NF-kappaB regulation of YY1 inhibits skeletal myogenesis through transcriptional silencing of myofibrillar genes

SIRT3, a mitochondrial NAD⁺-dependent deacetylase, is involved in the regulation of myoblast differentiation

Sirtuin 3 (SIRT3), one of the seven mammalian sirtuins, is a mitochondrial NAD+-dependent deacetylase known to control key metabolic pathways. SIRT3 deacetylases and activates a large number of mitochondrial enzymes involved in the respiratory chain, in ATP production, and in both the citric acid and urea cycles. We have previously shown that the regulation of myoblast differentiation is tightly linked to mitochondrial activity. Since SIRT3 modulates mitochondrial activity, we decide to address its role during myoblast differentiation. For this purpose, we first investigated the expression of endogenous SIRT3 during C2C12 myoblast differentiation. We further studied the impact of SIRT3 silencing on both the myogenic potential and the mitochondrial activity of C2C12 cells. We showed that SIRT3 protein expression peaked at the onset of myoblast differentiation. The inhibition of SIRT3 expression mediated by the stable integration of SIRT3 short inhibitory RNA (SIRT3shRNA) in C2C12 myoblasts, resulted in: 1) abrogation of terminal differentiation - as evidenced by a marked decrease in the myoblast fusion index and a significant reduction of Myogenin, MyoD, Sirtuin 1 and Troponin T protein expression - restored upon MyoD overexpression; 2) a decrease in peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) and citrate synthase protein expression reflecting an alteration of mitochondrial density; and 3) an increased production of reactive oxygen species (ROS) mirrored by the decreased activity of manganese superoxide dismutase (MnSOD). Altogether our data demonstrate that SIRT3 mainly regulates myoblast differentiation via its influence on mitochondrial activity.

Transcriptional regulation of important cellular processes in skeletal myogenesis through interferon-γ

The purpose of the present study was to investigate the effect of interferon (IFN)-γ on the transcriptomic profile of differentiating mouse C2C12 myogenic cells. Global gene expression was evaluated using whole mouse genome oligonucleotide microarrays, and the results were validated through real-time PCR. IFN-γ (1 ng/mL) increased myoblast proliferation but decreased cell respiration and myosin heavy chain content and slightly decreased the fusion index in differentiating C2C12 cell cultures. The genes upregulated through IFN-γ were involved in cell cycle; regulation of cell proliferation; programmed cell death; chemotaxis; and cytokine, growth factor, and peptidase activity, whereas the genes downregulated through IFN-γ primarily contributed to the regulation of transcription, cell-cell signaling, nitrogen compound biosynthesis, ser/thr protein kinase signaling, and regulation of the Wnt pathway. In conclusion, IFN-γ affects the expression of numerous genes associated with the regulation of several processes in myogenesis. The effects of IFN-γ on cellular transcription include (1) alteration of cytokine/growth factor expression, promoting cell proliferation and migration but inhibiting differentiation, (2) impairment of pro-myogenic transcription, (3) disruption of cell adhesion and sarcolemma/cytoskeleton organization, and (4) increased peptidase activity leading to enhanced proteolysis and apoptosis.

Vitamin E in sarcopenia: current evidences on its role in prevention and treatment

Sarcopenia is a geriatric syndrome that is characterized by gradual loss of muscle mass and strength with increasing age. Although the underlying mechanism is still unknown, the contribution of increased oxidative stress in advanced age has been recognized as one of the risk factors of sarcopenia. Thus, eliminating reactive oxygen species (ROS) can be a strategy to combat sarcopenia. In this review, we discuss the potential role of vitamin E in the prevention and treatment of sarcopenia. Vitamin E is a lipid soluble vitamin, with potent antioxidant properties and current evidence suggesting a role in the modulation of signaling pathways. Previous studies have shown its possible beneficial effects on aging and age-related diseases. Although there are evidences suggesting an association between vitamin E and muscle health, they are still inconclusive compared to other more extensively studied chronic diseases such as neurodegenerative diseases and cardiovascular diseases. Therefore, we reviewed the role of vitamin E and its potential protective mechanisms on muscle health based on previous and current in vitro and in vivo studies.

The role of miR-29 family members in malignant hematopoiesis

AIMS

MicroRNAs of the miR-29 family members were one of the first microRNAs identified as possible therapeutic agents in malignant hematopoiesis. The aim of our review is to summarize the current state of knowledge on miR-29 family members.

METHODS

We performed literature searches involving miR-29 family members and their relationship to individual hematological malignancies, namely acute myeloid leukemia (AML), chronic lymphoblastic leukemia (CLL) and chronic myeloid leukemia (CML). We also searched for subgroups of hematological malignancies, e.g. multiple myeloma, that are regarded as members of the acute or chronic types of leukemias.

RESULTS

A number of genes appear to be regulated by miR-29 family members in various physiological and pathological situations. In our view regulation of Tcl-1, Mcl-1 and DNA methyltransferases is relevant in case of hematological malignancies, hence these are the focus of this review. miR-29 family members also function during normal T-cell and B-cell development.

CONCLUSION

MiR-29 family members appear to govern some general features in commonly heterogenous hematological malignancies and therefore form a potential target for treatment.

Overexpression of Yin Yang 1 in bone marrow-derived human multiple myeloma and its clinical significance

Multiple myeloma (MM) patients initially respond to conventional therapy, however, many develop resistance and have recurrences. We have reported in other tumors that the transcription factor Yin Yang 1 (YY1) is a resistant factor and, thus, we hypothesized that YY1 may be over-expressed in MM. Significantly, higher expression (staining intensity and cell frequency) of YY1 in MM cell lines and in bone marrow-derived (BM) MM from 22 MM patients was observed as compared to expression in normal BM. Higher nuclear YY1 staining was associated with disease progression. Bioinformatic analyses of mRNA in data sets corroborated the above findings and showed significant overexpression of YY1 in MM compared to normal tissues and other hematopoietic disorders. The role of YY1 expression in the regulation of drug resistance was exemplified in a drug-resistant MM cell line transfected with YY1 siRNA and which was shown to be sensitized to bortezomib-induced apoptosis. These findings highlight the potential prognostic significance of YY1 expression level in MM patients and as a therapeutic target.

Long noncoding RNAs, emerging players in muscle differentiation and disease

The vast majority of the mammalian genome is transcribed giving rise to many different types of noncoding RNAs. Among them, long noncoding RNAs are the most numerous and functionally versatile class. Indeed, the lncRNA repertoire might be as rich as the proteome. LncRNAs have emerged as key regulators of gene expression at multiple levels. They play important roles in the regulation of development, differentiation and maintenance of cell identity and they also contribute to disease. In this review, we present recent advances in the biology of lncRNAs in muscle development and differentiation. We will also discuss the contribution of lncRNAs to muscle disease with a particular focus on Duchenne and facioscapulohumeral muscular dystrophies.

FOXO3A regulation by miRNA-29a Controls chondrogenic differentiation of mesenchymal stem cells and cartilage formation

Skeletal development and cartilage formation require stringent regulation of gene expression for mesenchymal stem cells (MSCs) to progress through stages of differentiation. Since microRNAs (miRNAs) regulate biological processes, the objective of the present study was to identify novel miRNAs involved in the modulation of chondrogenesis. We performed miRNA profiling and identify miR-29a as being one of the most down-regulated miRNAs during the chondrogenesis. Using chromatin immunoprecipitation, we showed that SOX9 down-regulates its transcription. Moreover, the over-expression of miR-29a strongly inhibited the expression of chondrocyte-specific markers during in vitro chondrogenic differentiation of MSCs. We identified FOXO3A as a direct target of miR-29a and showed a down- and up-regulation of FOXO3a protein levels after transfection of, respectively, premiR- and antagomiR-29a oligonucleotides. Finally, we showed that using the siRNA or premiR approach, chondrogenic differentiation was inhibited to a similar extent. Together, we demonstrate that the down-regulation of miR-29a, concomitantly with FOXO3A up-regulation, is essential for the differentiation of MSCs into chondrocytes and in vivo cartilage/bone formation. The delivery of miRNAs that modulate MSC chondrogenesis may be applicable for cartilage regeneration and deserves further investigation.

Shared signaling systems in myeloid cell-mediated muscle regeneration

Much of the focus in muscle regeneration has been placed on the identification and delivery of stem cells to promote regenerative capacity. As those efforts have advanced, we have learned that complex features of the microenvironment in which regeneration occurs can determine success or failure. The immune system is an important contributor to that complexity and can determine the extent to which muscle regeneration succeeds. Immune cells of the myeloid lineage play major regulatory roles in tissue regeneration through two general, inductive mechanisms: instructive mechanisms that act directly on muscle cells; and permissive mechanisms that act indirectly to influence regeneration by modulating angiogenesis and fibrosis. In this article, recent discoveries that identify inductive actions of specific populations of myeloid cells on muscle regeneration are presented, with an emphasis on how processes in muscle and myeloid cells are co-regulated.

Yin Yang 1 is a repressor of glutamate transporter EAAT2, and it mediates manganese-induced decrease of EAAT2 expression in astrocytes

Impairment of astrocytic glutamate transporter (GLT-1; EAAT2) function is associated with multiple neurodegenerative diseases, including Parkinson's disease (PD) and manganism, the latter being induced by chronic exposure to high levels of manganese (Mn). Mn decreases EAAT2 promoter activity and mRNA and protein levels, but the molecular mechanism of Mn-induced EAAT2 repression at the transcriptional level has yet to be elucidated. We reveal that transcription factor Yin Yang 1 (YY1) is critical in repressing EAAT2 and mediates the effects of negative regulators, such as Mn and tumor necrosis factor alpha (TNF-α), on EAAT2. YY1 overexpression in astrocytes reduced EAAT2 promoter activity, while YY1 knockdown or mutation of the YY1 consensus site of the EAAT2 promoter increased its promoter activity and attenuated the Mn-induced repression of EAAT2. Mn increased YY1 promoter activity and mRNA and protein levels via NF-κB activation. This led to increased YY1 binding to the EAAT2 promoter region. Epigenetically, histone deacetylase (HDAC) classes I and II served as corepressors of YY1, and, accordingly, HDAC inhibitors increased EAAT2 promoter activity and reversed the Mn-induced repression of EAAT2 promoter activity. Taken together, our findings suggest that YY1, with HDACs as corepressors, is a critical negative transcriptional regulator of EAAT2 and mediates Mn-induced EAAT2 repression.

Roles of enhancer of zeste homolog 2: from skeletal muscle differentiation to rhabdomyosarcoma carcinogenesis

Polycomb group proteins represent a global silencing system involved in embryonic development and stem-cell maintenance that regulates the transition from proliferation to differentiation during organogenesis. Two main complexes have been discovered: the polycomb repressive complex (PRC) 1 and 2, able to induce gene silencing by a synergistic mechanism or independently by each other. Enhancer of zeste homolog 2 (EZH2), the catalytic subunit of PRC2, represses gene transcription through the tri-methylation of histone H3 lysine 27. EZH2 deregulation is frequently associated with tumorigenesis, metastatic character, and poor prognosis in various cancer types. This review explores the role of EZH2 in normal development and in carcinogenesis. We reviewed the polycomb-mediated silencing mechanisms, the regulation of EZH2 activity and its recruitment to target genes. We also analyzed the role of EZH2 in normal muscle differentiation and in rhabdomyosarcoma, considering EZH2 blockade as a new strategy for developing specific therapies.

Yin Yang 1 promotes hepatic steatosis through repression of farnesoid X receptor in obese mice

BACKGROUND

Non-alcoholic fatty liver disease (NAFLD) is characterised by accumulation of excessive triglycerides in the liver. Obesity is usually associated with NAFLD through an unknown mechanism.

OBJECTIVE

To investigate the roles of Yin Yang 1 (YY1) in the progression of obesity-associated hepatosteatosis.

METHODS

Expression levels of hepatic YY1 were identified by microarray analysis in high-fat-diet (HFD)-induced obese mice. Liver triglyceride metabolism was analysed in mice with YY1 overexpression and suppression.

RESULTS

YY1 expression was markedly upregulated in HFD-induced obese mice and NAFLD patients. Overexpression of YY1 in healthy mice promoted hepatosteatosis under high-fat dietary conditions, whereas liver-specific ablation of YY1 using adenoviral shRNA ameliorated triglyceride accumulation in obese mice. At the molecular level, YY1 suppressed farnesoid X receptor (FXR) expression through binding to the YY1 responsive element at intron 1 of the FXR gene.

CONCLUSIONS

These findings indicate that YY1 plays a crucial role in obesity-associated hepatosteatosis, through repression of FXR expression.

Mechanisms of muscle injury, repair, and regeneration

Skeletal muscle continuously adapts to changes in its mechanical environment through modifications in gene expression and protein stability that affect its physiological function and mass. However, mechanical stresses commonly exceed the parameters that induce adaptations, producing instead acute injury. Furthermore, the relatively superficial location of many muscles in the body leaves them further vulnerable to acute injuries by exposure to extreme temperatures, contusions, lacerations or toxins. In this article, the molecular, cellular, and mechanical factors that underlie muscle injury and the capacity of muscle to repair and regenerate are presented. Evidence shows that muscle injuries that are caused by eccentric contractions result from direct mechanical damage to myofibrils. However, muscle pathology following other acute injuries is largely attributable to damage to the muscle cell membrane. Many feaures in the injury-repair-regeneration cascade relate to the unregulated influx of calcium through membrane lesions, including: (i) activation of proteases and hydrolases that contribute muscle damage, (ii) activation of enzymes that drive the production of mitogens and motogens for muscle and immune cells involved in injury and repair, and (iii) enabling protein-protein interactions that promote membrane repair. Evidence is also presented to show that the myogenic program that is activated by acute muscle injury and the inflammatory process that follows are highly coordinated, with myeloid cells playing a central role in modulating repair and regeneration. The early-invading, proinflammatory M1 macrophages remove debris caused by injury and express Th1 cytokines that play key roles in regulating the proliferation, migration, and differentiation of satellite cells. The subsequent invasion by anti-inflammatory, M2 macrophages promotes tissue repair and attenuates inflammation. Although this system provides an effective mechanism for muscle repair and regeneration following acute injury, it is dysregulated in chronic injuries. In this article, the process of muscle injury, repair and regeneration that occurs in muscular dystrophy is used as an example of chronic muscle injury, to highlight similarities and differences between the injury and repair processes that occur in acutely and chronically injured muscle.

Transcriptional networks regulating the costamere, sarcomere, and other cytoskeletal structures in striated muscle

Structural abnormalities in striated muscle have been observed in numerous transcription factor gain- and loss-of-function phenotypes in animal and cell culture model systems, indicating that transcription is important in regulating the cytoarchitecture. While most characterized cytoarchitectural defects are largely indistinguishable by histological and ultrastructural criteria, analysis of dysregulated gene expression in each mutant phenotype has yielded valuable information regarding specific structural gene programs that may be uniquely controlled by each of these transcription factors. Linking the formation and maintenance of each subcellular structure or subset of proteins within a cytoskeletal compartment to an overlapping but distinct transcription factor cohort may enable striated muscle to control cytoarchitectural function in an efficient and specific manner. Here we summarize the available evidence that connects transcription factors, those with established roles in striated muscle such as MEF2 and SRF, as well as other non-muscle transcription factors, to the regulation of a defined cytoskeletal structure. The notion that genes encoding proteins localized to the same subcellular compartment are coordinately transcriptionally regulated may prompt rationally designed approaches that target specific transcription factor pathways to correct structural defects in muscle disease.

Genome-wide survey by ChIP-seq reveals YY1 regulation of lincRNAs in skeletal myogenesis

Skeletal muscle differentiation is orchestrated by a network of transcription factors, epigenetic regulators, and non-coding RNAs. The transcription factor Yin Yang 1 (YY1) silences multiple target genes in myoblasts (MBs) by recruiting Ezh2 (Enhancer of Zeste Homologue2). To elucidate genome-wide YY1 binding in MBs, we performed chromatin immunoprecipitation (ChIP)-seq and found 1820 specific binding sites in MBs with a large portion residing in intergenic regions. Detailed analysis demonstrated that YY1 acts as an activator for many loci in addition to its known repressor function. No significant co-occupancy was found between YY1 and Ezh2, suggesting an additional Ezh2-independent function for YY1 in MBs. Further analysis of intergenic binding sites showed that YY1 potentially regulates dozens of large intergenic non-coding RNAs (lincRNAs), whose function in myogenesis is underexplored. We characterized a novel muscle-associated lincRNA (Yam-1) that is positively regulated by YY1. Yam-1 is downregulated upon differentiation and acts as an inhibitor of myogenesis. We demonstrated that Yam-1 functions through in cis regulation of miR-715, which in turn targets Wnt7b. Our findings not only provide the first genome-wide picture of YY1 association in muscle cells, but also uncover the functional role of lincRNA Yam-1.

Mechanisms of impaired differentiation in rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma of childhood, with presumed skeletal muscle origins, because of its myogenic phenotype. RMS is composed of two main subtypes, embryonal RMS (eRMS) and alveolar RMS (aRMS). Whereas eRMS histologically resembles embryonic skeletal muscle, the aRMS subtype is more aggressive and has a poorer prognosis. In addition, whereas the genetic profile of eRMS is not well established, aRMS is commonly associated with distinct chromosome translocations that fuse domains of the transcription factors Pax3 and Pax7 to the forkhead family member FOXO1A. Both eRMS and aRMS tumor cells express myogenic markers such as MyoD, but their ability to complete differentiation is impaired. How this impairment occurs is the subject of this review, which will focus on several themes, including signaling pathways that converge on Pax-forkhead gene targets, alterations in MyoD function, epigenetic modifications of myogenic promoters, and microRNAs whose expression patterns in RMS alter key regulatory circuits to help maintain tumor cells in an opportunistically less differentiated state.

Transcriptional repression of Bim by a novel YY1-RelA complex is essential for the survival and growth of Multiple Myeloma

Multiple Myeloma (MM) is an incurable plasma cell cancer that is caused by several chromosomal translocations and gene deletions. Although deregulation of several signaling pathways including the Nuclear Factor-Kappa B (NF-κB) pathway has been reported in MM, the molecular requirement and the crosstalk between NF-κB and its target genes in MM cell survival has been largely unclear. Here, we report that Yin Yang1 (YY1), a target gene for NF-κB, is hyperexpressed in most MM tumor cells obtained from human patients, exhibits constitutive nuclear localization, and is essential for survival of MM cells. Mechanistically, we report a novel YY1-RelA complex formation, which is essential to transcriptionally repress a proapoptotic gene Bim. In line with this, depletion of YY1 or RelA resulted in elevated levels of Bim and apoptosis. Moreover, both YY1 and RelA are recruited to the Bim promoter and are required to repress the Bim promoter. Importantly, depletion of YY1 or RelA almost completely impaired the colony forming ability of MM progenitor cells suggesting that both RelA and YY1 are essential for the survival and growth of MM progenitor cells. Moreover, depletion of either YY1 or RelA completely inhibited MM tumor growth in xenograft models for human myeloma. Thus, a novel RelA-YY1 transcriptional repression complex is an attractive drug target in MM.

miR-29 targets Akt3 to reduce proliferation and facilitate differentiation of myoblasts in skeletal muscle development

MicroRNAs (miRNAs) are a type of endogenous noncoding small RNAs involved in the regulation of multiple biological processes. Recently, miR-29 was found to participate in myogenesis. However, the underlying mechanisms by which miR-29 promotes myogenesis have not been identified. We found here that miR-29 was significantly upregulated with age in postnatal mouse skeletal muscle and during muscle differentiation. Overexpression of miR-29 inhibited mouse C2C12 myoblast proliferation and promoted myotube formation. miR-29 specifically targeted Akt3, a member of the serine/threonine protein kinase family responsive to growth factor cell signaling, to result in its post-transcriptional downregulation. Furthermore, knockdown of Akt3 by siRNA significantly inhibited the proliferation of C2C12 cells, and conversely, overexpression of Akt3 suppressed their differentiation. Collectively and given the inverse endogenous expression pattern of rising miR-29 levels and decreasing Akt3 protein levels with age in mouse skeletal muscle, we propose a novel mechanism in which miR-29 modulates growth and promotes differentiation of skeletal muscle through the post-transcriptional downregulation of Akt3.

Idiopathic inflammatory myopathies: pathogenic mechanisms of muscle weakness

Idiopathic inflammatory myopathies (IIMs) are a heterogenous group of complex muscle diseases of unknown etiology. These diseases are characterized by progressive muscle weakness and damage, together with involvement of other organ systems. It is generally believed that the autoimmune response (autoreactive lymphocytes and autoantibodies) to skeletal muscle-derived antigens is responsible for the muscle fiber damage and muscle weakness in this group of disorders. Therefore, most of the current therapeutic strategies are directed at either suppressing or modifying immune cell activity. Recent studies have indicated that the underlying mechanisms that mediate muscle damage and dysfunction are multiple and complex. Emerging evidence indicates that not only autoimmune responses but also innate immune and non-immune metabolic pathways contribute to disease pathogenesis. However, the relative contributions of each of these mechanisms to disease pathogenesis are currently unknown. Here we discuss some of these complex pathways, their inter-relationships and their relation to muscle damage in myositis. Understanding the relative contributions of each of these pathways to disease pathogenesis would help us to identify suitable drug targets to alleviate muscle damage and also improve muscle weakness and quality of life for patients suffering from these debilitating muscle diseases.

Heightened muscle inflammation susceptibility may impair regenerative capacity in aging humans

The regenerative response of skeletal muscle to mechanically induced damage is impaired with age. Previous work in our laboratory suggests this may result from higher proinflammatory signaling in aging muscle at rest and/or a greater inflammatory response to damage. We, therefore, assessed skeletal muscle proinflammatory signaling at rest and 24 h after unaccustomed, loaded knee extension contractions that induced modest muscle damage (72% increase in serum creatine kinase) in a cohort of 87 adults across three age groups (AGE40, AGE61, and AGE76). Vastus lateralis muscle gene expression and protein cell signaling of the IL-6 and TNF-α pathways were determined by quantitative PCR and immunoblot analysis. For in vitro studies, cell signaling and fusion capacities were compared among primary myoblasts from young (AGE28) and old (AGE64) donors treated with TNF-α. Muscle expression was higher (1.5- to 2.1-fold) in AGE76 and AGE61 relative to AGE40 for several genes involved in IL-6, TNF-α, and TNF-like weak inducer of apoptosis signaling. Indexes of activation for the proinflammatory transcription factors signal transducer and activator of transcription-3 and NF-κB were highest in AGE76. Resistance loading reduced gene expression of IL-6 receptor, muscle RING finger 1, and atrogin-1, and increased TNF-like weak inducer of apoptosis receptor expression. Donor myoblasts from AGE64 showed impaired differentiation and fusion in standard media and greater NF-κB activation in response to TNF-α treatment (compared with AGE28). We show for the first time that human aging is associated with muscle inflammation susceptibility (i.e., higher basal state of proinflammatory signaling) that is present in both tissue and isolated myogenic cells and likely contributes to the impaired regenerative capacity of skeletal muscle in the older population.

Reining in nuclear factor-kappaB in skeletal muscle disorders

PURPOSE OF REVIEW

Nuclear factor-kappaB (NF-κB) activation is associated with a wide range of muscle-related diseases. Here, we review the evidence implicating specific NF-κB components in different disease pathologies and discuss therapies designed to target aberrant NF-κB signaling for the treatment of those pathologies.

RECENT FINDINGS

Many components of the NF-κB signaling pathway contribute to muscle pathologies, presumably through activation of the transcription factor. In addition, an increasing number of upstream factors have been connected to disease progression. Genetic models and therapeutic approaches affecting these upstream targets associate with ameliorating disease progression.

SUMMARY

Dissecting the crosstalk between NF-κB, its upstream mediators, and other signaling pathways is vital to our understanding of how activation of this signaling pathway is mediated in various diseases. The strides made in therapeutically inhibiting the NF-κB pathway provide some promise for the treatment of these diseases.

miR-411 is up-regulated in FSHD myoblasts and suppresses myogenic factors

BACKGROUND

Facioscapulohumeral muscular dystrophy (FSHD) is an autosomal dominant muscle disorder, which is linked to the contraction of the D4Z4 array at chromosome 4q35. Recent studies suggest that this shortening of the D4Z4 array leads to aberrant expression of double homeobox protein 4 (DUX4) and causes FSHD. In addition, misregulation of microRNAs (miRNAs) has been reported in muscular dystrophies including FSHD. In this study, we identified a miRNA that is differentially expressed in FSHD myoblasts and investigated its function.

METHODS

To identify misregulated miRNAs and their potential targets in FSHD myoblasts, we performed expression profiling of both miRNA and mRNA using TaqMan Human MicroRNA Arrays and Affymetrix Human Genome U133A plus 2.0 microarrays, respectively. In addition, we over-expressed miR-411 in C₂C₁₂ cells to determine the effect of miR-411 on myogenic markers.

RESULTS

Using miRNA and mRNA expression profiling, we identified 8 miRNAs and 1,502 transcripts that were differentially expressed in FSHD myoblasts during cell proliferation. One of the 8 differentially expressed miRNAs, miR-411, was validated by quantitative RT-PCR in both primary (2.1 fold, p<0.01) and immortalized (2.7 fold, p<0.01) myoblasts. In situ hybridization showed cytoplasmic localization of miR-411 in FSHD myoblasts. By analyzing both miRNA and mRNA data using Partek Genomics Suite, we identified 4 mRNAs potentially regulated by miR-411 including YY1 associated factor 2 (YAF2). The down-regulation of YAF2 in immortalized myoblasts was validated by immunoblotting (-3.7 fold, p<0.01). C₂C₁₂ cells were transfected with miR-411 to determine whether miR-411 affects YAF2 expression in myoblasts. The results showed that over-expression of miR-411 reduced YAF2 mRNA expression. In addition, expression of myogenic markers including Myod, myogenin, and myosin heavy chain 1 (Myh1) were suppressed by miR-411.

CONCLUSIONS

The study demonstrated that miR-411 was differentially expressed in FSHD myoblasts and may play a role in regulating myogenesis.

Oxygen consumption and usage during physical exercise: the balance between oxidative stress and ROS-dependent adaptive signaling

The complexity of human DNA has been affected by aerobic metabolism, including endurance exercise and oxygen toxicity. Aerobic endurance exercise could play an important role in the evolution of Homo sapiens, and oxygen was not important just for survival, but it was crucial to redox-mediated adaptation. The metabolic challenge during physical exercise results in an elevated generation of reactive oxygen species (ROS) that are important modulators of muscle contraction, antioxidant protection, and oxidative damage repair, which at moderate levels generate physiological responses. Several factors of mitochondrial biogenesis, such as peroxisome proliferator-activated receptor-γ coactivator 1α (PGC-1α), mitogen-activated protein kinase, and SIRT1, are modulated by exercise-associated changes in the redox milieu. PGC-1α activation could result in decreased oxidative challenge, either by upregulation of antioxidant enzymes and/or by an increased number of mitochondria that allows lower levels of respiratory activity for the same degree of ATP generation. Endogenous thiol antioxidants glutathione and thioredoxin are modulated with high oxygen consumption and ROS generation during physical exercise, controlling cellular function through redox-sensitive signaling and protein-protein interactions. Endurance exercise-related angiogenesis, up to a significant degree, is regulated by ROS-mediated activation of hypoxia-inducible factor 1α. Moreover, the exercise-associated ROS production could be important to DNA methylation and post-translation modifications of histone residues, which create heritable adaptive conditions based on epigenetic features of chromosomes. Accumulating data indicate that exercise with moderate intensity has systemic and complex health-promoting effects, which undoubtedly involve regulation of redox homeostasis and signaling.

YY1TargetDB: an integral information resource for Yin Yang 1 target loci

Yin Yang 1 (YY1), a ubiquitously expressed transcription factor, plays a critical role in regulating cell development, differentiation, cellular proliferation and tumorigenesis. Previous studies identified many YY1-regulated target genes in both human and mouse. Emerging global mapping by Chromatin ImmnoPrecipitation (ChIP)-based high-throughput experiments indicate that YY1 binds to a vast number of loci genome-wide. However, the information is widely scattered in many disparate poorly cross-indexed literatures; a large portion was only published recently by the ENCODE consortium with limited annotation. A centralized database, which annotates and organizes YY1-binding loci and target motifs in a systematic way with easy access, will be valuable resources for the research community. We therefore implemented a web-based YY1 Target loci Database (YY1TargetDB). This database contains YY1-binding loci (binding peaks) from ChIP-seq and ChIP-on-chip experiments, computationally predicated YY1 and cofactor motifs within each locus. It also collects the experimentally verified YY1-binding motifs from individual researchers. The current version of YY1TargetDB contains 92 314 binding loci identified by ChIP-based experiments; 157 200 YY1-binding motifs in which 42 are experimentally verified and 157 158 are computationally predicted; and 130 759 binding motifs for 47 cofactors.

Database URL:http://www.myogenesisdb.org/YY1TargetDB

NF-κB1 p50 promotes p53 protein translation through miR-190 downregulation of PHLPP1

The biological function of NF-κB1 (p50) in the regulation of protein expression is far from well understood owing to the lack of a transcriptional domain. Here, we report a novel function of p50 in its regulation of p53 protein translation under stress conditions. We found that the deletion of p50 (p50-/-) impaired arsenite-induced p53 protein expression, which could be restored after reconstitutive expression of HA-p50 in p50-/- cells, p50-/-(Ad-HA-p50). Further studies indicated that the amounts of p53 mRNA, p53 promoter-driven transcription activity and p53 protein degradation were comparable between wild-type and p50-/- cells. Moreover, we found that p50 was crucial for Akt/S6 ribosomal protein activation via inhibition of the translation of the PH domain and leucine-rich repeat protein phosphatases 1 (PHLPP1), a phosphatase of Akt. Further studies showed that p50-mediated upregulation of miR-190 was responsible for the inhibition of PHLPP1 translation by targeting the 3'-untranslated region of its mRNA. Collectively, we have identified a novel function of p50 in modulating p53 protein translation via regulation of the miR-190/PHLPP1/Akt-S6 ribosomal protein pathway.

Epigenetic regulation of muscle phenotype and adaptation: a potential role in COPD muscle dysfunction

Quadriceps muscle dysfunction occurs in one-third of patients with chronic obstructive pulmonary disease (COPD) in very early stages of their condition, even prior to the development of airway obstruction. Among several factors, deconditioning and muscle mass loss are the most relevant contributing factors leading to this dysfunction. Moreover, epigenetics, defined as the process whereby gene expression is regulated by heritable mechanisms that do not affect DNA sequence, could be involved in the susceptibility to muscle dysfunction, pathogenesis, and progression. Herein, we review the role of epigenetic mechanisms in muscle development and adaptation to environmental factors such as immobilization and exercise, and their implications in the pathophysiology and susceptibility to muscle dysfunction in COPD. The epigenetic modifications identified so far include DNA methylation, histone acetylation and methylation, and non-coding RNAs such as microRNAs (miRNAs). In the present review, we describe the specific contribution of epigenetic mechanisms to the regulation of embryonic myogenesis, muscle structure and metabolism, immobilization, and exercise, and in muscles of COPD patients. Events related to muscle development and regeneration and the response to exercise and immobilization are tightly regulated by epigenetic mechanisms. These environmental factors play a key role in the outcome of muscle mass and function as well as in the susceptibility to muscle dysfunction in COPD. Future research remains to be done to shed light on the specific target pathways of miRNA function and other epigenetic mechanisms in the susceptibility, pathogenesis, and progression of COPD muscle dysfunction.

Nuclear factor κB down-regulates human UDP-glucuronosyltransferase 1A1: a novel mechanism involved in inflammation-associated hyperbilirubinaemia

Jaundice or hyperbilirubinaemia is a common complication of sepsis. UGT1A1 (UDP-glucuronosyltransferase 1A1) is a critical gene for bilirubin metabolism and irinotecan detoxification. However, the molecular pathogenesis of hyperbilirubinaemia during inflammation needs to be further clarified. Human hepatic UGT1A1 expression was analysed by RT (reverse transcription)–PCR, qRT-PCR (quantitative real-time PCR) and Western blotting in response to LPS (lipopolysaccharide) stimulation. Transcription regulatory elements in the upstream promoter region of the human UGT1A1 gene were determined using EMSA (electrophoretic mobility-shift assay) and ChIP (chromatin immunoprecipitation). The important role of the transcription regulatory element was examined using a luciferase assay, and was determined by qRT-PCR using a transcription factor activation inhibitor. LPS down-regulated the UGT1A1 mRNA expression in human hepatoma cell lines. A newly identified NF-κB (nuclear factor κB)-binding site was located on the upstream promoter region (−725/−716) of the human UGT1A1 gene. LPS-induced NF-κB activation and specific binding to the NF-κB-binding site can suppress human UGT1A1 promoter activity and human UGT1A1 expression. We demonstrated that LPS mediates the suppression of human UGT1A1 expression through specific binding of NF-κB to this newly identified NF-κB-binding site in the upstream promoter of the human UGT1A1 gene. The present study may partly explain the molecular pathogenesis of inflammation-associated hyperbilirubinaemia.

Epigenetic deregulation of microRNAs in rhabdomyosarcoma and neuroblastoma and translational perspectives

Gene expression control mediated by microRNAs and epigenetic remodeling of chromatin are interconnected processes often involved in feedback regulatory loops, which strictly guide proper tissue differentiation during embryonal development. Altered expression of microRNAs is one of the mechanisms leading to pathologic conditions, such as cancer. Several lines of evidence pointed to epigenetic alterations as responsible for aberrant microRNA expression in human cancers. Rhabdomyosarcoma and neuroblastoma are pediatric cancers derived from cells presenting features of skeletal muscle and neuronal precursors, respectively, blocked at different stages of differentiation. Consistently, tumor cells express tissue markers of origin but are unable to terminally differentiate. Several microRNAs playing a key role during tissue differentiation are often epigenetically downregulated in rhabdomyosarcoma and neuroblastoma and behave as tumor suppressors when re-expressed. Recently, inhibition of epigenetic modulators in adult tumors has provided encouraging results causing re-expression of anti-tumor master gene pathways. Thus, a similar approach could be used to correct the aberrant epigenetic regulation of microRNAs in rhabdomyosarcoma and neuroblastoma. The present review highlights the current insights on epigenetically deregulated microRNAs in rhabdomyosarcoma and neuroblastoma and their role in tumorigenesis and developmental pathways. The translational clinical implications and challenges regarding modulation of epigenetic chromatin remodeling/microRNAs interconnections are also discussed.

The transcription factor YY1 is a novel substrate for Aurora B kinase at G2/M transition of the cell cycle

Yin Yang 1 (YY1) is a ubiquitously expressed and highly conserved multifunctional transcription factor that is involved in a variety of cellular processes. Many YY1-regulated genes have crucial roles in cell proliferation, differentiation, apoptosis, and cell cycle regulation. Numerous mechanisms have been shown to regulate the function of YY1, such as DNA binding affinity, subcellular localization, and posttranslational modification including phosphorylation. Polo-like kinase 1(Plk1) and Casein kinase 2α (CK2 α) were the first two kinases identified to phosphorylate YY1. In this study, we identify a third kinase. We report that YY1 is a novel substrate of the Aurora B kinase both in vitro and in vivo. Serine 184 phosphorylation of YY1 by Aurora B is cell cycle regulated and peaks at G2/M and is rapidly dephosphorylated, likely by protein phosphatase 1 (PP1) as the cells enter G1. Aurora A and Aurora C can also phosphorylate YY1 in vitro, but at serine/threonine residues other than serine 184. We present evidence that phosphorylation of YY1 in the central glycine/alanine (G/A)-rich region is important for DNA binding activity, with a potential phosphorylation/acetylation interplay regulating YY1 function. Given their importance in mitosis and overexpression in human cancers, Aurora kinases have been identified as promising therapeutic targets. Increasing our understanding of Aurora substrates will add to the understanding of their signaling pathways.

Mammalian target of rapamycin (mTOR) signaling network in skeletal myogenesis

Mammalian (or mechanistic) target of rapamycin (mTOR) regulates a wide range of cellular and developmental processes by coordinating signaling responses to mitogens, nutrients, and various stresses. Over the last decade, mTOR has emerged as a master regulator of skeletal myogenesis, controlling multiple stages of the myofiber formation process. In this minireview, we present an emerging view of the signaling network underlying mTOR regulation of myogenesis, which contrasts with the well established mechanisms in the regulation of cell and muscle growth. Current questions for future studies are also highlighted.

TWEAK and cIAP1 regulate myoblast fusion through the noncanonical NF-κB signaling pathway

The fusion of mononucleated muscle progenitor cells (myoblasts) into multinucleated muscle fibers is a critical aspect of muscle development and regeneration. We identified the noncanonical nuclear factor κB (NF-κB) pathway as a signaling axis that drives the recruitment of myoblasts into new muscle fibers. Loss of cellular inhibitor of apoptosis 1 (cIAP1) protein led to constitutive activation of the noncanonical NF-κB pathway and an increase in the number of nuclei per myotube. Knockdown of essential mediators of NF-κB signaling, such as p100, RelB, inhibitor of κB kinase α, and NF-κB-inducing kinase, attenuated myoblast fusion in wild-type myoblasts. In contrast, the extent of myoblast fusion was increased when the activity of the noncanonical NF-κB pathway was enhanced by increasing the abundance of p52 and RelB or decreasing the abundance of tumor necrosis factor (TNF) receptor-associated factor 3, an inhibitor of this pathway. Low concentrations of the cytokine TNF-like weak inducer of apoptosis (TWEAK), which preferentially activates the noncanonical NF-κB pathway, also increased myoblast fusion, without causing atrophy or impairing myogenesis. These results identify roles for TWEAK, cIAP1, and noncanonical NF-κB signaling in the regulation of myoblast fusion and highlight a role for cytokine signaling during adult skeletal myogenesis.

Impaired myogenesis in estrogen-related receptor γ (ERRγ)-deficient skeletal myocytes due to oxidative stress

Specialized contractile function and increased mitochondrial number and oxidative capacity are hallmark features of myocyte differentiation. The estrogen-related receptors (ERRs) can regulate mitochondrial biogenesis or mitochondrial enzyme expression in skeletal muscle, suggesting that ERRs may have a role in promoting myogenesis. Therefore, we characterized myogenic programs in primary myocytes isolated from wild-type (M-ERRγWT) and muscle-specific ERRγ(-/-) (M-ERRγ(-/-)) mice. Myotube maturation and number were decreased throughout differentiation in M-ERRγ(-/-) primary myocytes, resulting in myotubes with reduced mitochondrial content and sarcomere assembly. Compared with M-ERRγWT myocytes at the same differentiation stage, the glucose oxidation rate was reduced by 30% in M-ERRγ(-/-) myotubes, while medium-chain fatty acid oxidation was increased by 34% in M-ERRγ(-/-) myoblasts and 36% in M-ERRγ(-/-) myotubes. Concomitant with increased reliance on mitochondrial β-oxidation, H(2)O(2) production was significantly increased by 40% in M-ERRγ(-/-) myoblasts and 70% in M-ERRγ(-/-) myotubes compared to M-ERRγWT myocytes. ROS activation of FoxO and NF-κB and their downstream targets, atrogin-1 and MuRF1, was observed in M-ERRγ(-/-) myocytes. The antioxidant N-acetyl cysteine rescued myotube formation and atrophy gene induction in M-ERRγ(-/-) myocytes. These results suggest that loss of ERRγ causes metabolic defects and oxidative stress that impair myotube formation through activation of skeletal muscle atrophy pathways.

Regulation of interlocking gene regulatory network subcircuits by a small molecule inhibitor of retinoblastoma protein (RB) phosphorylation: cancer cell expression of HLA-DR

The induction of the major histocompatibility (MHC), antigen-presenting class II molecules by interferon-gamma, in solid tumor cells, requires the retinoblastoma tumor suppressor protein (Rb). In the absence of Rb, a repressosome blocks the access of positive-acting, promoter binding proteins to the MHC class II promoter. However, a complete molecular linkage between Rb expression and the disassembly of the MHC class II repressosome has been lacking. By treating A549 lung carcinoma cells with a novel small molecule that prevents phosphorylation-mediated, Rb inactivation, we demonstrate that Rb represses the synthesis of an MHC class II repressosome component, YY1. The reduction in YY1 synthesis correlates with the advent of MHC class II inducibility; with loss of YY1 binding to the promoter of the HLA-DRA gene, the canonical human MHC class II gene; and with increased Rb binding to the YY1 promoter. These results support the concept that the Rb gene regulatory network (GRN) subcircuit that regulates cell proliferation is linked to a GRN subcircuit regulating a tumor cell immune function.

Growth and Development Symposium: Impacts of inflammation on cattle growth and carcass merit

Inflammation caused by bovine respiratory disease (BRD) continues to be one of the greatest challenges facing beef cattle producers and feedlot managers. Inflammation decreases DMI, ADG, and G:F in feedlot calves, decreasing growth rate and increasing days on feed, which results in economic losses during the feeding period. During the past decade, marketing of feedlot animals has changed from selling cattle on a live basis to a grid-based marketing system. When cattle are marketed on a live basis, the economic effects of BRD stop at increased health cost and decreased feedlot performance, carcass weight, and death loss. However, when cattle are marketed in a grid-based system, inflammation has the potential to also affect carcass cutability and quality. The effects of inflammation on feedlot cattle in regards to performance are well understood; however, specific effects on cattle growth and ultimately carcass merit are not as well described. Recent studies in feedlot cattle have indicated that the incidence of BRD decreases both HCW and marbling; however, mechanisms are not understood. Research in other species has demonstrated that during the acute phase response, pro-inflammatory cytokines promote skeletal muscle catabolism to supply AA and energy substrates for immune tissues. Further, during this early immune response, the liver changes its metabolic priorities to the production of acute phase proteins for use in host defense. Together these dramatic shifts in systemic metabolism may explain the detrimental effects on performance and carcass traits commonly associated with BRD in feedlot calves. Moreover, recent studies relative to human health have revealed complex multilevel interactions between the metabolic and immune systems, and highlighted inflammation as being a significant contributor to major metabolic diseases. The objective of this paper is to review data to help explain the economical and physiological effects of inflammation on cattle growth and carcass merit.

p130Cas-dependent actin remodelling regulates myogenic differentiation

Actin dynamics are implicated in various cellular processes, not only through the regulation of cytoskeletal organization, but also via the control of gene expression. In the present study we show that the Src family kinase substrate p130Cas (Cas is Crk-associated substrate) influences actin remodelling and concomitant muscle-specific gene expression, thereby regulating myogenic differentiation. In C2C12 myoblasts, silencing of p130Cas expression by RNA interference impaired F-actin (filamentous actin) formation and nuclear localization of the SRF (serum-response factor) co-activator MAL (megakaryocytic acute leukaemia) following the induction of myogenic differentiation. Consequently, formation of multinucleated myotubes was abolished. Re-introduction of wild-type p130Cas, but not its phosphorylation-defective mutant, into p130Cas-knockdown myoblasts restored F-actin assembly, MAL nuclear localization and myotube formation. Depletion of the adhesion molecule integrin β3, a key regulator of myogenic differentiation as well as actin cytoskeletal organization, attenuated p130Cas phosphorylation and MAL nuclear localization during C2C12 differentiation. Moreover, knockdown of p130Cas led to the activation of the F-actin-severing protein cofilin. The introduction of a dominant-negative mutant of cofilin into p130Cas-knockdown myoblasts restored muscle-specific gene expression and myotube formation. The results of the present study suggest that p130Cas phosphorylation, mediated by integrin β3, facilitates cofilin inactivation and promotes myogenic differentiation through modulating actin cytoskeleton remodelling.

Structural and functional analysis of myostatin-2 promoter alleles from the marine fish Sparus aurata: evidence for strong muscle-specific promoter activity and post-transcriptional regulation

Myostatin (MSTN) is a negative regulator of skeletal muscle growth. In contrast to mammals, fish possess at least two paralogs of MSTN: MSTN-1 and MSTN-2. In this study, we analyzed the structural-functional features of the four variants of Sparus aurata MSTN-2 5'-flanking region: saMSTN-2a, saMSTN-2as, saMSTN-2b and saMSTN-2c. In silico analysis revealed numerous putative cis regulatory elements including several E-boxes known as binding sites to myogenic transcription factors. Transient transfection experiments using non-muscle and muscle cell lines showed surprisingly high transcriptional activity in muscle cells, suggesting the presence of regulatory elements unique to differentiated myotubes. These observations were confirmed by in situ intramuscular injections of promoter DNA followed by reporter gene assays. Moreover, high promoter activity was found in differentiated neural cell, in agreement with MSTN-2 expression in brain. Progressive 5'-deletion analysis, using reporter gene assays, showed that the core promoter is located within the first -127 bp upstream of the ATG, and suggested the presence of regulatory elements that either repress or induce transcriptional activity. Transient transgenic zebrafish provided evidence for saMSTN-2 promoter ability to direct GFP expression to myofibers. Finally, our data shows that although no mature saMSTN-2 mRNA is observed in muscle; unspliced forms accumulate, confirming high level of transcription. In conclusion, our study shows for the first time that MSTN-2 promoter is a very robust promoter, especially in muscle cells.

A novel miR-193a-5p-YY1-APC regulatory axis in human endometrioid endometrial adenocarcinoma

Aberrant expression and altered function of transcription factors (TFs) have vital roles in many aspects of tumor development and progression. In this study, we investigated the functional significance of a TF, Yin Yang1 (YY1) in tumorigenesis of endometrioid endometrial carcinoma (EEC). We demonstrated that YY1 is upregulated in EEC cell lines and primary tumors; and its expression is associated with tumor stages. Depletion of YY1 inhibits EEC cell proliferation and migration both in vitro and in vivo, whereas overexpression of YY1 promotes EEC cell growth. These results suggest that YY1 functions as an oncogenic factor in EEC. Transcriptome analysis revealed a significant effect of YY1 on critical aspects of EEC tumorigenesis through inhibition of APC expression. Further mechanistic investigation uncovered a new epigenetic silencing mode of APC by YY1 through recruitment of EZH2 and trimethylation of histone 3 lysine 27 on its promoter region. Moreover, YY1 overexpression was found to be a consequence of miR-193a-5p downregulation through direct miR-193a-5p-YY1 interplay. Our results therefore establish a novel miR-193a-5p-YY1-APC axis, which contributes to EEC development, and may serve as future intervention target.

Implications of glucocorticoid therapy in idiopathic inflammatory myopathies

Glucocorticoids are the cornerstone of therapy in patients with idiopathic inflammatory myopathies (IIM), despite adverse effects and suboptimal therapy success rates. Glucocorticoids are used in patients with IIM to suppress inflammatory and immune responses implicated in the pathogenesis of these diseases. Nevertheless, potential inhibitory effects of glucocorticoids on skeletal muscle mass, myogenesis and immune responses that promote skeletal muscle regeneration after muscle injury warrant attention. Glucocorticoids lead to skeletal muscle catabolism by modulating major pathways involved in regulating muscle mass. Glucocorticoids also inhibit muscle regeneration by decreasing myogenic cell proliferation and differentiation. Finally, glucocorticoids might have inhibitory effects on immune cells that have been shown to be an important component of the muscle regenerative response. Better understanding of the signalling pathways involved in restorative versus adverse effects of glucocorticoids in IIM could yield additional insight into the aetiopathogenesis of persistent muscle weakness in patients with IIM after glucocorticoid treatment, and help in the development of novel, targeted treatment options with fewer adverse effects.

A novel target of microRNA-29, Ring1 and YY1-binding protein (Rybp), negatively regulates skeletal myogenesis

Skeletal muscle cell differentiation (myogenesis) is a process orchestrated by a complex network involving transcription factors, epigenetic regulators, and microRNAs. Previous studies identified miR-29 as a pro-myogenic factor that interacts with components of Polycomb repressive complex, YY1 and Ezh2. In a genome-wide survey of miR-29-mediated transcriptome changes in C2C12 myoblasts, many epigenetic factors were found to be down-regulated by miR-29. Among them, Rybp was shown to be a direct target of miR-29 through binding to its 3' UTR. Functional studies demonstrated that Rybp is down-regulated during myogenesis and acts as a negative regulator of skeletal myogenesis both in vitro during C2C12 differentiation and in vivo in injury-induced muscle regeneration. Furthermore, we found that Rybp and YY1 co-occupy several myogenic loci, including miR-29 itself, to silence their expression, thus forming a Rybp-miR-29 feedback loop. Rybp overexpression was found to enhance the enrichment of Ezh2 and trimethylation of H3K27 at target loci, suggesting it may facilitate the recruitment or stabilization of the Polycomb repressive complex. Collectively, our results identify Rybp as a novel regulator of myogenesis that co-acts with YY1 to silence miR-29 and other myogenic loci.

Myogenesis in dysferlin-deficient myoblasts is inhibited by an intrinsic inflammatory response

Limb-girdle muscular dystrophy type 2B results from mutations in dysferlin, a membrane-associated protein involved in cellular membrane repair. Primary myoblast cultures derived from dysferlinopathy patients show reduced myogenic potential, suggesting that dysferlin may regulate myotube fusion and be required for muscle regeneration. These observations contrast with the findings that muscle develops normally in pre-symptomatic dysferlinopathy patients. To better understand the role of dysferlin in myogenesis, we investigated this process in vitro using cells derived from two mouse models of dysferlinopathy: SJL/J and A/J mice. We observed that myotubes derived from dysferlin-deficient muscle were of significantly smaller diameters, contained fewer myonuclei, and displayed reduced myogenic gene expression compared to dysferlin-sufficient cells. Together, these findings suggest that the absence of dysferlin from myoblasts is detrimental to myogenesis. Pro-inflammatory NFκB signaling was upregulated in dysferlin-deficient myotubes; the anti-inflammatory agent celastrol reduced the NFκB activation and improved myogenesis in dysferlin-deficient cultures. The results suggest that decreased myotube fusion in dysferlin deficiency is attributable to intrinsic inflammatory activation and can be improved using anti-inflammatory mediators.

IKKα and alternative NF-κB regulate PGC-1β to promote oxidative muscle metabolism

Alternative NF-κB signaling modulates the activity of PGC-1β to promote oxidative metabolism in skeletal muscle.

Although the physiological basis of canonical or classical IκB kinase β (IKKβ)–nuclear factor κB (NF-κB) signaling pathway is well established, how alternative NF-κB signaling functions beyond its role in lymphoid development remains unclear. In particular, alternative NF-κB signaling has been linked with cellular metabolism, but this relationship is poorly understood. In this study, we show that mice deleted for the alternative NF-κB components IKKα or RelB have reduced mitochondrial content and function. Conversely, expressing alternative, but not classical, NF-κB pathway components in skeletal muscle stimulates mitochondrial biogenesis and specifies slow twitch fibers, suggesting that oxidative metabolism in muscle is selectively controlled by the alternative pathway. The alternative NF-κB pathway mediates this specificity by direct transcriptional activation of the mitochondrial regulator PPAR-γ coactivator 1β (PGC-1β) but not PGC-1α. Regulation of PGC-1β by IKKα/RelB also is mammalian target of rapamycin (mTOR) dependent, highlighting a cross talk between mTOR and NF-κB in muscle metabolism. Together, these data provide insight on PGC-1β regulation during skeletal myogenesis and reveal a unique function of alternative NF-κB signaling in promoting an oxidative metabolic phenotype.

Stevioside enhances satellite cell activation by inhibiting of NF-κB signaling pathway in regenerating muscle after cardiotoxin-induced injury

Stevioside, a noncaloric sweetener isolated from Stevia rebaudiana, exhibits anti-inflammatory and immunomodulatory effects through interference of nuclear factor (NF)-kappa B pathway. We investigated whether this anti-inflammatory property of stevioside could improve muscle regeneration following cardiotoxin-induced muscle injury. Adult male Wistar rats received stevioside orally at an accepted daily dosage of 10 mg kg⁻¹ for 7 days before cardiotoxin injection at the tibialis anterior (TA) muscle of the right hindlimb (the left hindlimb served as control), and stevioside administration was continued for 3 and 7 days. TA muscle was examined at days 3 and 7 postinjury. Although stevioside treatment had no significant effect in enhancing muscle regeneration as indicated by the absence of decreased muscle inflammation or improved myofibrillar protein content compared with vehicle treated injured group at day 7 postinjury, the number of MyoD-positive nuclei were increased (P < 0.05), with a corresponding decrease in NF-κB nuclear translocation (P < 0.05). This is the first study to demonstrate that stevioside could enhance satellite cell activation by modulation of the NF-κB signaling pathway in regenerating muscle following injury. Thus, stevioside may be beneficial as a dietary supplementation for promoting muscle recovery from injury. However, its pharmacological effect on muscle function recovery warrants further investigation.

Loss of miR-29 in myoblasts contributes to dystrophic muscle pathogenesis

microRNAs (miRNAs) are noncoding RNAs that regulate gene expression in post-transcriptional fashion, and emerging studies support their importance in a multitude of physiological and pathological processes. Here, we describe the regulation and function of miR-29 in Duchenne muscular dystrophy (DMD) and its potential use as therapeutic target. Our results demonstrate that miR-29 expression is downregulated in dystrophic muscles of mdx mice, a model of DMD. Restoration of its expression by intramuscular and intravenous injection improved dystrophy pathology by both promoting regeneration and inhibiting fibrogenesis. Mechanistic studies revealed that loss of miR-29 in muscle precursor cells (myoblasts) promotes their transdifferentiation into myofibroblasts through targeting extracellular molecules including collagens and microfibrillar-associated protein 5 (Mfap5). We further demonstrated that miR-29 is under negative regulation by transforming growth factor-β (TGF-β) signaling. Together, these results not only identify TGF-β-miR-29 as a novel regulatory axis during myoblasts conversion into myofibroblasts which constitutes a novel contributing route to muscle fibrogenesis of DMD but also implicate miR-29 replacement therapy as a promising treatment approach for DMD.

The oncogenic role of Yin Yang 1

Yin Yang 1 (YY1) is a transcription factor with diverse and complex biological functions. YY1 either activates or represses gene transcription, depending on the stimuli received by the cells and its association with other cellular factors. Since its discovery, a biological role for YY1 in tumor development and progression has been suggested because of its regulatory activities toward multiple cancer-related proteins and signaling pathways and its overexpression in most cancers. In this review, we primarily focus on YY1 studies in cancer research, including the regulation of YY1 as a transcription factor, its activities independent of its DNA binding ability, the functions of its associated proteins, and mechanisms regulating YY1 expression and activities. We also discuss the correlation of YY1 expression with clinical outcomes of cancer patients and its target potential in cancer therapy. Although there is not a complete consensus about the role of YY1 in cancers based on its activities of regulating oncogene and tumor suppressor expression, most of the currently available evidence supports a proliferative or oncogenic role of YY1 in tumorigenesis.

Inhibition of miR-29 by TGF-beta-Smad3 signaling through dual mechanisms promotes transdifferentiation of mouse myoblasts into myofibroblasts

MicroRNAs (miRNAs) are non-coding RNAs that regulate gene expression in post-transcriptional fashion, and emerging studies support their importance in regulating many biological processes, including myogenic differentiation and muscle development. miR-29 is a promoting factor during myogenesis but its full spectrum of impact on muscle cells has yet to be explored. Here we describe an analysis of miR-29 affected transcriptome in C2C12 muscle cells using a high throughput RNA-sequencing platform. The results reveal that miR-29 not only functions to promote myogenic differentiation but also suppresses the transdifferentiation of myoblasts into myofibroblasts. miR-29 inhibits the fibrogenic differentiation through down-regulating both extracellular matrix genes and cell adhesion genes. We further demonstrate that miR-29 is under negative regulation by TGF-beta (TGF-β)-Smad3 signaling via dual mechanisms of both inhibiting MyoD binding and enhancing Yin Yang 1 (YY1)-recruited Polycomb association. Together, these results identify miR-29 as a pleiotropic molecule in both myogenic and fibrogenic differentiation of muscle cells.

Resistance exercise increases NF-κB activity in human skeletal muscle

Intense resistance exercise causes a significant inflammatory response. NF-κB has been identified as a prospective key transcription factor mediating the postexercise inflammatory response. The purpose of this study was to determine whether a single bout of intense resistance exercise regulates NF-κB signaling in human skeletal muscle. Muscle biopsy samples were obtained from the vastus lateralis of five recreationally active, but not strength-trained, males (21.9 ± 1.3 yr) prior to, and at 2 and 4 h following, a single bout of intense resistance exercise. A further five subjects (4 males, 1 female) (23 ± 0.89 yr) were recruited as a nonexercise control group to examine the effect of the muscle biopsy protocol on key markers of skeletal muscle inflammation. Protein levels of IκBα and phosphorylated NF-κB (p65), as well as the mRNA expression of inflammatory myokines monocyte chemoattractant protein 1 (MCP-1), IL-6, and IL-8 were measured. Additionally, NF-κB (p65) DNA binding to the promoter regions of MCP-1, IL-6, and IL-8 was investigated. IκBα protein levels decreased, while p-NF-κB (p65) protein levels increased 2 h postexercise and returned to near-baseline levels by 4-h postexercise. Immunohistochemical data verified these findings, illustrating an increase in p-NF-κB (p65) protein levels, and nuclear localization at 2 h postexercise. Furthermore, NF-κB DNA binding to MCP-1, IL-6, and IL-8 promoter regions increased significantly 2 h postexercise as did mRNA levels of these myokines. No significant change was observed in the nonexercise control group. These novel data provide evidence that intense resistance exercise transiently activates NF-κB signaling in human skeletal muscle during the first few hours postexercise. These findings implicate NF-κB in the transcriptional control of myokines known to be central to the postexercise inflammatory response.

Yin Yang-1 suppresses differentiation of hepatocellular carcinoma cells through the downregulation of CCAAT/enhancer-binding protein alpha

As a member of the GLI-Kruppel family of transcriptional factors, Yin Yang-1 (YY1) functions as an oncogene in various types of cancers. However, the role of YY1 in hepatocellular carcinogenesis remains unknown. In this report, we investigated the relevance of YY1 to hepatocellular carcinoma (HCC) development. We found that YY1 was upregulated in HCC cell lines. Ectopic YY1 expression promoted the growth of non-tumor liver cells that expressed low level of YY1. In contrast, YY1 depletion inhibited the growth of HCC cells which was accompanied with distinct morphological changes. Moreover, the phenotypic changes induced by YY1 depletion were attributed to cellular differentiation rather than cellular senescence. CCAAT/enhancer-binding protein alpha (CEBPA) which was important to regulate differentiation of hepatocytes was found as the direct target downregulated by YY1. Restoration of CEBPA in YY1-expressing HCC cells induced cellular differentiation and growth inhibition while knockdown of CEBPA expression in non-tumor liver cells promoted cell growth. In summary, our study demonstrated that YY1 could promote hepatocellular carcinogenesis and inhibit cellular differentiation through the downregulation of CEBPA expression.