle-specific deletion of SOCS3 does not reduce the anabolic response to leucine in a mouse model of acute inflammation

Excessive inflammation reduces skeletal muscle protein synthesis leading to wasting and weakness. The janus kinase/signal transducers and activators of transcription-3 (JAK/STAT3) pathway is important for the regulation of inflammatory signaling. As such, suppressor of cytokine signaling-3 (SOCS3), the negative regulator of JAK/STAT signaling, is thought to be important in the control of muscle homeostasis. We hypothesized that muscle-specific deletion of SOCS3 would impair the anabolic response to leucine during an inflammatory insult. Twelve week old (n=8 per group) SOCS3 muscle-specific knockout mice (SOCS3-MKO) and littermate controls (WT) were injected with lipopolysaccharide (LPS, 1mg/kg) or saline and were studied during fasted conditions or after receiving 0.5g/kg leucine 3h after the injection of LPS. Markers of inflammation, anabolic signaling, and protein synthesis were measured 4h after LPS injection. LPS injection robustly increased mRNA expression of inflammatory molecules (Socs3, Socs1, Il-6, Ccl2, Tnfα and Cd68). In muscles from SOCS3-MKO mice, the Socs3 mRNA response to LPS was significantly blunted (∼6-fold) while STAT3 Tyr705 phosphorylation was exacerbated (18-fold). Leucine administration increased protein synthesis in both WT (∼1.6-fold) and SOCS3-MKO mice (∼1.5-fold) compared to basal levels. LPS administration blunted this effect, but there were no differences between WT and SOCS3-MKO mice. Muscle-specific SOCS3 deletion did not alter the response of AKT, mTOR, S6 or 4EBP1 under any treatment conditions. Therefore, SOCS3 does not appear to mediate the early inflammatory or leucine-induced changes in protein synthesis in skeletal muscle.

JAK-STAT pathway and myogenic differentiation

Myogenic differentiation plays an important role in muscle regeneration and is regulated by two transcription factor families, MRFs and MEF2, which induce differentiation of myoblasts through expression of the muscle-specific gene, myogenin. In addition, many intracellular signaling pathways are also involved in myogenic differentiation, including p38 MAPK, ERK/MAPK and PI3K/AKT. The JAK-STAT pathway is activated by various cytokines and positively or negatively regulates the differentiation of myoblasts. JAK1 plays a notable role in proliferation; whereas, JAK2 and JAK3 function mainly in differentiation. The STATs, molecules downstream of JAK, regulate myogenesis. With JAK1, STAT1 promotes proliferation, while STAT3 has a dual effect on proliferation and differentiation. The JAK-STAT negative regulator, SOCS, is also associated with myogenesis; although, its role is controversial. In this review, we will discuss the role of the JAK-STAT pathway on myogenic differentiation.

BRCA1 is a novel regulator of metabolic function in skeletal muscle

Breast cancer type 1 (BRCA1) susceptibility protein is expressed across multiple tissues including skeletal muscle. The overall objective of this investigation was to define a functional role for BRCA1 in skeletal muscle using a translational approach. For the first time in both mice and humans, we identified the presence of multiple isoforms of BRCA1 in skeletal muscle. In response to an acute bout of exercise, we found increases in the interaction between the native forms of BRCA1 and the phosphorylated form of acetyl-CoA carboxylase. Decreasing BRCA1 content using a shRNA approach in cultured primary human myotubes resulted in decreased oxygen consumption by the mitochondria and increased reactive oxygen species production. The decreased BRCA1 content also resulted in increased storage of intracellular lipid and reduced insulin signaling. These results indicate that BRCA1 plays a critical role in the regulation of metabolic function in skeletal muscle. Collectively, these data reveal BRCA1 as a novel target to consider in our understanding of metabolic function and risk for development of metabolic-based diseases.

Interleukin-6-induced satellite cell proliferation is regulated by induction of the JAK2/STAT3 signalling pathway through cyclin D1 targeting

OBJECTIVES

To determine whether interleukin-6 (IL-6) stimulates rat muscle satellite cell proliferation in culture, and if so, to clarify the signalling mechanisms.

MATERIALS AND METHODS

Primary satellite cells were isolated from thirty male F344 rats, 11 weeks of age. IL-6 at concentrations of 0.01, 0.1, 1, 10 or 100 ng/ml was added to culture media.

RESULTS

IL-6 at 0.01-1 ng/ml induced dose-dependent increase in cell proliferation. After treatment with 1 ng/ml IL-6, cell proliferation increased by 31%, and p-STAT3(+) /MyoD(+) cells increased in number compared to those in control media (P < 0.05). Inhibitors of JAK2 (AG 490) and STAT3 (STAT3 peptide) blocked the increase in BrdUrd(+) cell numbers at 6 h post stimulation with 1 ng/ml IL-6 (P < 0.05). Furthermore, cyclin D1 mRNA expression and cyclin D1(+) /MyoD(+) cell numbers significantly increased in cultures treated with 1 ng/ml IL-6 compared to those in control media (P < 0.05). In contrast, treatment with 10 and 100 ng/ml IL-6 did not stimulate cell proliferation. Treatment with 10 ng/ml IL-6 induced greater SOCS3 mRNA expression than with 1 ng/ml IL-6 and control media. Moreover, co-localization of SOCS3 and myogenin was observed after treatment with 10 ng/ml IL-6.

CONCLUSIONS

IL-6 induced dose-dependent increase in satellite cell proliferation by activating the JAK2/STAT3/cyclin D1 pathway.

Characterization and regulation of mechanical loading-induced compensatory muscle hypertrophy

In mammalian systems, skeletal muscle exists in a dynamic state that monitors and regulates the physiological investment in muscle size to meet the current level of functional demand. This review attempts to consolidate current knowledge concerning development of the compensatory hypertrophy that occurs in response to a sustained increase in the mechanical loading of skeletal muscle. Topics covered include: defining and measuring compensatory hypertrophy, experimental models, loading stimulus parameters, acute responses to increased loading, hyperplasia, myofiber-type adaptations, the involvement of satellite cells, mRNA translational control, mechanotransduction, and endocrinology. The authors conclude with their impressions of current knowledge gaps in the field that are ripe for future study.

Striated muscle activator of Rho signaling is required for myotube survival but does not influence basal protein synthesis or degradation

Skeletal muscle mass is regulated by sensing and transmitting extracellular mechanical stress signals to intracellular signaling pathways controlling protein synthesis and degradation. Striated muscle activator of Rho signaling (STARS) is a muscle-specific actin-binding protein that is sensitive to extracellular stress signals. STARS stimulates actin polymerization and influences serum response factor (SRF) and peroxisome proliferator-activated receptor-γ coactivator (PGC)-1α transcription of genes involved in muscle growth, structure, and contraction. The role of STARS in skeletal muscle cells is not well understood. This study investigated whether STARS influenced C2C12 myotube growth by regulating protein synthesis and degradation. The influence of STARS on Pgc-1α, Srf, and Errα mRNA levels, as well as several of their downstream targets involved in muscle cell growth, contraction, and metabolism, was also investigated. STARS overexpression increased actin polymerization, with no effect on protein synthesis, protein degradation, or Akt phosphorylation. STARS overexpression increased Pgc-1α, Srf, Ckmt2, Cpt-1β, and Mhc1 mRNA. STARS knockdown reduced actin polymerization and increased cell death and dead cell protease activity. It also increased markers of inflammation (Casp1, Il-1β, and Mcp-1), regeneration (Socs3 and Myh8), and fast myosin isoforms (Mhc2a and Mhc2x). We show for the first time in muscle cells that STARS overexpression increases actin polymerization and shifts the muscle cell to a more oxidative phenotype. The suppression of STARS causes cell death and increases markers of necrosis, inflammation, and regeneration. As STARS levels are suppressed in clinical models associated with increased necrosis and inflammation, such as aging and limb immobilization, rescuing STARS maybe a future therapeutic strategy to maintain skeletal muscle function and attenuate contraction-induced muscle damage.

Interleukin-6 myokine signaling in skeletal muscle: a double-edged sword?

Interleukin (IL)-6 is a cytokine with pleiotropic functions in different tissues and organs. Skeletal muscle produces and releases significant levels of IL-6 after prolonged exercise and is therefore considered as a myokine. Muscle is also an important target of the cytokine. IL-6 signaling has been associated with stimulation of hypertrophic muscle growth and myogenesis through regulation of the proliferative capacity of muscle stem cells. Additional beneficial effects of IL-6 include regulation of energy metabolism, which is related to the capacity of actively contracting muscle to synthesize and release IL-6. Paradoxically, deleterious actions for IL-6 have also been proposed, such as promotion of atrophy and muscle wasting. We review the current evidence for these apparently contradictory effects, the mechanisms involved and discuss their possible biological implications.

Interleukin-6 promotes myogenic differentiation of mouse skeletal muscle cells: role of the STAT3 pathway

Myogenic differentiation of skeletal muscle cells is characterized by a sequence of events that include activation of signal transducer and activator of transcription 3 (STAT3) and enhanced expression of its target gene Socs3. Autocrine effects of IL-6 may contribute to the activation of the STAT3-Socs3 cascade and thus to myogenic differentiation. The importance of IL-6 and STAT3 for the differentiation process was studied in C2C12 cells and in primary mouse wild-type and IL-6−/− skeletal muscle cells. In differentiating C2C12 myoblasts, the upregulation of IL-6 mRNA expression and protein secretion started after increased phosphorylation of STAT3 on tyrosine 705 and increased mRNA expression of Socs3 was observed. Knockdown of STAT3 and IL-6 mRNA in differentiating C2C12 myoblasts impaired the expression of the myogenic markers myogenin and MyHC IIb and subsequently myotube fusion. However, the knockdown of IL-6 did not prevent the induction of STAT3 tyrosine phosphorylation. The IL-6-independent activation of STAT3 was verified in differentiating primary IL-6−/− myoblasts. The phosphorylation of STAT3 and the expression levels of STAT3, Socs3, and myogenin during differentiation were comparable in the primary myoblasts independent of the genotype. However, IL-6−/− cells failed to induce MyHC IIb expression to the same level as in wild-type cells and showed reduced myotube formation. Supplementation of IL-6 could partially restore the fusion of IL-6−/− cells. These data demonstrate that IL-6 depletion during myogenic differentiation does not reduce the activation of the STAT3-Socs3 cascade, while IL-6 and STAT3 are both necessary to promote myotube fusion.

Deletion of skeletal muscle SOCS3 prevents insulin resistance in obesity

Obesity is associated with chronic low-grade inflammation that contributes to defects in energy metabolism and insulin resistance. Suppressor of cytokine signaling (SOCS)-3 expression is increased in skeletal muscle of obese humans. SOCS3 inhibits leptin signaling in the hypothalamus and insulin signal transduction in adipose tissue and the liver. Skeletal muscle is an important tissue for controlling energy expenditure and whole-body insulin sensitivity; however, the physiological importance of SOCS3 in this tissue has not been examined. Therefore, we generated mice that had SOCS3 specifically deleted in skeletal muscle (SOCS MKO). The SOCS3 MKO mice had normal muscle development, body mass, adiposity, appetite, and energy expenditure compared with wild-type (WT) littermates. Despite similar degrees of obesity when fed a high-fat diet, SOCS3 MKO mice were protected against the development of hyperinsulinemia and insulin resistance because of enhanced skeletal muscle insulin receptor substrate 1 (IRS1) and Akt phosphorylation that resulted in increased skeletal muscle glucose uptake. These data indicate that skeletal muscle SOCS3 does not play a critical role in regulating muscle development or energy expenditure, but it is an important contributing factor for inhibiting insulin sensitivity in obesity. Therapies aimed at inhibiting SOCS3 in skeletal muscle may be effective in reversing obesity-related glucose intolerance and insulin resistance.

Wheel running prevents the accumulation of monounsaturated fatty acids in the liver of ovariectomized mice by attenuating changes in SCD-1 content

Decreases in female sex steroids enhance the accumulation of visceral fat mass, leading to a predisposition to developing metabolic diseases. The purpose of this study was to determine whether loss of ovarian function alters the amount and (or) the fatty acid (FA) composition of triacylglycerol (TAG) levels in the liver of ovary-intact (SHAM) or ovariectomized (OVX) mice. We also sought to determine whether voluntary wheel running could attenuate the associated changes in the liver. Twenty-two C57/BL6 female mice were divided into 2 groups (SHAM, OVX) and were then subdivided into sedentary and exercising groups (SHAM-Sed, SHAM-Ex, OVX-Sed, OVX-Ex). Visceral fat mass significantly increased in the OVX-Sed animals; however, the effect was attenuated in the OVX-Ex animals. Total hepatic TAG content did not significantly increase in the OVX-Sed animals; however, SHAM-Ex and OVX-Ex animals demonstrated significant decreases in TAG levels. A significant increase in the FA desaturase index (18:1/18:0 and 16:1/16:0) was detected in the OVX-Sed animals compared with all other groups, which corresponded to increases in stearoyl-CoA desaturase (SCD-1) content. These results indicate that loss of ovarian function alters FA composition of hepatic TAG mediated by increases in SCD-1. These data indicate that female sex steroids influence lipid metabolism in the liver and provide important insight concerning the influence of exercise on hepatic function.

Impact of SOCS3 overexpression on human skeletal muscle development in vitro

The Janus kinase/signal transducers and activators of transcription (JAK/STAT) signaling cascade has been identified as a crucial factor for myogenesis. The STAT3 isoform is essential for satellite cell migration and myogenic differentiation as it mediates the expression of muscle specific myogenic factors. The SOCS (suppressors of cytokine signaling) family of proteins down-regulates STAT activation. Primary human skeletal muscle cells were isolated and cultured to investigate the effect of SOCS3 adenoviral overexpression on myotube maturation. It was demonstrated that STAT3 inhibition did not influence myotube development or survival. Moreover, SOCS3 overexpression enhances the mRNA expression of downstream targets of STAT3, c-FOS and VEGF. These increases were correlated with enhanced mRNA expression of genes associated with muscle maturation and hypertrophy. Thus SOCS3 influences myoblast differentiation and SOCS3 may be significant in regulating the activity of genes previously identified as transcriptionally regulated by STAT3.

JAK/STAT signaling and human in vitro myogenesis

BACKGROUND

A population of satellite cells exists in skeletal muscle. These cells are thought to be primarily responsible for postnatal muscle growth and injury-induced muscle regeneration. The Janus kinase/signal transducers and activators of transcription (JAK/STAT) signaling cascade has a crucial role in regulating myogenesis. In rodent skeletal muscle, STAT3 is essential for satellite cell migration and myogenic differentiation, regulating the expression of myogenic factors. The aim of the present study was to investigate and compare the expression profile of JAK/STAT family members, using cultured primary human skeletal muscle cells.

RESULTS

Near confluent proliferating myoblasts were induced to differentiate for 1, 5 or 10 days. During these developmental stages, members of the JAK/STAT family were examined, along with factors known to regulate myogenesis. We demonstrate the phosphorylation of JAK1 and STAT1 only during myoblast proliferation, while JAK2 and STAT3 phosphorylation increases during differentiation. These increases were correlated with the upregulation of genes associated with muscle maturation and hypertrophy.

CONCLUSIONS

Taken together, these results provide insight into JAK/STAT signaling in human skeletal muscle development, and confirm recent observations in rodents.

Sodium arsenite delays the differentiation of C2C12 mouse myoblast cells and alters methylation patterns on the transcription factor myogenin

Epidemiological studies have correlated arsenic exposure with cancer, skin diseases, and adverse developmental outcomes such as spontaneous abortions, neonatal mortality, low birth weight, and delays in the use of musculature. The current study used C2C12 mouse myoblast cells to examine whether low concentrations of arsenic could alter their differentiation into myotubes, indicating that arsenic can act as a developmental toxicant. Myoblast cells were exposed to 20 nM sodium arsenite, allowed to differentiate into myotubes, and expression of the muscle-specific transcription factor myogenin, along with the expression of tropomyosin, suppressor of cytokine signaling 3 (Socs3), prostaglandin I2 synthesis (Ptgis), and myocyte enhancer 2 (Mef2), was investigated using QPCR and immunofluorescence. Exposing C2C12 cells to 20 nM sodium arsenite delayed the differentiation process, as evidenced by a significant reduction in the number of multinucleated myotubes, a decrease in myogenin mRNA expression, and a decrease in the total number of nuclei expressing myogenin protein. The expression of mRNA involved in myotube formation, such as Ptgis and Mef2 mRNA, was also significantly reduced by 1.6-fold and 4-fold during differentiation. This was confirmed by immunofluorescence for Mef2, which showed a 2.6-fold reduction in nuclear translocation. Changes in methylation patterns in the promoter region of myogenin (-473 to +90) were examined by methylation-specific PCR and bisulfite genomic sequencing. Hypermethylated CpGs were found at -236 and -126 bp, whereas hypomethylated CpGs were found at -207 bp in arsenic-exposed cells. This study indicates that 20 nM sodium arsenite can alter myoblast differentiation by reducing the expression of the transcription factors myogenin and Mef2c, which is likely due to changes in promoter methylation patterns. The delay in muscle differentiation may lead to developmental abnormalities.

Constitutive expression of suppressor of cytokine signalling-3 in skeletal muscle leads to reduced mobility and overweight in mice

AIMS/HYPOTHESIS

Due to their ability to regulate various signalling pathways (cytokines, hormones, growth factors), the suppressor of cytokine signalling (SOCS) proteins are thought to be promising therapeutic targets for metabolic and inflammatory disorders. Hence, their role in vivo has to be precisely determined.

METHODS

We generated transgenic mice constitutively producing SOCS-3 in skeletal muscle to define whether the sole abundance of SOCS-3 is sufficient to induce metabolic disorders and whether SOCS-3 is implicated in physiological roles distinct from metabolism.

RESULTS

We demonstrate here that chronic expression of SOCS-3 in skeletal muscle leads to overweight in mice and worsening of high-fat diet-induced systemic insulin resistance. Counter-intuitively, insulin sensitivity in muscle of transgenic mice appears to be unaltered. However, following constitutive SOCS-3 production, several genes had deregulated expression, among them other members of the SOCS family. This could maintain the insulin signal into skeletal muscle. Interestingly, we found that SOCS-3 interacts with calcineurin, which has been implicated in muscle contractility. In Socs-3 transgenic muscle, this leads to delocalisation of calcineurin to the fibre periphery. Relevant to this finding, Socs-3 transgenic animals had dilatation of the sarcoplasmic reticulum associated with swollen mitochondria and decreased voluntary activity.

CONCLUSIONS/INTERPRETATION

Our results show that constitutive SOCS-3 production in skeletal muscle is not in itself sufficient to induce the establishment of metabolic disorders such as diabetes. In contrast, we reveal a novel role of SOCS-3, which appears to be important for muscle integrity and locomotor activity.

Effects of acute and chronic endurance exercise on intracellular nitric oxide in putative endothelial progenitor cells: role of NAPDH oxidase

We sought to delineate the effects of acute and chronic exercise on the regulation of intracellular nitric oxide (NO(i)) production in putative endothelial progenitor cells (EPCs). Putative EPC colony-forming units (CFU-EC) were cultured from blood drawn before and after 30 min of treadmill exercise at 75% of maximal oxygen uptake in active (n = 8) and inactive (n = 8) men. CFU-EC were similar between groups at baseline, but increased after exercise in active men only (P = 0.04). CFU-EC expressed lower NADPH oxidase subunit gp91(phox) mRNA and elevated endothelial nitric oxide synthase mRNA in active relative to inactive men at baseline (P < 0.05). Acute exercise reduced gp91(phox) mRNA in CFU-EC of both groups (P < 0.05), whereas p47(phox) mRNA levels were reduced in the inactive group only (P = 0.02). There were no differences between groups or with acute exercise in xanthine oxidase, superoxide dismutase isoforms, or gluthathione peroxidase-1 mRNA levels. NO(i) was significantly greater in CFU-EC of active men at baseline (P = 0.004). NO(i) increased in CFU-EC of inactive men with acute exercise, and in vitro experiments with apocynin indicated the increased NO(i) production was caused by suppression of NADPH oxidase. However, the increases in NO(i) with the different treatments in the inactive group did not reach the baseline levels in the active group (P < 0.05). We conclude that acute exercise increases NO(i) in cells generated by the CFU-EC assay through an NADPH oxidase-inhibition mechanism in sedentary men. However, differences due to chronic exercise must involve additional factors. Our findings support exercise as a means to improve putative EPC function and suggest a novel mechanism that may explain this effect.

Rapid muscle atrophy response to unloading: pretranslational processes involving MHC and actin

Skeletal muscles, especially weight-bearing muscles, are very sensitive to changes in loading state. The aim of this paper was to characterize the dynamic changes in the unloaded soleus muscle in vivo following a short bout of hindlimb suspension (HS), testing the hypothesis that transcriptional events respond early to the atrophic stimulus. In fact, we observed that after only 1 day of HS, primary transcript levels of skeletal alpha-actin and type I myosin heavy chain (MHC) genes were significantly reduced by more than 50% compared with ground control levels. The degree of the decline for the mRNA expression of actin and type I MHC lagged behind that of the pre-mRNA levels after 1 day of HS, but by 2 and 7 days of HS, large decreases were observed. Although the faster MHC isoforms, IIx and IIb, began to be expressed in soleus after 1 day of HS, a relatively significant shift in mRNA expression from the slow MHC isoform type I toward these fast MHC isoforms did not emerge until 7 days of HS. One day of HS was sufficient to show significant decreases in mRNA levels of putative signaling factors serum response factor (SRF), suppressor of cytokine signaling-3 (SOCS3), and striated muscle activator of Rho signaling (STARS), although transcription factors yin-yang-1 (YY1) and transcriptional enhancing factor-1 (TEF-1) were not significantly affected by HS. The protein levels of actin and type I MHC were significantly decreased after 2 days of HS, and SRF protein was significantly decreased after 7 days HS. Our results show that after only 1 day of unloading, pre-mRNA and mRNA expression of muscle proteins and muscle-specific signaling factors are significantly reduced, suggesting that the downregulation of the synthesis side of the protein balance equation that occurs in atrophying muscle is initiated rapidly.

SOCS1, SOCS3, and PIAS1 promote myogenic differentiation by inhibiting the leukemia inhibitory factor-induced JAK1/STAT1/STAT3 pathway

We recently showed that a leukemia inhibitory factor (LIF)-engaged signaling pathway consisting of JAK1, STAT1, and STAT3 plays dual roles in myogenic differentiation: while it participates in myoblast proliferation, it also actively represses differentiation. Downregulation of this pathway is required at the onset of differentiation. However, it remained unclear how this is achieved mechanistically. We now show that SOCS1, SOCS3, and PIAS1 promote myogenic differentiation by specifically inhibiting the LIF-induced JAK1/STAT1/STAT3 pathway via distinct targets; whereas SOCS1 and SOCS3 selectively bind and inhibit JAK1 and gp130, respectively, PIAS1 targets mainly the activated STAT1 and prevents its binding to DNA. We further demonstrated that the SUMO E3-ligase activity of PIAS1 is dispensable for its role in myogenic differentiation. Collectively, our current study revealed a molecular mechanism that explains how the LIF-induced JAK1/STAT1/STAT3 pathway is downregulated upon myogenic differentiation.

Alterations in peroxisome proliferator-activated receptor mRNA expression in skeletal muscle after acute and repeated bouts of exercise

Peroxisome proliferator-activated receptors (PPAR) exist in three different forms, alpha (alpha), beta/delta (beta/delta), or gamma (gamma), all of which are expressed in skeletal muscle and play a critical role in the regulation of oxidative metabolism. The purpose of this investigation was to determine the mRNA expression pattern of the different PPARs and peroxisome proliferator-activated receptor alpha coactivator-1 alpha (PGC-1alpha) in muscles that largely rely on either glycolytic (plantaris) or oxidative (soleus) metabolism. Further, we also examined the alterations in the PPARs mRNA expression after one bout of endurance exercise or after 12 weeks of exercise training in the different muscles. Female Sprague-Dawley rats (5-8 months) were either run on the treadmill once or exercised trained for 12 weeks. The muscles were removed 24 h after the last bout of exercise. The results demonstrated with the exception of PPAR beta/delta, the PPAR mRNAs are expressed to a greater extent in the soleus muscle than in the plantaris muscle in sedentary animals. PPARgamma was the least abundantly expressed PPAR in either the soleus or the plantaris muscle. With respect to exercise training, only PPARgamma mRNA expression increased in the soleus muscle, while PPARbeta/delta and gamma mRNA levels increased in the plantaris muscle. Minimal changes were detected in any of the PPARs with one bout of exercise training. These results suggest that PPARgamma mRNA levels are the lowest in skeletal muscle among all of the PPARs and PPARgamma mRNA is the most responsive to changes in physical activity levels.

Exercise-induced activation of STAT3 signaling is increased with age

Activation of the transcription factor signal transducers and activators of transcription (STAT) 3 is common to many inflammatory cytokines and growth factors, with recent evidence of involvement in skeletal muscle regeneration. The purpose of this study was to determine whether STAT3 signaling activation is regulated differentially, at rest and following intense resistance exercise, in aged human skeletal muscle. Skeletal muscle biopsies were harvested from healthy younger (n = 11, 20.4 +/- 0.8 years) and older men (n = 10, 67.4 +/- 1.3 years) under resting conditions and 2 h after the completion of resistance exercise. No differences were evident at rest, whereas the phosphorylation of STAT3 was significantly increased in old (23-fold) compared to young (5-fold) subjects after exercise. This correlated with significantly higher induction of the STAT3 target genes including; interleukin-6 (IL-6), JUNB, c-MYC, and suppressor of cytokine signaling (SOCS) 3 mRNA in older subjects following exercise. Despite increased SOCS3 mRNA, cellular protein abundance was suppressed. SOCS3 protein is an important negative regulator of STAT3 activation and cytokine signaling. Thus, in aged human muscle, elevated responsiveness of the STAT3 signaling pathway and suppressed SOCS3 protein are evident following resistance exercise. These data suggest that enhanced STAT3 signaling responsiveness to proinflammatory factors may impact on mechanisms of muscle repair and regeneration.

SOCS-3 inhibits E2F/DP-1 transcriptional activity and cell cycle progression via interaction with DP-1

Recent studies using SOCS family knock-out mice have suggested that SOCS proteins have multiple biological functions in addition to their role as negative regulators of JAK-STAT signaling. To explore these other functions of this family of proteins, we used yeast two-hybrid screening to find proteins interacting with human SOCS-3. We identified the transcriptional factor DP-1 as a SOCS-3-interacting protein involved in regulation of the cell cycle. Immunoprecipitation-Western blot assay showed that this interaction between these endogenous proteins occurred in cells both in vitro and in vivo. SOCS-3 interacted with the C-terminal region of DP-1, and amino acids 156-172 of SOCS-3 were required for this interaction. Confocal microscopy revealed that SOCS-3 and DP-1 were primarily colocalized in the cytoplasm. SOCS-3 inhibited E2F/DP-1 transcriptional activity under the cyclin-E promoter and actually inhibited cell cycle progression and cell growth under E2F/DP-1 control. In contrast, DP-1 almost completely eliminated the inhibitory action of SOCS-3 on LIF-stimulated STAT-3 transcriptional activity in JAK-STAT signaling. Interestingly, the alternative regulatory action of SOCS-3 and DP-1 was dramatically eliminated by each siRNA. Taken together, these findings demonstrate that SOCS-3 acts as a negative regulator of the cell cycle progression under E2F/DP-1 control by interfering with heterodimer formation between DP-1 and E2F and also that DP-1 plays an important role in controlling JAK-STAT signaling.

Myogenin regulates a distinct genetic program in adult muscle stem cells

In contrast to the detailed understanding we have for the regulation of skeletal muscle gene expression in embryos, similar insights into postnatal muscle growth and regeneration are largely inferential or do not directly address gene regulatory mechanisms. Muscle stem cells (satellite cells) are chiefly responsible for providing new muscle during postnatal and adult life. The purpose of this study was to determine the role that the myogenic basic helix-loop-helix regulatory factor myogenin has in postnatal muscle growth and adult muscle stem cell gene expression. We found that myogenin is absolutely required for skeletal muscle development and survival until birth, but it is dispensable for postnatal life. However, Myog deletion after birth led to reduced body size implying a role for myogenin in regulating body homeostasis. Despite a lack of skeletal muscle defects in Myog-deleted mice during postnatal life and the efficient differentiation of cultured Myog-deleted adult muscle stem cells, the loss of myogenin profoundly altered the pattern of gene expression in cultured muscle stem cells and adult skeletal muscle. Remarkably, these changes in gene expression were distinct from those found in Myog-null embryonic skeletal muscle, indicating that myogenin has separate functions during postnatal life.

Role of beta-adrenoceptor signaling in skeletal muscle: implications for muscle wasting and disease

The importance of beta-adrenergic signaling in the heart has been well documented, but it is only more recently that we have begun to understand the importance of this signaling pathway in skeletal muscle. There is considerable evidence regarding the stimulation of the beta-adrenergic system with beta-adrenoceptor agonists (beta-agonists). Although traditionally used for treating bronchospasm, it became apparent that some beta-agonists could increase skeletal muscle mass and decrease body fat. These so-called "repartitioning effects" proved desirable for the livestock industry trying to improve feed efficiency and meat quality. Studying beta-agonist effects on skeletal muscle has identified potential therapeutic applications for muscle wasting conditions such as sarcopenia, cancer cachexia, denervation, and neuromuscular diseases, aiming to attenuate (or potentially reverse) the muscle wasting and associated muscle weakness, and to enhance muscle growth and repair after injury. Some undesirable cardiovascular side effects of beta-agonists have so far limited their therapeutic potential. This review describes the physiological significance of beta-adrenergic signaling in skeletal muscle and examines the effects of beta-agonists on skeletal muscle structure and function. In addition, we examine the proposed beneficial effects of beta-agonist administration on skeletal muscle along with some of the less desirable cardiovascular effects. Understanding beta-adrenergic signaling in skeletal muscle is important for identifying new therapeutic targets and identifying novel approaches to attenuate the muscle wasting concomitant with many diseases.

Endogenous interferon-gamma is required for efficient skeletal muscle regeneration

The inflammatory response is thought to play important roles in tissue healing. The hypothesis of this study was that the inflammatory cytokine interferon (IFN)-gamma is produced endogenously following skeletal muscle injury and promotes efficient healing. We show that IFN-gamma is expressed at both mRNA and protein levels in skeletal muscle following injury, and that the time course of IFN-gamma expression correlated with the accumulation of macrophages, T-cells, and natural killer cells, as well as myoblasts, in damaged muscle. Cells of each type were isolated from injured muscle, and IFN-gamma expression was detected in each cell type. We also demonstrate that administration of an IFN-gamma receptor blocking antibody to wild-type mice impaired induction of interferon response factor-1, reduced cell proliferation, and decreased formation of regenerating fibers. IFN-gamma null mice showed similarly impaired muscle healing associated with impaired macrophage function and development of fibrosis. In vitro studies demonstrated that IFN-gamma and its receptor are expressed in the C2C12 muscle cell line, and that the IFN-gamma receptor blocking antibody reduced proliferation and fusion of these muscle cells. In summary, our results indicate that IFN-gamma promotes muscle healing, in part, by stimulating formation of new muscle fibers.

The effects of leptin on airway smooth muscle responses

Obesity is associated with asthma and airway hyperresponsiveness. Leptin modulates some of the proinflammatory effects observed in obesity. The objective of this study was to determine the effects of leptin on airway smooth muscle responses. The effect of leptin (0.1-100 ng/ml) on migration (toward platelet-derived growth factor [PDGF], 10 ng/ml, across collagen-coated membrane in Transwell culture plates), proliferation (by BrDU incorporation), and cytokine production (by Bioplex bead assay) of cultured human airway smooth muscle cells from nine nonasthmatic donors was assessed. Effects of leptin on the contractile responses were studied in bovine tracheal smooth muscle rings. Leptin receptor expression and activation of STAT-3, Src kinase, Suppressor of Cytokine Signaling-3 (SOCS-3), and COX were evaluated by Western blotting and PCR. PGE(2) levels in supernatant were assessed by enzyme immunoassay. Human airway smooth muscle cells express leptin receptor, which, when engaged, phosphorylated STAT-3. Leptin inhibited PDGF-induced human airway smooth muscle migration and proliferation and IL-13-induced eotaxin production. Leptin did not stimulate cytokine synthesis and did not evoke contractile responses or inhibit isoproterenol-induced relaxation of carbachol-induced contraction of bovine tracheal rings. The inhibitory effects on migration and eotaxin production are not due to activation of SOCS-3 but are partly due to increased production of PGE(2) because they were attenuated by indomethacin. In conclusion, leptin inhibited human airway smooth muscle proliferation, migration toward PDGF, and IL-13-induced eotaxin production. This is partly mediated by PGE(2) secretion from smooth muscle cells induced by leptin. The association between obesity and asthma is unlikely to be due to a direct effect of leptin on airway smooth muscle.

Neuroendocrine hormones such as growth hormone and prolactin are integral members of the immunological cytokine network

Neuroendocrine hormones such as growth hormone (GH) and prolactin (PRL) have been demonstrated to accelerate the recovery of the immune response after chemotherapy and bone marrow transplantation and to enhance the restoration of immunity in individuals infected with HIV and in normal individuals with compromised immune systems associated with aging. As the mechanism of action of these hormones has been elucidated, it has become clear that they are integral members of the immunological cytokine/chemokine network and share regulatory mechanisms with a wide variety of cytokines and chemokines. The members of this cytokine network induce and can be regulated by members of the suppressor of cytokine signaling (SOCS) family of intracellular proteins. In order to take advantage of the potential beneficial effects of hormones such as GH or PRL, it is essential to take into consideration the overall cytokine network and the regulatory effects of SOCS proteins.

Transcriptional activation of the suppressor of cytokine signaling-3 (SOCS-3) gene via STAT3 is increased in F9 REX1 (ZFP-42) knockout teratocarcinoma stem cells relative to wild-type cells

Rex1 (Zfp42), first identified as a gene that is transcriptionally repressed by retinoic acid (RA), encodes a zinc finger transcription factor expressed at high levels in F9 teratocarcinoma stem cells, embryonic stem cells, and other stem cells. Loss of both alleles of Rex1 by homologous recombination alters the RA-induced differentiation of F9 cells, a model of pluripotent embryonic stem cells. We identified Suppressor of Cytokine Signaling-3 (SOCS-3) as a gene that exhibits greatly increased transcriptional activation in RA, cAMP, and theophylline (RACT)-treated F9 Rex1(-/-) cells (approximately 25-fold) as compared to wild-type (WT) cells ( approximately 2.5-fold). By promoter deletion, mutation, and transient transfection analyses, we have shown that this transcriptional increase is mediated by the STAT3 DNA-binding elements located between -99 to -60 in the SOCS-3 promoter. Overexpression of STAT3 dominant-negative mutants greatly diminishes this SOCS-3 transcriptional increase in F9 Rex1(-/-) cells. This increase in SOCS-3 transcription is associated with a four- to fivefold higher level of tyrosine-phosphorylated STAT3 in the RACT-treated F9 Rex1(-/-) cells as compared to WT. Dominant-negative Src tyrosine kinase, Jak2, and protein kinase A partially reduce the transcriptional activation of the SOCS 3 gene in RACT-treated F9 Rex1 null cells. In contrast, parathyroid hormone peptide enhances the effect of RA in F9 Rex1(-/-) cells, but not in F9 WT. Thus, Rex1, which is highly expressed in stem cells, inhibits signaling via the Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway, thereby modulating the differentiation of F9 cells.

Suppressor of Cytokine Signaling, Skeletal Muscle, and Chronic Health Conditions

The suppressor of cytokine gene family (suppressor of cytokine signaling (SOCS)) is a recently described gene family that has the ability to affect a variety of physiological functions in the human body. This review will discuss the potential role that the SOCS proteins may have in skeletal muscle and the virtually unexplored interaction with exercise training.

Different transcription profiles of SOCS-3, ob and IGF-I genes and their possible correlations in obese and lean pigs

Pig breeds have significant differences in fat deposition and muscle development ability. However, the molecular mechanism behind these differences is still unknown. In this study, the expression patterns of three candidate genes, suppressor of cytokine signaling 3 (SOCS-3), obesity (ob) and insulin-like growth factor I (IGF-I), which are involved in adipose metabolism or muscle development, were analyzed. Total RNA was extracted from dorsal subcutaneous adipose tissue and longissimus of 8-month-old Bamei and Largewhite pigs. Semiquantitative reverse transcription-polymerase chain reaction was used to determine the expression levels of the SOCS-3 and ob genes in adipose tissue, and SOCS-3 and IGF-I genes in muscle tissue. The results showed that in adipose tissue the expression level of SOCS-3 was significantly higher in Bamei (obese) pigs than that in Largewhite (lean) pigs (P<0.01). However, in muscle tissue it was significantly lower in Bamei than that in Largewhite pigs (P<0.01). Furthermore, the expression of SOCS-3 was positively correlated to that of ob in adipose tissue and that of IGF-I in muscle tissue. These findings suggest that the difference in SOCS-3 gene expression levels in adipose and muscle tissues, the relationship between SOCS-3 and ob in adipose tissue, and that between SOCS-3 and IGF-I in muscle tissue, might contribute to the different fat deposition and muscle development ability between obese and lean pigs.

Regulation of SOCS-3 expression by leptin and its co-localization with insulin receptor in rat skeletal muscle cells

Obesity is a well-defined risk factor for the development of insulin resistance in target tissues, such as skeletal muscle, and thus type 2 diabetes. This may occur due to endocrine effects mediated by adipokines including leptin, the product of the obese (ob) gene, whose circulating levels positively correlate with body mass index. Induction of suppressor of cytokine-3 (SOCS-3) protein expression has been implicated as a possible mechanism of leptin-induced insulin resistance. Here, we show that treatment of rat skeletal muscle cells with leptin activated the SOCS-3 gene promoter and caused a time-dependent increase in both SOCS-3 mRNA and protein content. Confocal microscopy demonstrated increased co-localization of SOCS-3 with insulin receptor in leptin-treated cells and we confirmed a direct interaction between these two proteins by showing increased coimmunoprecipitation of SOCS-3 and insulin receptor after exposure of cells to leptin. However, the expected functional consequences were not observed, as we saw no change in basal or insulin-stimulated glucose uptake and phosphorylation of GSK3beta, Akt (T308 and S473) or ERK1/2. In summary, leptin induced SOCS-3 expression and its association with the insulin receptor in rat skeletal muscle cells but functional significance of this increase was not apparent upon measuring glucose uptake.

Suppressor of cytokine signaling-3 antagonizes cAMP effects on proliferation and apoptosis and is expressed in human prostate cancer

Interleukin-6, levels of which are elevated in prostate cancer, activates different signal transduction pathways including that of Janus kinases/signal transducer and activator of transcription (STAT)3. However, phosphorylation of STAT3 has been reported to be associated with either stimulatory or inhibitory effects on cellular proliferation. To better understand the mechanisms of STAT3 regulation in benign and malignant prostate, we have investigated the role of suppressor of cytokine signaling (SOCS)-3. Cell lines that did not express phosphorylated STAT3 were found to be SOCS-3-positive. SOCS-3 was re-expressed in LNCaP cells after treatment with a demethylating agent. SOCS-3 immunohistochemistry revealed a negative or weak reaction in benign areas, whereas its expression was detected in tumor tissue. To investigate the involvement of SOCS-3 in regulation of cellular events, we incubated cancer cells with a cAMP derivative. This treatment yielded higher SOCS-3 levels, reduced [3H]thymidine incorporation, and increased percentage of apoptotic cells. However, down-regulation of SOCS-3 by a short interfering RNA approach resulted in inhibition of proliferation and an increased apoptotic rate. Collectively, our results show that SOCS-3 antagonizes regulation of cellular events by cAMP and is expressed in human prostate cancer.

Similar acute molecular responses to equivalent volumes of isometric, lengthening, or shortening mode resistance exercise

The present study was undertaken to test the hypothesis that the contraction mode of action [static-isometric (Iso), shortening-concentric (Con), or lengthening-eccentric (Ecc)] used to stress the muscle provides a differential mechanical stimulus eliciting greater or lesser degrees of anabolic response at the initiation of a resistance training program. We performed an acute resistance training study in which different groups of rodents completed four training sessions in either the Iso, Con, or Ecc mode of contraction under conditions of activation and movement specifically designed to elicit equivalent volumes of force accumulation. The results of this experiment indicate that the three modes of contraction produced nearly identical cell signaling, indicative of an anabolic response involving factors such as increased levels of mRNA for IGF-I, procollagen III alpha1, decreased myostatin mRNA, and increased total RNA concentration. The resulting profiles collectively provide evidence that pure mode of muscle action, in and of itself, does not appear to be a primary variable in determining the efficacy of increased loading paradigms with regard to the initiation of selected muscle anabolic responses.

Exercise increases SOCS-3 expression in rat skeletal muscle: potential relationship to IL-6 expression

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-kappaB) luciferase construct into cultured C2C12 myotubes. SOCS-3 overexpression increased NF-kappaB transcriptional activity by 27-fold. The proximal region of the IL-6 gene promoter contains a NF-kappaB 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-kappaB IL-6 luciferase construct. SOCS-3 overexpression increased IL-6 transcriptional activity by 15-fold, however, when the NF-kappaB 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 IkappaBalpha and increased phosphorylation of Ikappakappa suggesting that NF-kappaB 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-kappaB activity.

SOCS-2 interferes with myotube formation and potentiates osteoblast differentiation through upregulation of JunB in C2C12 cells

Suppressor of cytokine signaling (SOCS)-2 regulates normal postnatal growth and its deficiency in mice causes gigantism with increased bone length and proportional enlargement in skeletal muscles. Using C2C12 mesenchymal precursor cell line as a model, we investigated a possible role of SOCS-2 in the differentiation process of mesenchymal precursors. Stable transfection of SOCS-2 into C2C12 cells resulted in the acceleration of proliferation and survival, and inhibition of spontaneous myotube formation. In addition, SOCS-2 potentiated bone morphogenic protein (BMP)-induced transdifferentiation of C2C12 cells into osteoblast phenotypes. These effects of SOCS-2 on C2C12 cells differed strikingly from that of SOCS-1, another member of SOCS family, and its mechanisms were evaluated. SOCS-2 did not alter BMP-induced phosphorylation and nuclear accumulation of Smad1, nor the expression of inhibitory-Smads mRNA. However, SOCS-2 enhanced BMP-induced transcriptional activation of the Smad-responsive reporter gene, suggesting that the action of SOCS-2 is exerted at the transcriptional level. Interestingly, SOCS-2 overexpression in C2C12 cells increased the endogenous JunB protein, one of the key transcriptional factors in the control of BMP/Smad signaling responsiveness. In addition, the proteasome inhibitor enhanced JunB protein expression in C2C12 cells. Moreover, we found that SOCS-2 reduced JunB ubiquitination in COS-7 cells. Although SOCS-2 is a modulator of growth hormone (GH) signaling, the upregulation of JunB by SOCS-2 did not require GH signaling. Taken together, these results suggest that SOCS-2 positively regulates endogenous JunB protein expression in C2C12 cells through inhibition of JunB destabilization by the ubiquitin-proteasome pathway, and thereby regulates the cell fate of mesenchymal precursors.