Insulin-like growth factor-mediated muscle cell survival: central roles for Akt and cyclin-dependent kinase inhibitor p21

Establishment and Characterization of SV40 T-Antigen Immortalized Porcine Muscle Satellite Cell

Muscle satellite cells (MuSCs) are crucial for muscle development and regeneration. The primary pig MuSCs (pMuSCs) is an ideal in vitro cell model for studying the pig's muscle development and differentiation. However, the long-term in vitro culture of pMuSCs results in the gradual loss of their stemness, thereby limiting their application. To address this conundrum and maintain the normal function of pMuSCs during in vitro passaging, we generated an immortalized pMuSCs (SV40 T-pMuSCs) by stably expressing SV40 T-antigen (SV40 T) using a lentiviral-based vector system. The SV40 T-pMuSCs can be stably sub-cultured for over 40 generations in vitro. An evaluation of SV40 T-pMuSCs was conducted through immunofluorescence staining, quantitative real-time PCR, EdU assay, and SA-β-gal activity. Their proliferation capacity was similar to that of primary pMuSCs at passage 1, and while their differentiation potential was slightly decreased. SiRNA-mediated interference of SV40 T-antigen expression restored the differentiation capability of SV40 T-pMuSCs. Taken together, our results provide a valuable tool for studying pig skeletal muscle development and differentiation.

Inonotus obliquus upregulates muscle regeneration and augments function through muscle oxidative metabolism

Skeletal muscle wasting related to aging or pathological conditions is critically associated with the increased incidence and prevalence of secondary diseases including cardiovascular diseases, metabolic syndromes, and chronic inflammations. Much effort is made to develop agents to enhance muscle metabolism and function. Inonotus obliquus (I. obliquus; IO) is a mushroom popularly called chaga and has been widely employed as a folk medicine for inflammation, cardiovascular diseases, diabetes, and cancer in Eastern Europe and Asia. However, its effect on muscle health has not been explored. Here, we aimed to investigate the beneficial effect of IO extract in muscle regeneration and metabolism. The treatment of IO in C2C12 myoblasts led to increased myogenic differentiation and alleviation of dexamethasone-induced myotube atrophy. Network pharmacological analysis using the identified specific chemical constituents of IO extracts predicted protein kinase B (AKT)-dependent mechanisms to promote myogenesis and muscle regeneration. Consistently, IO treatment resulted in the activation of AKT, which suppressed muscle-specific ubiquitin E3 ligases induced by dexamethasone. IO treatment in mice improved the regeneration of cardiotoxin-injured muscles accompanied by elevated proliferation and differentiation of muscle stem cells. Furthermore, it elevated the mitochondrial content and muscle oxidative metabolism accompanied by the induction of peroxisome proliferator-activated receptor γ coactivator α (PGC-1α). Our current data suggest that IO is a promising natural agent in enhancing muscle regenerative capacity and oxidative metabolism thereby preventing muscle wasting.

Hormesis mediates platelet-rich plasma and wound healing

Platelet-rich plasma (PRP) has become an accepted and general wound healing approach with an extremely wide range of applications. Despite considerable diversity in the composition of platelet-rich plasma products that are applied in specific wound healing usage, it is widely recognised that such diverse platelet-rich plasma complex mixtures routinely display hormetic-like biphasic concentrations that are independent of the tissue treated and endpoints measured. The present paper is the first to place the area of platelet-rich plasma-biomedical research and applications within an hormetic framework. The platelet-rich plasma area is also unique as it represents the application of the hormetic concept to the issue of complex biological mixtures.

The roles of growth factors and hormones in the regulation of muscle satellite cells for cultured meat production

Cultured meat is a potential sustainable food generated by the in vitro myogenesis of muscle satellite (stem) cells (MSCs). The self-renewal and differentiation properties of MSCs are of primary interest for cultured meat production. MSC proliferation and differentiation are influenced by a variety of growth factors such as insulin-like growth factors (IGF-1 and IGF-2), transforming growth factor beta (TGF-β), fibroblast growth factors (FGF-2 and FGF-21), platelet-derived growth factor (PDGF) and hepatocyte growth factor (HGF) and by hormones like insulin, testosterone, glucocorticoids, and thyroid hormones. In this review, we investigated the roles of growth factors and hormones during cultured meat production because these factors provide signals for MSC growth and structural stability. The aim of this article is to provide the important idea about different growth factors such as FGF (enhance the cell proliferation and differentiation), IGF-1 (increase the number of myoblasts), PDGF (myoblast proliferation), TGF-β1 (muscle repair) and hormones such as insulin (cell survival and growth), testosterone (muscle fiber size), dexamethasone (myoblast proliferation and differentiation), and thyroid hormones (amount and diameter of muscle fibers and determine the usual pattern of fiber distributions) as media components during myogenesis for cultured meat production.

The mitochondrial adenine nucleotide transporters in myogenesis

Adenine Nucleotide Translocator isoforms (ANTs) exchange ADP/ATP across the inner mitochondrial membrane, are also voltage-activated proton channels and regulate mitophagy and apoptosis. The ANT1 isoform predominates in heart and muscle while ANT2 is systemic. Here, we report the creation of Ant mutant mouse myoblast cell lines with normal Ant1 and Ant2 genes, deficient in either Ant1 or Ant2, and deficient in both the Ant1 and Ant2 genes. These cell lines are immortal under permissive conditions (IFN-γ + serum at 32 °C) permitting expansion but return to normal myoblasts that can be differentiated into myotubes at 37 °C. With this system we were able to complement our Ant1 mutant studies by demonstrating that ANT2 is important for myoblast to myotube differentiation and myotube mitochondrial respiration. ANT2 is also important in the regulation of mitochondrial biogenesis and antioxidant defenses. ANT2 is also associated with increased oxidative stress response and modulation for Ca⁺⁺ sequestration and activation of the mitochondrial permeability transition (mtPTP) pore during cell differentiation.

Role of Protein Arginine Methyltransferases and Inflammation in Muscle Pathophysiology

Arginine methylation mediated by protein arginine methyltransferases (PRMTs) is a post-translational modification of both histone and non-histone substrates related to diverse biological processes. PRMTs appear to be critical regulators in skeletal muscle physiology, including regeneration, metabolic homeostasis, and plasticity. Chronic inflammation is commonly associated with the decline of skeletal muscle mass and strength related to aging or chronic diseases, defined as sarcopenia. In turn, declined skeletal muscle mass and strength can exacerbate chronic inflammation. Thus, understanding the molecular regulatory pathway underlying the crosstalk between skeletal muscle function and inflammation might be essential for the intervention of muscle pathophysiology. In this review, we will address the current knowledge on the role of PRMTs in skeletal muscle physiology and pathophysiology with a specific emphasis on its relationship with inflammation.

BST204 Protects Dexamethasone-Induced Myotube Atrophy through the Upregulation of Myotube Formation and Mitochondrial Function

BST204 is a purified ginseng dry extract that has an inhibitory effect on lipopolysaccharide-induced inflammatory responses, but its effect on muscle atrophy is yet to be investigated. In this study, C2C12 myoblasts were induced to differentiate for three days followed by the treatment of dexamethasone (DEX), a corticosteroid drug, with vehicle or BST204 for one day and subjected to immunoblotting, immunocytochemistry, qRT-PCR and biochemical analysis for mitochondrial function. BST204 alleviates the myotube atrophic effect mediated by DEX via the activation of protein kinase B/mammalian target of rapamycin (Akt/mTOR) signaling. Through this pathway, BST204 suppresses the expression of muscle-specific E3 ubiquitin ligases contributing to the enhanced myotube formation and enlarged myotube diameter in DEX-treated myotubes. In addition, BST204 treatment significantly decreases the mitochondrial reactive oxygen species production in DEX-treated myotubes. Furthermore, BST204 improves mitochondrial function by upregulating the expression of peroxisome proliferator-activated receptor-γ coactivator-1α (PGC1α) in DEX-induced myotube atrophy. This study provides a mechanistic insight into the effect of BST204 on DEX-induced myotube atrophy, suggesting that BST204 has protective effects against the toxicity of a corticosteroid drug in muscle and promising potential as a nutraceutical remedy for the treatment of muscle weakness and atrophy.

Guanine nucleotide binding protein like-1 (GNL1) promotes cancer cell proliferation and survival through AKT/p21 CIP1 signaling cascade

Human guanine nucleotide binding protein like 1 (GNL1) is an evolutionary conserved putative nucleolar GTPase belonging to the HSR1_MMR1 subfamily of GTPases. GNL1 was found to be highly up-regulated in various cancers. Here, we report for the first time that GNL1 inhibits apoptosis by modulating the expression of Bcl2 family of proteins and the cleavage of caspases 7 and 8. Furthermore, GNL1 protects colon cancer cells from chemo-drug-induced apoptosis. Interestingly, GNL1 up-regulates the expression of p53 and its transcriptional target, p21 but the up-regulation of p21 was found to be p53 dependent as well as independent mechanisms. Our results further demonstrate that GNL1 promotes cell growth and survival by inducing cytoplasmic retention and stabilization of p21 through AKT-mediated phosphorylation. In addition, GNL1 failed to inhibit apoptosis under p21 knockdown conditions which suggests the critical role of p21 in GNL1-mediated cell survival. Finally, an inverse correlation of GNL1, p21, and AKT expression in primary colon and breast cancer with patient survival suggests their critical role in tumorigenesis. Collectively, our study reveals that GNL1 executes its antiapoptotic function by a novel mechanism and suggests that it may function as a regulatory component of the PI3K/AKT/p21 signaling network to promote cell proliferation and survival in cancers.

Insulin Promotes Mitochondrial Respiration and Survival through PI3K/AKT/GSK3 Pathway in Human Embryonic Stem Cells

Insulin is an essential growth factor for the survival and self-renewal of human embryonic stem cells (hESCs). Although it is best known as the principal hormone promoting glycolysis in somatic cells, insulin's roles in hESC energy metabolism remain unclear. In this report, we demonstrate that insulin is essential to sustain hESC mitochondrial respiration that is rapidly decreased upon insulin removal. Insulin-dependent mitochondrial respiration is stem cell specific, and mainly relies on pyruvate and glutamine, while glucose suppresses excessive oxidative phosphorylation. Pharmacologic and genetic manipulations reveal that continuous insulin signal sustains mitochondrial respiration through PI3K/AKT activation and downstream GSK3 inhibition. We further show that insulin acts through GSK3 inhibition to suppress caspase activation and rescue cell survival. This study uncovers a critical role of the AKT/GSK3 pathway in the regulation of mitochondrial respiration and cell survival, highlighting insulin as an essential factor for accurate assessment of mitochondrial respiration in hESCs.

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Insulin is continuously required to sustain mitochondrial respiration in hESCs

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Insulin-dependent mitochondrial respiration is substrate specific

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GSK3 is a major regulator of insulin-dependent respiration and cell survival

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Insulin is essential for accurate assessment of mitochondrial respiration in hESCs

BST204, a Rg3 and Rh2 Enriched Ginseng Extract, Upregulates Myotube Formation and Mitochondrial Function in TNF-α-Induced Atrophic Myotubes

The loss of skeletal muscle mass and function is a serious consequence of chronic diseases and aging. BST204 is a purified ginseng (the root of Panax ginseng) extract that has been processed using ginsenoside-β-glucosidase and acid hydrolysis to enrich ginsenosides Rg3 and Rh2 from the crude ginseng. BST204 has a broad range of health benefits, but its effects and mechanism on muscle atrophy are currently unknown. In this study, we have examined the effects and underlying mechanisms of BST204 on myotube formation and myotube atrophy induced by tumor necrosis factor-α (TNF-α). BST204 promotes myogenic differentiation and multinucleated myotube formation through Akt activation. BST204 prevents myotube atrophy induced by TNF-α through the activation of Akt/mTOR signaling and down-regulation of muscle-specific ubiquitin ligases, MuRF1, and Atrogin-1. Furthermore, BST204 treatment in atrophic myotubes suppresses mitochondrial reactive oxygen species (ROS) production and regulates mitochondrial transcription factors such as NRF1 and Tfam, through enhancing the activity and expression of peroxisome proliferator-activated receptor-γ coactivator1α (PGC1α). Collectively, our findings indicate that BST204 improves myotube formation and PGC1α-mediated mitochondrial function, suggesting that BST204 is a potential therapeutic or neutraceutical remedy to intervene muscle weakness and atrophy.

Rates of myogenesis and myofiber numbers are reduced in late gestation IUGR fetal sheep

Intrauterine growth-restricted (IUGR) fetuses are born with reduced skeletal muscle mass. We hypothesized that reduced rates of myogenesis would contribute to fewer and smaller myofibers in IUGR fetal hindlimb muscle compared to the normally growing fetus. We tested this hypothesis in IUGR fetal sheep with progressive placental insufficiency produced by exposing pregnant ewes to elevated ambient temperatures from 38 to 116 days gestation (dGA; term = 147 dGA). Surgically catheterized control (CON, n = 8) and IUGR (n = 13) fetal sheep were injected with intravenous 5-bromo-2′-deoxyuridine (BrdU) prior to muscle collection (134 dGA). Rates of myogenesis, defined as the combined processes of myoblast proliferation, differentiation, and fusion into myofibers, were determined in biceps femoris (BF), tibialis anterior (TA), and flexor digitorum superficialis (FDS) muscles. Total myofiber number was determined for the entire cross-section of the FDS muscle. In IUGR fetuses, the number of BrdU+ myonuclei per myofiber cross-section was lower in BF, TA, and FDS (P < 0.05), total myonuclear number per myofiber cross-section was lower in BF and FDS (P < 0.05), and total myofiber number was lower in FDS (P < 0.005) compared to CON. mRNA expression levels of cyclins, cyclin-dependent protein kinases, and myogenic regulatory factors were lower (P < 0.05), and inhibitors of the cell cycle were higher (P < 0.05) in IUGR BF compared to CON. Markers of apoptosis were not different in IUGR BF muscle. These results show that in IUGR fetuses, reduced rates of myogenesis produce fewer numbers of myonuclei, which may limit hypertrophic myofiber growth. Fewer myofibers of smaller size contribute to smaller muscle mass in the IUGR fetus.

The Intestinotrophic Effects of Glucagon-Like Peptide-2 in Relation to Intestinal Neoplasia

CONTEXT

Glucagon-like peptide-2 (GLP-2) is a gastrointestinal hormone with intestinotrophic and antiapoptotic effects. The hormone's therapeutic potential in intestinal diseases and relation to intestinal neoplasia has raised great interest among researchers. This article reviews and discusses published experimental and clinical studies concerning the growth-stimulating and antiapoptotic effects of GLP-2 in relation to intestinal neoplasia.

EVIDENCE ACQUISITION

The data used in this narrative review were collected through literature research in PubMed using English keywords. All studies to date examining GLP-2's relation to intestinal neoplasms have been reviewed in this article, as the studies on the matter are sparse.

EVIDENCE SYNTHESIS

GLP-2 has been found to stimulate intestinal growth through secondary mediators and through the involvement of Akt phosphorylation. Studies on rodents have shown that exogenously administered GLP-2 increases the growth and incidence of adenomas in the colon, suggesting that GLP-2 may play an important role in the progression of intestinal tumors. Clinical studies have found that exogenous GLP-2 treatment is well tolerated for up to 30 months, but the tolerability for even longer periods of treatment has not been examined.

CONCLUSION

Exogenous GLP-2 is currently available as teduglutide for the treatment of short bowel syndrome. However, the association between exogenous GLP-2 treatment and intestinal neoplasia in humans has not been fully identified. This leads to a cause for concern regarding the later risk of the development or progression of intestinal tumors with long-term GLP-2 treatment. Therefore, further research regarding GLP-2's potential relation to intestinal cancers is needed.

Evaluation of serum insulin-like growth factor-1 and 26S proteasome concentrations in healthy dogs and dogs with chronic diseases depending on body condition score

In patients suffering from chronic diseases, the objective assessment of metabolic states could be of interest for disease prognosis and therapeutic options. Therefore, the aim of this study was to assess insulin-like growth factor-1 (IGF-1) and 26S proteasome (26SP) in healthy dogs and dogs suffering from chronic diseases depending on their body condition score (BCS) and to examine their potential for objective assessment of anabolic and catabolic states. Serum concentrations of IGF-1, an anabolic hormone, and 26SP, a multiprotein complex which is part of the ubiquitin-proteasome pathway, by which the majority of endogenous proteins including the muscle proteins are degraded, were measured in 21 healthy dogs and 20 dogs with chronic diseases by canine ELISA. The concentrations of IGF-1, 26SP and their ratio (IGF-1/26SP) were set in relationship to the BCS of the dogs. When examining healthy and chronically diseased dogs separately, a positive correlation between IGF-1 and the BCS was observed in the healthy group and a negative correlation between 26SP and the BCS was noted in dogs with chronic diseases. Further, dogs suffering from chronic diseases showed higher 26SP concentrations and lower values for IGF-1/26SP than the healthy dogs. Overall, we detected a negative correlation between 26SP and the BCS and a positive correlation between IGF-1/26SP and the BCS. The results of our study indicate usability of IGF-1 for description of anabolic states, while 26SP could be useful for detection and description of catabolic states. Finally, the ratio IGF-1/26SP seems to be promising for assessment of metabolic states.

Role of Oxidative Stress as Key Regulator of Muscle Wasting during Cachexia

Skeletal muscle atrophy is a pathological condition mainly characterized by a loss of muscular mass and the contractile capacity of the skeletal muscle as a consequence of muscular weakness and decreased force generation. Cachexia is defined as a pathological condition secondary to illness characterized by the progressive loss of muscle mass with or without loss of fat mass and with concomitant diminution of muscle strength. The molecular mechanisms involved in cachexia include oxidative stress, protein synthesis/degradation imbalance, autophagy deregulation, increased myonuclear apoptosis, and mitochondrial dysfunction. Oxidative stress is one of the most common mechanisms of cachexia caused by different factors. It results in increased ROS levels, increased oxidation-dependent protein modification, and decreased antioxidant system functions. In this review, we will describe the importance of oxidative stress in skeletal muscles, its sources, and how it can regulate protein synthesis/degradation imbalance, autophagy deregulation, increased myonuclear apoptosis, and mitochondrial dysfunction involved in cachexia.

1,25-Dihydroxyvitamin D3 Increases the Transplantation Success of Human Muscle Precursor Cells in SCID Mice

Human muscle precursor cell (hMPC) transplantation is a potential therapy for severe muscle trauma or myopathies. Some previous studies demonstrated that 1,25-dihydroxyvitamin-D3 (1,25-D3) acted directly on myoblasts, regulating their proliferation and fusion. 1,25-D3 is also involved in apoptosis modulation of other cell types and may thus contribute to protect the transplanted hMPCs. We have therefore investigated whether 1,25-D3 could improve the hMPC graft success. The 1,25-D3 effects on hMPC proliferation, fusion, and survival were initially monitored in vitro. hMPCs were also grafted in the tibialis anterior of SCID mice treated or not with 1,25-D3 to determine its in vivo effect. Graft success, proliferation, and viability of transplanted hMPCs were evaluated. 1,25-D3 enhanced proliferation and fusion of hMPCs in vitro and in vivo. However, 1,25-D3 did not protect hMPCs from various proapoptotic factors (in vitro) or during the early posttransplantation period. 1,25-D3 enhanced hMPC graft success because the number of muscle fibers expressing human dystrophin was significantly increased in the TA sections of 1,25-D3-treated mice (166.75 ± 20.64) compared to the control mice (97.5 ± 16.58). This result could be partly attributed to the improvement of the proliferation and differentiation of hMPCs in the presence of 1,25-D3. Thus, 1,25-D3 administration could improve the clinical potential of hMPC transplantation currently developed for muscle trauma or myopathies.

Ginsenoside Rg1 from Panax ginseng enhances myoblast differentiation and myotube growth

BACKGROUND

Ginsenoside Rg1 belongs to protopanaxatriol-type ginsenosides and has diverse pharmacological activities. In this report, we investigated whether Rg1 could upregulate muscular stem cell differentiation and muscle growth.

METHODS

C2C12 myoblasts, MyoD-transfected 10T1/2 embryonic fibroblasts, and HEK293T cells were treated with Rg1 and differentiated for 2 d, subjected to immunoblotting, immunocytochemistry, or immunoprecipitation.

RESULTS

Rg1 activated promyogenic kinases, p38MAPK (mitogen-activated protein kinase) and Akt signaling, that in turn promote the heterodimerization with MyoD and E proteins, resulting in enhancing myogenic differentiation. Through the activation of Akt/mammalian target of rapamycin pathway, Rg1 induced myotube growth and prevented dexamethasone-induced myotube atrophy. Furthermore, Rg1 increased MyoD-dependent myogenic conversion of fibroblast.

CONCLUSION

Rg1 upregulates promyogenic kinases, especially Akt, resulting in improvement of myoblast differentiation and myotube growth.

Catechins activate muscle stem cells by Myf5 induction and stimulate muscle regeneration

Muscle weakness is one of the most common symptoms in aged individuals and increases risk of mortality. Thus, maintenance of muscle mass is important for inhibiting aging. In this study, we investigated the effect of catechins, polyphenol compounds in green tea, on muscle regeneration. We found that (-)-epicatechin gallate (ECG) and (-)-epigallocatechin-3-gallate (EGCG) activate satellite cells by induction of Myf5 transcription factors. For satellite cell activation, Akt kinase was significantly induced after ECG treatment and ECG-induced satellite cell activation was blocked in the presence of Akt inhibitor. ECG also promotes myogenic differentiation through the induction of myogenic markers, including Myogenin and Muscle creatine kinase (MCK), in satellite and C2C12 myoblast cells. Finally, EGCG administration to mice significantly increased muscle fiber size for regeneration. Taken together, the results suggest that catechins stimulate muscle stem cell activation and differentiation for muscle regeneration.

The Interplay between Oncogenic Signaling Networks and Mitochondrial Dynamics

Mitochondria are dynamic organelles that alter their organization in response to a variety of cellular cues. Mitochondria are central in many biologic processes, such as cellular bioenergetics and apoptosis, and mitochondrial network morphology can contribute to those physiologic processes. Some of the biologic processes that are in part governed by mitochondria are also commonly deregulated in cancers. Furthermore, patient tumor samples from a variety of cancers have revealed that mitochondrial dynamics machinery may be deregulated in tumors. In this review, we will discuss how commonly mutated oncogenes and their downstream effector pathways regulate the mitochondrial dynamics machinery to promote changes in mitochondrial morphology as well as the physiologic consequences of altered mitochondrial morphology for tumorigenic growth.

Vitamin D induces myogenic differentiation in skeletal muscle derived stem cells

Skeletal muscle wasting is a serious disorder associated with health conditions such as aging, chronic kidney disease and AIDS. Vitamin D is most widely recognized for its regulation of calcium and phosphate homeostasis in relation to bone development and maintenance. Recently, vitamin D supplementation has been shown to improve muscle performance and reduce the risk of falls in vitamin D deficient older adults. However, little is known of the underlying molecular mechanism(s) or the role it plays in myogenic differentiation. We examined the effect of 1,25-D₃ on myogenic cell differentiation in skeletal muscle derived stem cells. Primary cultures of skeletal muscle satellite cells were isolated from the tibialis anterior, soleus and gastrocnemius muscles of 8-week-old C57/BL6 male mice and then treated with 1,25-D₃ The efficiency of satellite cells isolation determined by PAX7+ cells was 81%, and they expressed VDR. Incubation of satellite cells with 1,25-D₃ induces increased expression of: (i) MYOD, (ii) MYOG, (iii) MYC2, (iv) skeletal muscle fast troponin I and T, (v) MYH1, (vi) IGF1 and 2, (vii) FGF1 and 2, (viii) BMP4, (ix) MMP9 and (x) FST. It also promotes myotube formation and decreases the expression of MSTN. In conclusion, 1,25-D₃ promoted a robust myogenic effect on satellite cells responsible for the regeneration of muscle after injury or muscle waste. This study provides a mechanistic justification for vitamin D supplementation in conditions characterized by loss of muscle mass and also in vitamin D deficient older adults with reduced muscle mass and strength, and increased risk of falls.

Spontaneous Formation of Extensive Vessel-Like Structures in Murine Engineered Heart Tissue

Engineered heart tissue (EHT) from primary heart cells contains endothelial cells (ECs), but the extent to which ECs organize into vessel-like structures or even functional vessels remains unknown and is difficult to study by conventional methods. In this study, we generated fibrin-based mini-EHTs from a transgenic mouse line (Cdh5-CreERT2 × Rosa26-LacZ), in which ECs were specifically and inducibly labeled by applying tamoxifen (EC(iLacZ)). EHTs were generated from an unpurified cell mix of newborn mouse hearts and were cultured under standard serum-containing conditions. Cre expression in 15-day-old EHTs was induced by addition of o-hydroxytamoxifen to the culture medium for 48 h, and ECs were visualized by X-gal staining. EC(iLacZ) EHTs showed a dense X-gal-positive vessel-like network with distinct tubular structures. Immunofluorescence revealed that ECs were mainly associated with cardiomyocytes within the EHT. EC(iLacZ) EHT developed spontaneous and regular contractility with forces up to 0.1 mN. Coherent contractility and the presence of an extensive vessel-like network were both dependent on the presence of animal sera in the culture medium. Contractile EC(iLacZ) EHTs successfully served as grafts in implantation studies onto the hearts of immunodeficient mice. Four weeks after implantation, EHTs showed X-gal-positive lumen-forming vessel structures connected to the host myocardium circulation as they contained erythrocytes on a regular basis. Taken together, genetic labeling of ECs revealed the extensive formation of vessel-like structures in EHTs in vitro. The EC(iLacZ) EHT model could help simultaneously study biological effects of compounds on cardiomyocyte function and tissue vascularization.

PKN2 and Cdo interact to activate AKT and promote myoblast differentiation

Skeletal myogenesis is coordinated by multiple signaling pathways that control cell adhesion/migration, survival and differentiation accompanied by muscle-specific gene expression. A cell surface protein Cdo is involved in cell contact-mediated promyogenic signals through activation of p38MAPK and AKT. Protein kinase C-related kinase 2 (PKN2/PRK2) is implicated in regulation of various biological processes, including cell migration, adhesion and death. It has been shown to interact with and inhibit AKT thereby inducing cell death. This led us to investigate the role of PKN2 in skeletal myogenesis and the crosstalk between PKN2 and Cdo. Like Cdo, PKN2 was upregulated in C2C12 myoblasts during differentiation and decreased in cells with Cdo depletion caused by shRNA or cultured on integrin-independent substratum. This decline of PKN2 levels resulted in diminished AKT activation during myoblast differentiation. Consistently, PKN2 overexpression-enhanced C2C12 myoblast differentiation, whereas PKN2-depletion impaired it, without affecting cell survival. PKN2 formed complexes with Cdo, APPL1 and AKT via its C-terminal region and this interaction appeared to be important for induction of AKT activity as well as myoblast differentiation. Furthermore, PKN2-enhanced MyoD-responsive reporter activities by mediating the recruitment of BAF60c and MyoD to the myogenin promoter. Taken together, PKN2 has a critical role in cell adhesion-mediated AKT activation during myoblast differentiation.

Myoprotective effects of bFGF on skeletal muscle injury in pressure-related deep tissue injury in rats

BACKGROUND

Pressure ulcers (PUs) are a major clinical problem that constitutes a tremendous economic burden on healthcare systems. Deep tissue injury (DTI) is a unique serious type of pressure ulcer that arises in skeletal muscle tissue. DTI arises in part because skeletal muscle tissues are more susceptible than skin to external compression. Unfortunately, few effective therapies are currently available for muscle injury. Basic fibroblast growth factor (bFGF), a potent mitogen and survival factor for various cells, plays a crucial role in the regulation of muscle development and homeostasis. The main purpose of this study was to test whether local administration of bFGF could accelerate muscle regeneration in a rat DTI model.

METHODS

Male Sprague Dawley (SD) rats (age 12 weeks) were individually housed in plastic cages and a DTI PU model was induced according to methods described before. Animals were randomly divided into three groups: a normal group, a PU group treated with saline, and a PU group treated with bFGF (10 μg/0.1 ml) subcutaneously near the wound.

RESULTS

We found that application of bFGF accelerated the rate of wound closure and promoted cell proliferation and tissue angiogenesis. In addition, compared to saline administration, bFGF treatment prevented collagen deposition, a measure of fibrosis, and up-regulated the myogenic marker proteins MyHC and myogenin, suggesting bFGF promoted injured muscle regeneration. Moreover, bFGF treatment increased levels of myogenesis-related proteins p-Akt and p-mTOR.

CONCLUSIONS

Our findings show that bFGF accelerated injured skeletal muscle regeneration through activation of the PI3K/Akt/mTOR signaling pathway and suggest that administration of bFGF is a potential therapeutic strategy for the treatment of skeletal muscle injury in PUs.

Evaluation of the angiogenic potency of a novel exopolysaccharide produced by the MK1 bacterial strain

Angiogenesis is an essential physiological step in wound healing and other regenerative processes. Here, we evaluated the angiogenic properties of an exopolysaccharide (EPS) secreted by MK1 (MK1-EPS), a novel bacterial strain isolated from Neungee mushrooms. MK1-EPS significantly increased human umbilical vein endothelial cell (HUVEC) proliferation, migration, and vascular tube formation. MK1-EPS enhanced the phosphorylation of extracellular signal-related kinase (ERK), c-Jun N-terminal kinase (JNK), and p38, which are mitogen-activated protein kinases. In addition, the expression of p21 and intercellular adhesion molecule 1 (ICAM1), and phosphorylation of signal transducer and activator of transcription 3 (STAT3), but not of protein kinase B (AKT), were increased. Specific inhibitors of p38 (SB203580), ERK (PD98059), and JNK (SP600125) inhibited MK1-EPS-induced HUVEC proliferation, tube formation, and cell migration, and partially attenuated MKI-EPS-induced expression of p21 and ICAM1, and STAT3 phosphorylation. After surgical implantation into rabbit calvarial bone defects, new blood vessel formation was significantly higher with MK1-EPS composite bone granules than with granules alone, and new bone formation increased significantly. Therefore, MK1-EPS induces angiogenesis and may have potential for use as a bone regeneration agent in bone tissue engineering applications.

Myocyte-derived Tnfsf14 is a survival factor necessary for myoblast differentiation and skeletal muscle regeneration

Adult skeletal muscle tissue has a uniquely robust capacity for regeneration, which gradually declines with aging or is compromised in muscle diseases. The cellular mechanisms regulating adult myogenesis remain incompletely understood. Here we identify the cytokine tumor necrosis factor superfamily member 14 (Tnfsf14) as a positive regulator of myoblast differentiation in culture and muscle regeneration in vivo. We find that Tnfsf14, as well as its cognate receptors herpes virus entry mediator (HVEM) and lymphotoxin β receptor (LTβR), are expressed in both differentiating myocytes and regenerating myofibers. Depletion of Tnfsf14 or either receptor inhibits myoblast differentiation and promotes apoptosis. Our results also suggest that Tnfsf14 regulates myogenesis by supporting cell survival and maintaining a sufficient pool of cells for fusion. In addition, we show that Akt mediates the survival and myogenic function of Tnfsf14. Importantly, local knockdown of Tnfsf14 is found to impair injury-induced muscle regeneration in a mouse model, affirming an important physiological role for Tnfsf14 in myogenesis in vivo. Furthermore, we demonstrate that localized overexpression of Tnfsf14 potently enhances muscle regeneration, and that this regenerative capacity of Tnfsf14 is dependent on Akt signaling. Taken together, our findings reveal a novel regulator of skeletal myogenesis and implicate Tnfsf14 in future therapeutic development.

Activation of muscle enhancers by MyoD and epigenetic modifiers

The early 1980s revelation of cis-acting genomic elements, known as transcriptional enhancers, is still regarded as one of the fundamental discoveries in the genomic field. However, only with the emergence of genome-wide techniques has the genuine biological scope of enhancers begun to be fully uncovered. Massive scientific efforts of multiple laboratories rapidly advanced the overall perception that enhancers are typified by common epigenetic characteristics that distinguish their activating potential. Broadly, chromatin modifiers and transcriptional regulators lay down the essential foundations necessary for constituting enhancers in their activated form. Basing on genome-wide ChIP-sequencing of enhancer-related marks we identified myogenic enhancers before and after muscle differentiation and discovered that MyoD was bound to nearly a third of condition-specific enhancers. Experimental studies that tested the deposition patterns of enhancer-related epigenetic marks in MyoD-null myoblasts revealed the high dependency that a specific set of muscle enhancers have towards this transcriptional regulator. Re-expression of MyoD restored the deposition of enhancer-related marks at myotube-specific enhancers and partially at myoblasts-specific enhancers. Our proposed mechanistic model suggests that MyoD is involved in recruitment of methyltransferase Set7, acetyltransferase p300 and deposition of H3K4me1 and H3K27ac at myogenic enhancers. In addition, MyoD binding at enhancers is associated with PolII occupancy and with local noncoding transcription. Modulation of muscle enhancers is suggested to be coordinated via transcription factors docking, including c-Jun and Jdp2 that bind to muscle enhancers in a MyoD-dependent manner. We hypothesize that distinct transcription factors may act as placeholders and mediate the assembly of newly formed myogenic enhancers.

Insulin-like growth factor-1 (IGF-1) promotes myoblast proliferation and skeletal muscle growth of embryonic chickens via the PI3K/Akt signalling pathway

During embryonic development, IGF-1 fulfils crucial roles in skeletal myogenesis. However, the involvement of IGF-1-induced myoblast proliferation in muscle growth is still unclear. In the present study, we have characterised the role of IGF-1 in myoblast proliferation both in vitro and in vivo and have revealed novel details of how exogenous IGF-1 influences myogenic genes in chicken embryos. The results show that IGF-1 significantly induces the proliferation of cultured myoblasts in a dose-dependent manner. Additionally, the IGF-1 treatment significantly promoted myoblasts entering a new cell cycle and increasing the mRNA expression levels of cell cycle-dependent genes. However, these effects were inhibited by the PI3K inhibitor LY294002 and the Akt inhibitor KP372-1. These data indicated that the pro-proliferative effect of IGF-1 was mediated in response to the PI3K/Akt signalling pathway. Moreover, we also showed that exogenous IGF-1 stimulated myoblast proliferation in vivo. IGF-1 administration obviously promoted the incorporation of BrdU and remarkably increased the number of PAX7-positive cells in the skeletal muscle of chicken embryos. Administration of IGF-1 also significantly induced the upregulation of myogenic factors gene, the enhancement of c-Myc and the inhibition of myostatin (Mstn) expression. These findings demonstrate that IGF-1 has strong activity as a promoter of myoblast expansion and muscle fiber formation during early myogenesis. Therefore, this study offers insight into the mechanisms responsible for IGF-1-mediated stimulation of embryonic skeletal muscle development, which could have important implications for the improvement of chicken meat production.

Naringenin modulates skeletal muscle differentiation via estrogen receptor α and β signal pathway regulation

Several experiments sustain healthful benefits of the flavanone naringenin (Nar) against chronic diseases including its protective effects against estrogen-related cancers. These experiments encourage Nar use in replacing estrogen treatment in post-menopausal women avoiding the serious side effects ascribed to this hormone. However, at the present, scarce data are available on the impact of Nar on E2-regulated cell functions. This study was aimed at determining the impact of Nar on the estrogen receptor (ERα and β)-dependent signals important for 17β-estradiol (E2) effect in muscle cells (rat L6 myoblasts, mouse C2C12 myoblasts, and mouse skeletal muscle satellite cells). Dietary relevant concentration of Nar delays the appearance of skeletal muscle differentiation markers (i.e., GLUT4 translocation, myogenin, and both fetal and slow MHC isoforms) and impairs E2 effects specifically hampering ERα ability to activate AKT. Intriguingly, Nar effects are specific for E2-initiating signals because IGF-I-induced AKT activation, and myoblast differentiation markers were not affected by Nar treatment. Only 7 days after Nar stimulation, early myoblast differentiation markers (i.e., myogenin, and fetal MHC) start to be accumulated in myoblasts. On the other hand, Nar stimulation activates, via ERβ, the phosphorylation of p38/MAPK involved in reducing the reactive oxygen species formation in skeletal muscle cells. As a whole, data reported here strongly sustain that although Nar action mechanisms include the impairment of ERα signals which drive muscle cells to differentiation, the effects triggered by Nar in the presence of ERβ could balance this negative effect avoiding the toxic effects produced by oxidative stress .

TREM-2 promotes macrophage-mediated eradication of Pseudomonas aeruginosa via a PI3K/Akt pathway

Triggering receptor expressed on myeloid cells 2 (TREM-2) is a cell surface receptor abundantly expressed on myeloid lineage cells such as macrophages and dendritic cells. It is reported that TREM-2 functions as an inflammatory inhibitor in macrophages and dendritic cells. However, the role of TREM-2 in bacterial killing remains unclear. This study explored the role of TREM-2 in bacterial eradication of Pseudomonas aeruginosa (PA), a Gram-negative bacterium which causes various opportunistic infections. Phagocytosis assay assessed by flow cytometry suggested that TREM-2 was not involved in the uptake of PA by macrophages, while bacterial plate count data showed that TREM-2 was required for macrophage-mediated intracellular killing of PA. Moreover, our results demonstrated that TREM-2 promoted macrophage killing by enhancing reactive oxygen species (ROS), but not nitric oxygen (NO) production. Treatment with N-acetylcysteine, a ROS scavenger, diminished the TREM-2-mediated intracellular killing of PA. To further investigate the underlined mechanisms of TREM-2-promoted bacterial killing, we examined the activation of downstream mitogen-activated protein kinases and PI3K/Akt pathway. Western blot data showed that silencing of TREM-2 inhibited phosphorylation of Akt, but not ERK, JNK or P38. In addition, pretreatment with PI3K active product PIP3 DiC16 reversed the elevation of intracellular bacterial load in TREM-2-silenced macrophages, while PI3K inhibitor wortmannin restored the decline of bacterial load in TREM-2-overexpressed macrophages. These data together suggested that the TREM-2-mediated bacterial killing is dependent on the activation of PI3K/Akt signalling, which may provide a better understanding of the host antibacterial immune defence.

Park7 expression influences myotube size and myosin expression in muscle

Callipyge sheep exhibit postnatal muscle hypertrophy due to the up-regulation of DLK1 and/or RTL1. The up-regulation of PARK7 was identified in hypertrophied muscles by microarray analysis and further validated by quantitative PCR. The expression of PARK7 in hypertrophied muscle of callipyge lambs was confirmed to be up-regulated at the protein level. PARK7 was previously identified to positively regulate PI3K/AKT pathway by suppressing the phosphatase activity of PTEN in mouse fibroblasts. The purpose of this study was to investigate the effects of PARK7 in muscle growth and protein accretion in response to IGF1. Primary myoblasts isolated from Park7 (+/+) and Park7 (−/−) mice were used to examine the effect of differential expression of Park7. The Park7 (+/+) myotubes had significantly larger diameters and more total sarcomeric myosin expression than Park7 (−/−) myotubes. IGF1 treatment increased the mRNA abundance of Myh4, Myh7 and Myh8 between 20-40% in Park7 (+/+) myotubes relative to Park7 (−/−). The level of AKT phosphorylation was increased in Park7 (+/+) myotubes at all levels of IGF1 supplementation. After removal of IGF1, the Park7 (+/+) myotubes maintained higher AKT phosphorylation through 3 hours. PARK7 positively regulates the PI3K/AKT pathway by inhibition of PTEN phosphatase activity in skeletal muscle. The increased PARK7 expression can increase protein synthesis and result in myotube hypertrophy. These results support the hypothesis that elevated expression of PARK7 in callipyge muscle would increase levels of AKT activity to cause hypertrophy in response to the normal IGF1 signaling in rapidly growing lambs. Increasing expression of PARK7 could be a novel mechanism to increase protein accretion and muscle growth in livestock or help improve muscle mass with disease or aging.

Cytokines from the tumor microenvironment modulate sirtinol cytotoxicity in A549 lung carcinoma cells

Cytokines in tumor microenvironment play an important role in the success or failure of molecular targeted therapies. We have chosen tumor necrosis factor α (TNF-α), TNF related apoptosis inducing ligand (TRAIL), insulin-like growth factor 1 (IGF-1) and transforming growth factor β (TGF-β) as representative pro-inflammatory, pro-apoptotic, anti-apoptotic and anti-inflammatory tumor derived cytokines. Analysis of Oncomine database revealed the differential expression of these cytokines in a subset of cancer patients. The effects of these cytokines on cytotoxicity of FDA approved drugs - cisplatin and taxol and inhibitors of epidermal growth factor receptor - AG658, Janus kinase - AG490 and SIRT1 - sirtinol were assessed in A549 lung cancer cells. TRAIL augmented cytotoxicity of sirtinol and IGF-1 had a sparing effect. Since TRAIL and IGF-1 differentially modulated sirtinol cytotoxicity, further studies were carried out to identify the mechanisms. Sirtinol or knockdown of SIRT1 increased the expression of death receptors DR4 and DR5 and sensitized A549 cells to TRAIL. Increased cell death in presence of TRAIL and sirtinol was caspase independent and demonstrated classical features of necroptosis. Inhibition of iNOS increased caspase activity and switched the mode of cell death to caspase mediated apoptosis. Interestingly, sirtinol or SIRT1 knockdown did not increase IGF-1R expression. Instead, it abrogated ligand induced downregulation of IGF-1R and increased cell survival through PI3K-AKT pathway. In conclusion, these findings reveal that the tumor microenvironment contributes to modulation of cytotoxicity of drugs and that combination therapy, with agents that increase TRAIL signaling and suppress IGF-1 pathway may potentiate anticancer effect.

The role of MyoD1 and histone modifications in the activation of muscle enhancers

MyoD1 is a key regulator that orchestrates skeletal muscle differentiation through the regulation of gene expression. Although many studies have focused on its role in transcriptional control at gene promoters, less is known regarding the role of MyoD1 in the assembly of active enhancers. Here, we discuss novel data that point to the ability of MyoD1 to mediate the assembly of active enhancers that augment the transcription of genes essential for muscle development and lineage specification. Based on genome-wide studies of epigenetic marks that typify active enhancers, we recently identified the compendium of distal regulatory elements that dictate transcriptional programs during myogenesis. Superimposition of MyoD1 binding sites upon the locations of muscle enhancers revealed its unequivocal binding to a core region of nearly a third of condition-specific muscle enhancers. Further studies exploring deposition of enhancer-related epigenetic marks in myoblasts lacking MyoD1 demonstrate the dependence of muscle enhancer assembly on the presence of MyoD1. We propose a model wherein MyoD1 mediates recruitment of Set7, H3K4me1, H3K27ac, p300, and RNAP II to MyoD1-bound enhancers to establish condition-specific activation of muscle genes. Moreover, muscle enhancers are modulated through coordinated binding of transcription factors, including c-Jun, Jdp2, Meis, and Runx1, which are recruited to muscle enhancers in a MyoD1-dependent manner. Thus, MyoD1 and enhancer-associated transcription factors function coordinately to assemble and regulate enhancers, thereby augmenting expression of muscle-related genes.

Cancer cachexia prevention via physical exercise: molecular mechanisms

Cancer cachexia is a debilitating consequence of disease progression, characterised by the significant weight loss through the catabolism of both skeletal muscle and adipose tissue, leading to a reduced mobility and muscle function, fatigue, impaired quality of life and ultimately death occurring with 25-30 % total body weight loss. Degradation of proteins and decreased protein synthesis contributes to catabolism of skeletal muscle, while the loss of adipose tissue results mainly from enhanced lipolysis. These mechanisms appear to be at least, in part, mediated by systemic inflammation. Exercise, by virtue of its anti-inflammatory effect, is shown to be effective at counteracting the muscle catabolism by increasing protein synthesis and reducing protein degradation, thus successfully improving muscle strength, physical function and quality of life in patients with non-cancer-related cachexia. Therefore, by implementing appropriate exercise interventions upon diagnosis and at various stages of treatment, it may be possible to reverse protein degradation, while increasing protein synthesis and lean body mass, thus counteracting the wasting seen in cachexia.

TREM-2 promotes host resistance against Pseudomonas aeruginosa infection by suppressing corneal inflammation via a PI3K/Akt signaling pathway

PURPOSE

To explore the role of triggering receptor expressed on myeloid cells 2 (TREM-2) in Pseudomonas aeruginosa (PA) keratitis.

METHODS

BALB/c mice were routinely infected with PA and evaluated at various postinfection time points for corneal expression of TREM-2, by real-time PCR, Western blot, and flow cytometry. Next, BALB/c and C57BL/6 mice were respectively treated with TREM-2 siRNA or agonistic anti-TREM-2 antibody, to determine the role of TREM-2 in PA keratitis. Bacterial load and neutrophil infiltration were tested by plate count and myeloperoxidase assay, respectively. Th1-/Th2-type and proinflammatory cytokine expression were tested by real-time PCR and ELISA after in vivo and in vitro silencing of TREM-2. Moreover, phosphorylated Akt levels were tested by Western blot in murine macrophages after treatment with agonistic anti-TREM-2 antibody. mRNA levels of proinflammatory cytokines were examined in murine macrophages after TREM-2 activation and lipopolysaccharide stimulation, following pretreatment with inhibitors for PI3K or Akt, to determine whether PI3K/Akt is required in TREM-2-mediated immune modulation. In addition, BALB/c mice were treated with wortmannin and analyzed for bacterial load and proinflammatory cytokine expression.

RESULTS

TREM-2 expression was elevated in the infected BALB/c corneas at 3 or 5 days postinfection. Silencing of TREM-2 accelerated disease progression by enhancing bacterial load and corneal inflammation, whereas activation of TREM-2 promoted host resistance to PA keratitis. PI3K/Akt signaling is required in the TREM-2-mediated immune modulation, and inhibition of PI3K resulted in worsened disease after PA corneal infection.

CONCLUSIONS

TREM-2 promoted host resistance to PA infection by suppressing corneal inflammation via activation of the PI3K/Akt pathway.

Live cell imaging reveals marked variability in myoblast proliferation and fate

Background

During the process of muscle regeneration, activated stem cells termed satellite cells proliferate, and then differentiate to form new myofibers that restore the injured area. Yet not all satellite cells contribute to muscle repair. Some continue to proliferate, others die, and others become quiescent and are available for regeneration following subsequent injury. The mechanisms that regulate the adoption of different cell fates in a muscle cell precursor population remain unclear.

Methods

We have used live cell imaging and lineage tracing to study cell fate in the C2 myoblast line.

Results

Analyzing the behavior of individual myoblasts revealed marked variability in both cell cycle duration and viability, but similarities between cells derived from the same parental lineage. As a consequence, lineage sizes and outcomes differed dramatically, and individual lineages made uneven contributions toward the terminally differentiated population. Thus, the cohort of myoblasts undergoing differentiation at the end of an experiment differed dramatically from the lineages present at the beginning. Treatment with IGF-I increased myoblast number by maintaining viability and by stimulating a fraction of cells to complete one additional cell cycle in differentiation medium, and as a consequence reduced the variability of the terminal population compared with controls.

Conclusion

Our results reveal that heterogeneity of responses to external cues is an intrinsic property of cultured myoblasts that may be explained in part by parental lineage, and demonstrate the power of live cell imaging for understanding how muscle differentiation is regulated.

Decorin activates Akt downstream of IGF-IR and promotes myoblast differentiation

Decorin, a small leucine-rich proteoglycan, plays an important role in cellular activities through modification of growth factors. It also acts as a signaling molecule to non-muscle cells through epidermal growth factor receptor or insulin-like growth factor I receptor (IGF-IR). However, it is unclear if decorin acts as a signaling molecule to myogenic cells. In this study, we investigated the effect of decorin on the differentiation of myoblasts and the signaling via IGF-IR to myogenic cells. C2C12 myoblasts cultured in media containing decorin for 72 h showed more extensive formation of multinucleated myotubes than control cells cultured in the same media without decorin. The protein expressions of myogenin and myosin heavy chian were higher in decorn-treated cells than in control cells. These results suggest that decorin enhances the differentiation of myoblasts. Western blot analysis and immunocytochemistry showed that IGF-IR was expressed in myoblasts and myotubes. Furthermore, Akt, which is downstream of IGF-IR, was more phosphorylated in myoblasts cultured in media containing decorin than those in media without decorin. These results suggest that decorin activates Akt downstream of IGF-IR and enhances the differentiation of myogenic cells.

Profiling of differentially expressed microRNA and the bioinformatic target gene analyses in bovine fast- and slow-type muscles by massively parallel sequencing

MicroRNA (miRNA) are highly conserved, noncoding small RNA involved in post-transcriptional gene regulation in a variety of biological processes. To elucidate roles of miRNA in bovine muscle type specification and maintenance, we sought to determine differentially expressed miRNA between semitendinosus (STD) and masseter (MS) muscles from 3 Japanese black cattle by massively parallel sequencing. Differential gene expression of myosin heavy chain (MyHC) isoforms confirmed that STD and MS were MyHC-2x- and MyHC-1-abundant muscles, respectively. In total, 192 known miRNA and 20 potential new bovine miRNA were obtained from the sequencing. The differentially expressed miRNA with more than 2-fold difference in each muscle were identified. In particular, miR-196a and miR-885 were exclusively expressed in STD muscle, which was validated by quantitative reverse transcription-PCR (P=0.045 and P<0.001, respectively), whereas a slow type-directing miR-208b was highly expressed in MS compared with STD (false discovery rate<0.05). In addition, 16 potential novel miRNA were mapped and confirmed for their precursor structures by computational analyses. The results of functional annotation combined with in silico target analysis showed that the predicted target genes of miR-196a/b and miR-885 enriched gene ontology (GO) terms related to skeletal system development and regulation of transcription, respectively. Moreover, GO terms enriched from predicted targets miRNA suggested that STD-abundant- and MS-abundant-miRNA were associated with embryonic body planning and organ/tissue pattern formation, respectively. The present results revealed that the differentially expressed miRNA between the STD and MS muscles may play key roles to determine muscle type-specific tissue formation and maintenance in cattle thorough attenuating putative target genes involved in different developmental events.

Role of phosphatidylinositol 3,4,5-trisphosphate (PIP3) 5-phosphatase skeletal muscle- and kidney-enriched inositol polyphosphate phosphatase (SKIP) in myoblast differentiation

Insulin-like growth factors (IGFs) are essential for the development, regeneration, and hypertrophy of skeletal muscles. IGF-II promotes myoblast differentiation through phosphatidylinositol 3-kinase (PI 3-kinase), Akt, and mTOR signaling. Here, we report that skeletal muscle- and kidney-enriched inositol polyphosphate phosphatase (SKIP) negatively regulates myogenesis through inhibition of IGF-II production and attenuation of the IGF-II-Akt-mTOR signaling pathway. We also demonstrate that SKIP expression, which was markedly elevated during differentiation, was controlled by MyoD in C2C12 cells. Expression of SKIP inhibited IGF-II at the transcription level. These results indicate that SKIP regulates MyoD-mediated muscle differentiation. Silencing of SKIP increased IGF-II transcription and myoblast differentiation. Furthermore, knockdown of SKIP resulted in thick myotubes with a larger number of nuclei than that in control C2C12 cells. Taken together, these data indicate that SKIP controls the IGF-II-PI 3-kinase-Akt-mTOR auto-regulation loop during myogenesis. Our findings identify SKIP as a key regulator of muscle cell differentiation.

The expression of phospho-AKT1 and phospho-MTOR is associated with a favorable prognosis independent of PTEN expression in intrahepatic cholangiocarcinomas

AKT1 signaling pathway is important for the regulation of protein synthesis and cell survival with implications in carcinogenesis. In this study, we explored the prognostic significance of AKT1 pathway in intrahepatic cholangiocarcinomas. We investigated the status of phosphatase and tensin homolog deleted on chromosome 10 (PTEN), phosphorylated (p) AKT1 (p-AKT1), p-mammalian target of rapamycin (p-MTOR), p-p70 ribosomal protein S6 kinase (p-RPS6KB2) and p-eukaryotic initiation factor 4E-binding protein-1 (p-EIF4EBP1) in 101 intrahepatic cholangiocarcinomas by immunohistochemistry. Western blot analysis was performed to verify the expression levels of p-AKT1 and p-MTOR. The relationship of protein expression with clinicopathological data and the correlations of protein expression levels were explored. The overexpression of p-AKT1, p-MTOR, and PTEN was associated with a better survival in patients with intrahepatic cholangiocarcinoma (P=0.0137, 0.0194, and 0.0337, respectively). In a multivariate analysis, PTEN was an independent prognostic factor, and p-AKT1 showed tendency (P=0.032 and 0.051, respectively). The overexpression of p-MTOR was correlated with well-to-moderately differentiated tumors (P<0.001) and tumors without metastasis (P=0.046). Expression levels of the AKT1 signaling pathway proteins in this study showed positive correlations with each other, except for PTEN. Aberrant expressions of p-AKT1 and p-MTOR in intrahepatic cholangiocarcinoma were associated with a favorable prognosis, possibly in a PTEN-independent manner. Our results indicate that dysregulation of the AKT1 pathway may have an important role in the development of intrahepatic cholangiocarcinoma, but not necessarily in the progression of the disease.

Regulation of DMD pathology by an ankyrin-encoded miRNA

Background

Duchenne muscular dystrophy (DMD) is an X-linked myopathy resulting from the production of a nonfunctional dystrophin protein. MicroRNA (miRNA) are small 21- to 24-nucleotide RNA that can regulate both individual genes and entire cell signaling pathways. Previously, we identified several mRNA, both muscle-enriched and inflammation-induced, that are dysregulated in the skeletal muscles of DMD patients. One particularly muscle-enriched miRNA, miR-486, is significantly downregulated in dystrophin-deficient mouse and human skeletal muscles. miR-486 is embedded within the ANKYRIN1(ANK1) gene locus, which is transcribed as either a long (erythroid-enriched) or a short (heart muscle- and skeletal muscle-enriched) isoform, depending on the cell and tissue types.

Results

Inhibition of miR-486 in normal muscle myoblasts results in inhibited migration and failure to repair a wound in primary myoblast cell cultures. Conversely, overexpression of miR-486 in primary myoblast cell cultures results in increased proliferation with no changes in cellular apoptosis. Using bioinformatics and miRNA reporter assays, we have identified platelet-derived growth factor receptor β, along with several other downstream targets of the phosphatase and tensin homolog deleted on chromosome 10/AKT (PTEN/AKT) pathway, as being modulated by miR-486. The generation of muscle-specific transgenic mice that overexpress miR-486 revealed that miR-486 alters the cell cycle kinetics of regenerated myofibers in vivo, as these mice had impaired muscle regeneration.

Conclusions

These studies demonstrate a link for miR-486 as a regulator of the PTEN/AKT pathway in dystrophin-deficient muscle and an important factor in the regulation of DMD muscle pathology.

1,25(OH)2vitamin D3 stimulates myogenic differentiation by inhibiting cell proliferation and modulating the expression of promyogenic growth factors and myostatin in C2C12 skeletal muscle cells

Skeletal muscle wasting is an important public health problem associated with aging, chronic disease, cancer, kidney dialysis, and HIV/AIDS. 1,25-Dihydroxyvitamin D (1,25-D3), the active form of vitamin D, is widely recognized for its regulation of calcium and phosphate homeostasis in relation to bone development and maintenance and for its calcemic effects on target organs, such as intestine, kidney, and parathyroid glands. Emerging evidence has shown that vitamin D administration improves muscle performance and reduces falls in vitamin D-deficient older adults. However, little is known of the underlying mechanism or the role 1,25-D3 plays in promoting myogenic differentiation at the cellular and/or molecular level. In this study, we examined the effect of 1,25-D3 on myoblast cell proliferation, progression, and differentiation into myotubes. C(2)C(12) myoblasts were treated with 1,25-D3 or placebo for 1, 3, 4, 7, and 10 d. Vitamin D receptor expression was analyzed by quantitative RT-PCR, Western blottings and immunofluorescence. Expression of muscle lineage, pro- and antimyogenic, and proliferation markers was assessed by immunocytochemistry, PCR arrays, quantitative RT-PCR, and Western blottings. Addition of 1,25-D3 to C(2)C(12) myoblasts 1) increased expression and nuclear translocation of the vitamin D receptor, 2) decreased cell proliferation, 3) decreased IGF-I expression, and 4) promoted myogenic differentiation by increasing IGF-II and follistatin expression and decreasing the expression of myostatin, the only known negative regulator of muscle mass, without changing growth differentiation factor 11 expression. This study identifies key vitamin D-related molecular pathways for muscle regulation and supports the rationale for vitamin D intervention studies in select muscle disorder conditions.

Growth of limb muscle is dependent on skeletal-derived Indian hedgehog

During embryogenesis, muscle and bone develop in close temporal and spatial proximity. We show that Indian Hedgehog, a bone-derived signaling molecule, participates in growth of skeletal muscle. In Ihh(-/-) embryos, skeletal muscle development appears abnormal at embryonic day 14.5 and at later ages through embryonic day 20.5, dramatic losses of hindlimb muscle occur. To further examine the role of Ihh in myogenesis, we manipulated Ihh expression in the developing chick hindlimb. Reduction of Ihh in chicken embryo hindlimbs reduced skeletal muscle mass similar to that seen in Ihh(-/-) mouse embryos. The reduction in muscle mass appears to be a direct effect of Ihh since ectopic expression of Ihh by RCAS retroviral infection of chicken embryo hindlimbs restores muscle mass. These effects are independent of bone length, and occur when Shh is not expressed, suggesting Ihh acts directly on fetal myoblasts to regulate secondary myogenesis. Loss of muscle mass in Ihh null mouse embryos is accompanied by a dramatic increase in myoblast apoptosis by a loss of p21 protein. Our data suggest that Ihh promotes fetal myoblast survival during their differentiation into secondary myofibers by maintaining p21 protein levels.

Secretome Analysis of Skeletal Myogenesis Using SILAC and Shotgun Proteomics

Myogenesis, the formation of skeletal muscle, is a multistep event that commences with myoblast proliferation, followed by cell-cycle arrest, and finally the formation of multinucleated myotubes via fusion of mononucleated myoblasts. Each step is orchestrated by well-documented intracellular factors, such as cytoplasmic signalling molecules and nuclear transcription factors. Regardless, the key step in getting a more comprehensive understanding of the regulation of myogenesis is to explore the extracellular factors that are capable of eliciting the downstream intracellular factors. This could further provide valuable insight into the acute cellular response to extrinsic cues in maintaining normal muscle development. In this paper, we survey the intracellular factors that respond to extracellular cues that are responsible for the cascades of events during myogenesis: myoblast proliferation, cell-cycle arrest of myoblasts, and differentiation of myoblasts into myotubes. This focus on extracellular perspective of muscle development illustrates our mass spectrometry-based proteomic approaches to identify differentially expressed secreted factors during skeletal myogenesis.

Identification of differentially regulated secretome components during skeletal myogenesis

Myogenesis is a well-characterized program of cellular differentiation that is exquisitely sensitive to the extracellular milieu. Systematic characterization of the myogenic secretome (i.e. the ensemble of secreted proteins) is, therefore, warranted for the identification of novel secretome components that regulate both the pluripotency of these progenitor mesenchymal cells, and also their commitment and passage through the differentiation program. Previously, we have successfully identified 26 secreted proteins in the mouse skeletal muscle cell line C2C12 (1). In an effort to attain a more comprehensive picture of the regulation of myogenesis by its extracellular milieu, quantitative profiling employing stable isotope labeling by amino acids in cell culture was implemented in conjunction with two parallel high throughput online reverse phase liquid chromatography-tandem mass spectrometry systems. In summary, 34 secreted proteins were quantified, 30 of which were shown to be differentially expressed during muscle development. Intriguingly, our analysis has revealed several novel up- and down-regulated secretome components that may have critical biological relevance for both the maintenance of pluripotency and the passage of cells through the differentiation program. In particular, the altered regulation of secretome components, including follistatin-like protein-1, osteoglycin, spondin-2, and cytokine-induced apoptosis inhibitor-1, along with constitutively expressed factors, such as fibulin-2, illustrate dynamic changes in the secretome that take place when differentiation to a specific lineage occurs.

Modulation of the apoptotic pathway in skeletal muscle models: the role of growth hormone

Despite numerous studies on the role of growth hormone (GH), its function in skeletal muscle apoptosis secondary to various stimuli is poorly understood. In this study, we used rodent muscle cell lines to analyse cell growth and survival as well as the morphological and molecular markers of cell death in C2C12 and L6C5 myoblasts. These cells were treated either in the presence or absence of GH under serum starvation conditions or in the pro-apoptotic concentrations of hydrogen peroxide (H2O2). Although the cells were responsive to the presence of GH, we did not observe GH modulation of cell growth and survival. The presence of GH did not affect the cell death programme or the expression of apoptotic markers in basal conditions or under oxidative stress. In conclusion, this study indicated that GH "by itself" is not effective in modulating the intracellular pathways leading to cell survival or cell death induced by apoptotic stimuli.

Olfactomedin 4 is a novel target gene of retinoic acids and 5-aza-2'-deoxycytidine involved in human myeloid leukemia cell growth, differentiation, and apoptosis

Clinical application of retinoic acids (RAs) and demethylation agents has proven to be effective in treating certain myeloid leukemia patients. However, the target genes that mediate these antileukemia activities are still poorly understood. In this study, we identified olfactomedin 4 (OLFM4), a myeloid-lineage-specific gene from the olfactomedin family, as a novel target gene for RAs and the demethylation agent, 5-aza-2'-deoxycytidine. We demonstrated that the retinoic acid receptor alpha/retinoic X receptor alpha heterodimer binds to a retinoic acid response-element (DR5) site in the OLFM4 promoter and mediates all-trans-retinoic acid (ATRA)-induced transactivation of the OLFM4 gene. OLFM4 overexpression in HL-60 cells led to growth inhibition, differentiation, and apoptosis, and potentiated ATRA induction of these effects. Conversely, down-regulation of endogenous OLFM4 in acute myeloid leukemia-193 cells compromised ATRA-induced growth inhibition, differentiation, and apoptosis. Overexpression of OLFM4 in HL-60 cells inhibited constitutive and ATRA-induced phosphorylation of the eukaryote initiation factor 4E-binding protein 1 (4E-BP1), whereas down-regulation of OLFM4 protein in acute myeloid leukemia-193 cells increased 4E-BP1 phosphorylation, suggesting that OLFM4 is a potent upstream inhibitor of 4E-BP1 phosphorylation/deactivation. Thus, our study demonstrates that OLFM4 plays an important role in myeloid leukemia cellular functions and induction of OLFM4-mediated effects may contribute to the therapeutic value of ATRA.