Down-regulation of transcription factors AP-1, Sp-1, and NF-kappa B precedes myocyte differentiation

The Upstream 1350~1250 Nucleotide Sequences of the Human ENDOU-1 Gene Contain Critical Cis-Elements Responsible for Upregulating Its Transcription during ER Stress

ENDOU-1 encodes an endoribonuclease that overcomes the inhibitory upstream open reading frame (uORF)-trap at 5'-untranslated region (UTR) of the CHOP transcript, allowing the downstream coding sequence of CHOP be translated during endoplasmic reticulum (ER) stress. However, transcriptional control of ENDOU-1 remains enigmatic. To address this, we cloned an upstream 2.1 kb (-2055~+77 bp) of human ENDOU-1 (pE2.1p) fused with reporter luciferase (luc) cDNA. The promoter strength driven by pE2.1p was significantly upregulated in both pE2.1p-transfected cells and pE2.1p-injected zebrafish embryos treated with stress inducers. Comparing the luc activities driven by pE2.1p and -1125~+77 (pE1.2p) segments, we revealed that cis-elements located at the -2055~-1125 segment might play a critical role in ENDOU-1 upregulation during ER stress. Since bioinformatics analysis predicted many cis-elements clustered at the -1850~-1250, we further deconstructed this segment to generate pE2.1p-based derivatives lacking -1850~-1750, -1749~-1650, -1649~-1486, -1485~-1350 or -1350~-1250 segments. Quantification of promoter activities driven by these five internal deletion plasmids suggested a repressor binding element within the -1649~-1486 and an activator binding element within the -1350~-1250. Since luc activities driven by the -1649~-1486 were not significantly different between normal and stress conditions, we herein propose that the stress-inducible activator bound at the -1350~-1250 segment makes a major contribution to the increased expression of human ENDOU-1 upon ER stresses.

Investigating proliferation and differentiation capacities of Hanwoo steer myosatellite cells at different passages for developing cell-cultured meat

The aim of study was to investigate proliferation and differentiation capacities of Hanwoo myosatellite cells for the development of Hanwoo cell cultures. From P1 to P19, the number of live cells decreased and the cell size increased. It was confirmed that the PAX7 mRNA was higher in P3 than P6 and P9 (p < 0.05). The maximum differentiation score was measured from P1 to P12. The maximum differentiation score maintained high from P1 to P10. Immunostaining was performed for both P1 and P10 cells to investigate differentiation characteristics. And there were no significant differences in differentiation characteristics between P1 and P10 cells. MYOG mRNA was low, whereas C-FOS mRNA was high (p < 0.05) in the late passage. Myosin and Tom20 protein also showed low values in the late passage (p < 0.05). In conclusion, our results suggest that it is appropriate to use P1 to P10 for the production of cultured meat using Hanwoo muscle cells. If cell culture meat production is performed without differentiation, the passage range may increase further. These results provide basic essential data required for further development of Hanwoo cell cultures, which could provide a valuable source of protein for human populations in the future.

Proteome-wide profiling of transcriptional machinery on accessible chromatin with biotinylated transposons

To directly and quantitatively identify the transcriptional protein complexes assembled on accessible chromatin, we develop an assay for transposase-accessible chromatin using mass spectrum (ATAC-MS) based on direct transposition of biotinylated adaptors into open chromatin. Coupling with activated gene sequence information by ATAC-seq, ATAC-MS can profile the accessible chromatin-protein machinery. ATAC-MS, combined with fractionation strategies (fATAC-MS), can provide a high-resolution chromatin-transcriptional machinery atlas. ATAC-MS with a novel Tn5-dCas9 fusion protein [dCas9-targeted ATAC-MS (ctATAC-MS)] further facilitates systematic pinpointing of the transcriptional machinery at specific open chromatin regions. We used ATAC-MS and ATAC-seq to investigate transcriptional regulation during C2C12 cell differentiation and demonstrated the role of RFX1 in regulating the proliferation and differentiation of C2C12 cells. Our strategy provides a universal toolbox including ATAC-MS, fATAC-MS, and ctATAC-MS, which enables us to portray the transcriptional regulation machinery atlas in genome scale and investigate the protein-DNA complex at a specific genomic locus.

Early Growth Response 3 (Egr3) Contributes a Maintenance of C2C12 Myoblast Proliferation

Satellite cell proliferation is a crucially important process for adult myogenesis. However, its regulatory mechanisms remain unknown. Early growth response 3 (Egr3) is a zinc-finger transcription factor that regulates different cellular functions. Reportedly, Egr3 interacts with multiple signaling molecules that are also known to regulate satellite cell proliferation. Therefore, it is possible that Egr3 is involved in satellite cell proliferation. Results of this study have demonstrated that Egr3 transcript levels are upregulated in regenerating mouse skeletal muscle after cardiotoxin injury. Using C2C12 myoblast culture (a model of activated satellite cells), results show that inhibition of Egr3 by shRNA impairs the myoblast proliferation rate. Results also show reduction of NF-кB transcriptional activity in Egr3-inhibited cells. Inhibition of Egr3 is associated with an increase in annexin V⁺ cell fraction and apoptotic protein activity including caspase-3 and caspase-7, and Poly-ADP ribose polymerase. By contrast, the reduction of cellular proliferation by inhibition of Egr3 was partially recovered by treatment of pan-caspase inhibitor Z-VAD-FMK. Collectively, these results suggest that Egr3 is involved in myoblast proliferation by interaction with survival signaling. J. Cell. Physiol. 232: 1114-1122, 2017. © 2016 Wiley Periodicals, Inc.

Activation of Mechanosensitive Transcription Factors in Murine C2C12 Mesenchymal Precursors by Focused Low-Intensity Pulsed Ultrasound (FLIPUS)

In this paper, we investigated the mechanoresponse of C2C12 mesenchymal precursor cells to focused low-intensity pulsed ultrasound (FLIPUS). The setup has been developed for in vitro stimulation of adherent cells in the defocused far field of the ultrasound propagating through the bottom of the well plate. Twenty-four-well tissue culture plates, carrying the cell monolayers, were incubated in a temperature-controlled water tank. The ultrasound was applied at 3.6-MHz frequency, pulsed at 100-Hz repetition frequency with a 27.8% duty cycle, and calibrated at an output intensity of I_SATA = 44.5 ±7.1 mW/cm². Numerical sound propagation simulations showed no generation of standing waves in the well plate. The response of murine C2C12 cells to FLIPUS was evaluated by measuring activation of mechanosensitive transcription factors, i.e., activator protein-1 (AP-1), specificity protein 1 (Sp1), and transcriptional enhancer factor (TEAD), and expression of mechanosensitive genes, i.e., c-fos, c-jun, heparin binding growth associated molecule (HB-GAM), and Cyr-61. FLIPUS induced 50% ( p ≤ 0.05 ) and 70% ( p ≤ 0.05 ) increases in AP-1 and TEAD promoter activities, respectively, when stimulated for 5 min. The Sp1 activity was enhanced by about 20% ( p ≤ 0.05 ) after 5-min FLIPUS exposure and the trend persisted for 30-min ( p ≤ 0.05 ) and 1-h ( p ≤ 0.05 ) stimulation times. Expressions of mechanosensitive genes c-fos ( p ≤ 0.05 ), c-jun ( p ≤ 0.05 ), HB-GAM ( p ≤ 0.05 ), and cystein-rich protein 61 ( p ≤ 0.05 ) were enhanced in response to 5-min FLIPUS stimulation. The increase in proliferation of C2C12s occurred after the FLIPUS stimulation ( p ≤ 0.05 ), with AP-1, Sp1, and TEAD possibly regulating the observed cellular activities.

NF-κB inhibition reveals a novel role for HGF during skeletal muscle repair

The transcription factor nuclear factor κB (NF-κB)/p65 is the master regulator of inflammation in Duchenne muscular dystrophy (DMD). Disease severity is reduced by NF-κB inhibition in the mdx mouse, a murine DMD model; however, therapeutic targeting of NF-κB remains problematic for patients because of its fundamental role in immunity. In this investigation, we found that the therapeutic effect of NF-κB blockade requires hepatocyte growth factor (HGF) production by myogenic cells. We found that deleting one allele of the NF-κB subunit p65 (p65+/-) improved the survival and enhanced the anti-inflammatory capacity of muscle-derived stem cells (MDSCs) following intramuscular transplantation. Factors secreted from p65+/- MDSCs in cell cultures modulated macrophage cytokine expression in an HGF-receptor-dependent manner. Indeed, we found that following genetic or pharmacologic inhibition of basal NF-κB/p65 activity, HGF gene transcription was induced in MDSCs. We investigated the role of HGF in anti-NF-κB therapy in vivo using mdx;p65+/- mice, and found that accelerated regeneration coincided with HGF upregulation in the skeletal muscle. This anti-NF-κB-mediated dystrophic phenotype was reversed by blocking de novo HGF production by myogenic cells following disease onset. HGF silencing resulted in increased inflammation and extensive necrosis of the diaphragm muscle. Proteolytic processing of matrix-associated HGF is known to activate muscle stem cells at the earliest stages of repair, but our results indicate that the production of a second pool of HGF by myogenic cells, negatively regulated by NF-κB/p65, is crucial for inflammation resolution and the completion of repair in dystrophic skeletal muscle. Our findings warrant further investigation into the potential of HGF mimetics for the treatment of DMD.

Acute increases in intramuscular inflammatory cytokines are necessary for the development of mechanical hypersensitivity in a mouse model of musculoskeletal sensitization

Musculoskeletal pain is a widespread health problem in the United States. Back pain, neck pain, and facial pain are three of the most prevalent types of chronic pain, and each is characterized as musculoskeletal in origin. Despite its prevalence, preclinical research investigating musculoskeletal pain is limited. Musculoskeletal sensitization is a preclinical model of muscle pain that produces mechanical hypersensitivity. In a rodent model of musculoskeletal sensitization, mechanical hypersensitivity develops at the hind paws after injection of acidified saline (pH 4.0) into the gastrocnemius muscle. Inflammatory cytokines contribute to pain during a variety of pathologies, and in this study we investigate the role of local, intramuscular cytokines in the development of mechanical hypersensitivity after musculoskeletal sensitization in mice. Local intramuscular concentrations of interleukin-1β (IL-1), IL-6 and tumor necrosis factor-α (TNF) were quantified following injection of normal (pH 7.2) or acidified saline into the gastrocnemius muscle. A cell-permeable inhibitor was used to determine the impact on mechanical hypersensitivity of inhibiting nuclear translocation of the transcription factor nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) prior to musculoskeletal sensitization. The role of individual cytokines in mechanical hypersensitivity following musculoskeletal sensitization was assessed using knockout mice lacking components of the IL-1, IL-6 or TNF systems. Collectively, our data demonstrate that acidified saline injection increases intramuscular IL-1 and IL-6, but not TNF; that intramuscular pre-treatment with an NF-κB inhibitor blocks mechanical hypersensitivity; and that genetic manipulation of the IL-1 and IL-6, but not TNF systems, prevents mechanical hypersensitivity following musculoskeletal sensitization. These data establish that actions of IL-1 and IL-6 in local muscle tissue play an acute regulatory role in the development of mechanical hypersensitivity following musculoskeletal sensitization.

NF-KB activity functions in primary pericytes in a cell- and non-cell-autonomous manner to affect myotube formation

INTRODUCTION

Skeletal muscle regeneration following damage relies on proliferation and differentiation of muscle precursor cells (MPCs). We recently observed increased NF-kB activity in vascular-associated muscle resident pericytes following muscle damage in humans. We determined how altered NF-kB activity in human primary pericytes (HPPs) affects their myogenic differentiation (cell-autonomous effects), as well as proliferation and differentiation of co-cultured MPCs (non-cell-autonomous effects).

METHODS

HPPs were transfected with vectors that increased or decreased NF-kB activity. Transfected HPPs were co-cultured with C2 C12 myoblasts under differentiation conditions, and HPP fusion to myotubes was measured. We also co-cultured HPPs with C2 C12 myoblasts and measured proliferation and myotube formation.

RESULTS

Inhibition of NF-kB activity increased HPP fusion to C2 C12 myotubes. Moreover, enhanced NF-kB activity in HPPs suppressed differentiation and enhanced proliferation of co-cultured myoblasts.

CONCLUSIONS

NF-kB activity acts cell-autonomously to inhibit HPP myogenic differentiation and non-cell-autonomously to promote MPC proliferation and suppress MPC differentiation in vitro.

Sox4-mediated caldesmon expression facilitates skeletal myoblast differentiation

Caldesmon (CaD), originally identified as an actin-regulatory protein, is involved in the regulation of diverse actin-related signaling processes, including cell migration and proliferation, in various cells. The cellular function of CaD has been studied primarily in the smooth muscle system; nothing is known about its function in skeletal muscle differentiation. In this study, we found that the expression of CaD gradually increased as C2C12 myoblast differentiation progressed. Silencing of CaD inhibited cell spreading and migration, resulting in a decrease in myoblast differentiation. Promoter analysis of the caldesmon gene (CALD1) and gel mobility shift assays identified Sox4 as a major trans-acting factor for the regulation of CALD1 expression during myoblast differentiation. Silencing of Sox4 decreased not only CaD protein synthesis but also myoblast fusion in C2C12 cells and myofibril formation in mouse embryonic muscle. Overexpression of CaD in Sox4-silenced C2C12 cells rescued the differentiation process. These results clearly demonstrate that CaD, regulated by Sox4 transcriptional activity, contributes to skeletal muscle differentiation.

The effect of the NF-kappa B inhibitors curcumin and lactacystin on myogenic differentiation of rhabdomyosarcoma cells

Rhabdomyosarcoma is a soft tissue sarcoma mainly seen in children. Despite considerable progress within the last few years, therapeutic approaches for this type of tumor are still limited. The respective tumor cells originate from myogenic precursor cells and are characterized by a blockade in their differentiation program. Interestingly, there is a direct inverse correlation between the differentiation status of a specific rhabdomyosarcoma cell and its metastatic potential. Thus, here, we tested whether the ubiquitous transcription factor NF-κB, which regulates myogenic differentiation and is also a promising therapeutic target in the treatment of other types of tumors, might be an interesting candidate for the development of novel rhabdomyosarcoma treatment strategies. For this purpose, we analyzed NF-κB activity (classical pathway) in myoblasts with different differentiation potential, specifically in three different rhabdomyosarcoma cell lines. In addition, we inhibited NF-κB activity in these cells and analyzed the effects on myogenic differentiation. We show that after the induction of differentiation, NF-κB activity declines rapidly in normal myoblasts, but only slightly in rhabdomyosarcoma cells. However, after treatment of the cells with two different small-molecule NF-κB-inhibiting compounds, the IKK inhibitor curcumin and the proteasome inhibitor lactacystin, we found that neither curcumin nor lactacystin promoted myogenic differentiation in either normal myoblasts or rhabdomyosarcoma cells. Taken together, our data suggest that treatment with curcumin or lactacystin might not be a suitable approach in the treatment of rhabdomyosarcoma.

NF-κB negatively impacts the myogenic potential of muscle-derived stem cells

Inhibition of the inhibitor of kappa B kinase (IKK)/nuclear factor-kappa B (NF-κB) pathway enhances muscle regeneration in injured and diseased skeletal muscle, but it is unclear exactly how this pathway contributes to the regeneration process. In this study, we examined the role of NF-κB in regulating the proliferation and differentiation of muscle-derived stem cells (MDSCs). MDSCs isolated from the skeletal muscles of p65(+/-) mice (haploinsufficient for the p65 subunit of NF-κB) had enhanced proliferation and myogenic differentiation compared to MDSCs isolated from wild-type (wt) littermates. In addition, selective pharmacological inhibition of IKKβ, an upstream activator of NF-κB, enhanced wt MDSC differentiation into myotubes in vitro. The p65(+/-) MDSCs also displayed a higher muscle regeneration index than wt MDSCs following implantation into adult mice with muscular dystrophy. Additionally, using a muscle injury model, we observed that p65(+/-) MDSC engraftments were associated with reduced inflammation and necrosis. These results suggest that inhibition of the IKK/NF-κB pathway represents an effective approach to improve the myogenic regenerative potential of MDSCs and possibly other adult stem cell populations. Moreover, our results suggest that the improved muscle regeneration observed following inhibition of IKK/NF-κB, is mediated, at least in part, through enhanced stem cell proliferation and myogenic potential.

Impact of SOCS3 overexpression on human skeletal muscle development in vitro

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

Tissue engineering of skin and cornea: Development of new models for in vitro studies

Human beings are greatly preoccupied with the unavoidable nature of aging. While the biological processes of senescence and aging are the subjects of intense investigations, the molecular mechanisms linking aging with disease and death are yet to be elucidated. Tissue engineering offers new models to study the various processes associated with aging. Using keratin 19 as a stem cell marker, our studies have revealed that stem cells are preserved in human skin reconstructed by tissue engineering and that the number of epithelial stem cells varies according to the donor's age. As with skin, human corneas can also be engineered in vitro. Among the epithelial cells used for reconstructing skin and corneas, significant age-dependent variations in the expression of the transcription factor Sp1 were observed. Culturing skin epithelial cells with a feeder layer extended their life span in culture, likely by preventing Sp1 degradation in epithelial cells, therefore demonstrating the pivotal role played by this transcription factor in cell proliferation. Finally, using the human tissue-engineered skin as a model, we linked Hsp27 activation with skin differentiation.

NF-kappaB signaling: a tale of two pathways in skeletal myogenesis

NF-kappaB is a ubiquitiously expressed transcription factor that plays vital roles in innate immunity and other processes involving cellular survival, proliferation, and differentiation. Activation of NF-kappaB is controlled by an IkappaB kinase (IKK) complex that can direct either canonical (classical) NF-kappaB signaling by degrading the IkappaB inhibitor and releasing p65/p50 dimers to the nucleus, or causes p100 processing and nuclear translocation of RelB/p52 via a noncanonical (alternative) pathway. Under physiological conditions, NF-kappaB activity is transiently regulated, whereas constitutive activation of this transcription factor typically in the classical pathway is associated with a multitude of disease conditions, including those related to skeletal muscle. How NF-kappaB functions in muscle diseases is currently under intense investigation. Insight into this role of NF-kappaB may be gained by understanding at a more basic level how this transcription factor contributes to skeletal muscle cell differentiation. Recent data from knockout mice support that the classical NF-kappaB pathway functions as an inhibitor of skeletal myogenesis and muscle regeneration acting through multiple mechanisms. In contrast, alternative NF-kappaB signaling does not appear to be required for myofiber conversion, but instead functions in myotube homeostasis by regulating mitochondrial biogenesis. Additional knowledge of these signaling pathways in skeletal myogenesis should aid in the development of specific inhibitors that may be useful in treatments of muscle disorders.

Prolyl hydroxylase EGLN3 regulates skeletal myoblast differentiation through an NF-kappaB-dependent pathway

The egg-laying abnormal-9 (EGLN) prolyl hydroxylases have been shown to regulate the stability and thereby the activity of the alpha subunits of hypoxia-inducible factor (HIF) through its ability to catalyze their hydroxylation. We have previously shown that EGLN3 promotes differentiation of C2C12 skeletal myoblasts. However, the mechanism underlying this effect remains to be fully elucidated. Here, we report that exposure of C2C12 cells to dimethyl oxalylglycine (DMOG), desferrioxamine, and hypoxia, all inhibitors of prolyl hydroxylase activity, led to repression of C2C12 myogenic differentiation. Inactivation of HIF by expression of a HIF dominant-negative mutant or deletion of HIF-1alpha by RNA interference did not affect the inhibitory effect of DMOG, suggesting that the effect of DMOG is HIF-independent. Pharmacologic inactivation of EGLN3 hydroxylase resulted in activation of the canonical NF-kappaB pathway. The inhibitory effect of DMOG on myogenic differentiation was markedly impaired in C2C12 cells expressing a dominant-negative mutant of IkappaBalpha. Exogenous expression of wild-type EGLN3, but not its catalytically inactive mutant, significantly inhibited NF-kappaB activation induced by overexpressed TRAF2 or IkappaB kinase 2. In contrast, deletion of EGLN3 by small interfering RNAs led to activation of NF-kappaB. These data suggest that EGLN3 is a negative regulator of NF-kappaB, and its prolyl hydroxylase activity is required for this effect. Furthermore, wild-type EGLN3, but not its catalytically inactive mutant, potentiated myogenic differentiation. This study demonstrates a novel role for EGLN3 in the regulation of NF-kappaB and suggests that it is involved in mediating myogenic differentiation, which is HIF-independent.

NF-kappaB functions in stromal fibroblasts to regulate early postnatal muscle development

Classical NF-kappaB activity functions as an inhibitor of the skeletal muscle myogenic program. Recent findings reveal that even in newborn RelA/p65(-/-) mice, myofiber numbers are increased over that of wild type mice, suggesting that NF-kappaB may be a contributing factor in early postnatal skeletal muscle development. Here we show that in addition to p65 deficiency, repression of NF-kappaB with the IkappaB alpha-SR transdominant inhibitor or with muscle-specific deletion of IKKbeta resulted in similar increases in total fiber numbers as well as an up-regulation of myogenic gene products. Upon further characterization of early postnatal muscle, we observed that NF-kappaB activity progressively declines within the first few weeks of development. At birth, the majority of this activity is compartmentalized to muscle fibers, but by neonatal day 8 NF-kappaB activity from the myofibers diminishes, and instead, stromal fibroblasts become the main cellular compartment within the muscle that contains active NF-kappaB. We find that NF-kappaB functions in these fibroblasts to regulate inducible nitric-oxide synthase expression, which we show is important for myoblast fusion during the growth and maturation process of skeletal muscle. Together, these data broaden our understanding of NF-kappaB during development by showing that in addition to its role as a negative regulator of myogenesis, NF-kappaB also regulates nitric-oxide synthase expression within stromal fibroblasts to stimulate myoblast fusion and muscle hypertrophy.

JNK1 determines the oncogenic or tumor-suppressive activity of the integrin-linked kinase in human rhabdomyosarcoma

Although most reports describe the protein kinase integrin-linked kinase (ILK) as a proto-oncogene, occasional studies detail opposing functions in the regulation of normal and transformed cell proliferation, differentiation, and apoptosis. Here, we demonstrated that ILK functions as an oncogene in the highly aggressive pediatric sarcoma alveolar rhabdomyosarcoma (ARMS) and as a tumor suppressor in the related embryonal rhabdomyosarcoma (ERMS). These opposing functions hinge on signaling through a noncanonical ILK target, JNK1, to the proto-oncogene c-Jun. RNAi-mediated depletion of ILK induced activation of JNK and its target, c-Jun, resulting in growth of ERMS cells, whereas in ARMS cells, it led to loss of JNK/c-Jun signaling and suppression of growth both in vitro and in vivo. Ectopic expression of the fusion gene characteristic of ARMS (paired box 3-forkhead homolog in rhabdomyosarcoma [PAX3-FKHR]) in ERMS cells was sufficient to convert them to an ARMS signaling phenotype and render ILK activity oncogenic. Furthermore, restoration of JNK1 in ARMS reestablished a tumor-suppressive function for ILK. These findings indicate what we believe to be a novel effector pathway regulated by ILK, provide a mechanism for interconversion of oncogenic and tumor-suppressor functions of a single regulatory protein based on the genetic background of the tumor cells, and suggest a rationale for tailored therapy of rhabdomyosarcoma based on the different activities of ILK.

Skeletal muscle diseases, inflammation, and NF-kappaB signaling: insights and opportunities for therapeutic intervention

Signaling through nuclear factor-kappa B (NF-kappaB) is emerging as an important regulator of muscle development, maintenance, and regeneration. Classic signaling modulates early muscle development by enhancing proliferation and inhibiting differentiation, and alternative signaling promotes myofiber maintenance and metabolism. Likewise, NF-kappaB signaling is critical for the development of immunity. Although these processes occur normally, dysregulation of NF-kappaB signaling has prohibitive effects on muscle growth and regeneration and can perpetuate inflammation in muscle diseases. Aberrant NF-kappaB signaling from immune and muscle cells has been detected and implicated in the pathologic progression of numerous dystrophies and myopathies, indicating that targeted NF-kappaB inhibitors may prove clinically beneficial.

Nuclear factor-kappa B signaling in skeletal muscle atrophy

Skeletal muscle atrophy/wasting is a serious complication of a wide range of diseases and conditions such as aging, disuse, AIDS, chronic obstructive pulmonary disease, space travel, muscular dystrophy, chronic heart failure, sepsis, and cancer. Emerging evidence suggests that nuclear factor-kappa B (NF-kappaB) is one of the most important signaling pathways linked to the loss of skeletal muscle mass in various physiological and pathophysiological conditions. Activation of NF-kappaB in skeletal muscle leads to degradation of specific muscle proteins, induces inflammation and fibrosis, and blocks the regeneration of myofibers after injury/atrophy. Recent studies employing genetic mouse models have provided strong evidence that NF-kappaB can serve as an important molecular target for the prevention of skeletal muscle loss. In this article, we have outlined the current understanding regarding the role of NF-kappaB in skeletal muscle with particular reference to different models of muscle wasting and the development of novel therapy.

IKK/NF-kappaB regulates skeletal myogenesis via a signaling switch to inhibit differentiation and promote mitochondrial biogenesis

Nuclear factor κB (NF-κB) is involved in multiple skeletal muscle disorders, but how it functions in differentiation remains elusive given that both anti- and promyogenic activities have been described. In this study, we resolve this by showing that myogenesis is controlled by opposing NF-κB signaling pathways. We find that myogenesis is enhanced in MyoD-expressing fibroblasts deficient in classical pathway components RelA/p65, inhibitor of κB kinase β (IKKβ), or IKKγ. Similar increases occur in myoblasts lacking RelA/p65 or IKKβ, and muscles from RelA/p65 or IKKβ mutant mice also contain higher fiber numbers. Moreover, we show that during differentiation, classical NF-κB signaling decreases, whereas the induction of alternative members IKKα, RelB, and p52 occurs late in myogenesis. Myotube formation does not require alternative signaling, but it is important for myotube maintenance in response to metabolic stress. Furthermore, overexpression or knockdown of IKKα regulates mitochondrial content and function, suggesting that alternative signaling stimulates mitochondrial biogenesis. Together, these data reveal a unique IKK/NF-κB signaling switch that functions to both inhibit differentiation and promote myotube homeostasis.

Bisperoxovanadium, a phospho-tyrosine phosphatase inhibitor, reprograms myogenic cells to acquire a pluripotent, circulating phenotype

Satellite cells are the main source of myogenic progenitors in postnatal skeletal muscle, but their use in cell therapy for muscle disorders is limited because these cells cannot be delivered through circulation and they are rapidly exhausted in severe myopathies. The search for alternative donor cells is ongoing, but none of the candidates so far show all the features required for successful colonization and repair of diseased muscle. In this study, we show that bisperoxovanadium, a phospho-tyrosine phosphatase inhibitor, induces myogenic cells to acquire a gene expression profile and a differentiation potential consistent with the phenotype of a circulating precursors, while maintaining their myogenic potential. These effects are mediated, at least in part, by NF-kappaB activation through the Tyr42-IkappaB-alpha phosphorylation, as shown by the expression of the dominant negative mutant form of the p50 NF-kappaB subunit. Moreover, when bisperoxovanadium-treated cells are injected into the femoral artery of alpha-sarcoglican null dystrophic mice, they are able to circulate and to reach muscle tissue; importantly, they contribute to muscle regeneration, as shown by the expression of alpha-sarcoglican in some fibers. Our observations indicate that bisperoxovanadium, or similar compounds, may prove very valuable to obtain and to expand, from committed cells, multipotent cell populations suitable for gene-cell therapy applications and may help to understand the molecular basis of genome reprogramming and "stem-ness."

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

NF-kappaB signaling is implicated as an important regulator of skeletal muscle homeostasis, but the mechanisms by which this transcription factor contributes to muscle maturation and turnover remain unclear. To gain insight into these mechanisms, gene expression profiling was examined in C2C12 myoblasts devoid of NF-kappaB activity. Interestingly, even in proliferating myoblasts, the absence of NF-kappaB caused the pronounced induction of several myofibrillar genes, suggesting that NF-kappaB functions as a negative regulator of late-stage muscle differentiation. Although several myofibrillar promoters contain predicted NF-kappaB binding sites, functional analysis using the troponin-I2 gene as a model revealed that NF-kappaB-mediated repression does not occur through direct DNA binding. In the search for an indirect mediator, the transcriptional repressor YinYang1 (YY1) was identified. While inducers of NF-kappaB stimulated YY1 expression in multiple cell types, genetic ablation of the RelA/p65 subunit of NF-kappaB in both cultured cells and adult skeletal muscle correlated with reduced YY1 transcripts and protein. NF-kappaB regulation of YY1 occurred at the transcriptional level, mediated by direct binding of the p50/p65 heterodimer complex to the YY1 promoter. Furthermore, YY1 was found associated with multiple myofibrillar promoters in C2C12 myoblasts containing NF-kappaB activity. Based on these results, we propose that NF-kappaB regulation of YY1 and transcriptional silencing of myofibrillar genes represent a new mechanism by which NF-kappaB functions in myoblasts to modulate skeletal muscle differentiation.

TGFbeta1 regulation of vimentin gene expression during differentiation of the C2C12 skeletal myogenic cell line requires Smads, AP-1 and Sp1 family members

Vimentin exhibits a complex pattern of developmental and tissue-specific expression regulated by such growth factors as TGFbeta1, PDGF, FGF, EGF and cytokines. Vimentin is expressed in the more migratory, mesenchymal cell and its expression is often down-regulated to make way for tissue-specific intermediate filaments proteins such as desmin in muscle. Here, we suggest a mechanism to explain how TGFbeta1 contributes to the up-regulation of vimentin expression while blocking myogenesis. TGFbeta1 binds to serine/threonine kinase receptors resulting in the phosphorylation of Smad2 and Smad3, followed by formation of a heteromeric complex with Smad4. The translocation of this complex to the nucleus modulates transcription of selected genes such as vimentin. However, the vimentin gene lacks a consensus TGFbeta1 response element. By transient transfection analysis of vimentin's various promoter elements fused to the CAT reporter gene, we have determined that tandem AP-1 sites surrounded by GC-boxes are required for TGFbeta1 induction. Mutations within this region eliminated the ability of Smad3 to induce reporter gene expression. DNA precipitation and ChIP assays suggest that c-Jun, c-Fos, Smad3 and Sp1/Sp3 interact over this region, but this interaction changes during myogenesis with TGFbeta1 induction.

Control of mitochondrial biogenesis during myogenesis

We used expression and reporter gene analysis to understand how changes in transcription factors impinge on mitochondrial gene expression during myogenesis of cultured murine myoblasts (C2C12 and Sol8). The mRNA levels for nuclear respiratory factor-1 (NRF-1) and NRF-2α increased 60% by the third day of myogenesis, whereas NRF-1 and NRF-2 reporter gene activity increased by fivefold over the same period. Although peroxisome proliferator activated receptor (PPARα) mRNA levels increased almost 10-fold, the activity of a PPAR reporter was unchanged during myogenesis. The PPAR coactivator PPAR-γ coactivator-1α (PGC1α), a master controller of mitochondrial biogenesis, was not expressed at detectable levels. However, the mRNA for both PGC1α-related coactivator and PGC1β was abundant, with the latter increasing by 50% over 3 days of differentiation. We also conducted promoter analysis of the gene for citrate synthase (CS), a common mitochondrial marker enzyme. The proximal promoter (∼2,100 bp) of the human CS lacks binding sites for PPAR, NRF-1, or NRF-2. Deletion mutants, a targeted mutation, and an Sp1 site-containing reporter construct suggest that changes in Sp1 regulation also participate in mitochondrial biogenesis during myogenesis. Because most mitochondrial genes are regulated by PPARs, NRF-1, and/or NRF-2, we conducted inhibitor studies to further support the existence of a distinct pathway for CS gene regulation in myogenesis. Although both LY-294002 (a phosphatidylinositol 3-kinase inhibitor) and SB-203580 (a p38-MAPK inhibitor) blocked myogenesis (as indicated by creatine phosphokinase activity), only SB-203580 prevented the myogenic increase in cytochrome oxidase activity, whereas only LY-294002 blocked the increase in CS (enzyme and reporter gene activities). Collectively, these studies help delineate the roles of some transcriptional regulators involved in mitochondrial biogenesis associated with myogenesis and underscore an import role for posttranscriptional regulation of transcription factor activity.

p75NTR-mediated signaling promotes the survival of myoblasts and influences muscle strength

During muscle development, the p75(NTR) is expressed transiently on myoblasts. The temporal expression pattern of the receptor raises the possibility that the receptor is influencing muscle development. To test this hypothesis, p75(NTR)-deficient mutant mice were tested for muscle strength by using a standard wire gripe strength test and were found to have significantly decreased strength relative to that of normal mice. When normal mybolasts were examined in vivo for expression of NGF receptors, p75(NTR) was detected on myoblasts but the high affinity NGF receptor, trk A, was not co-expressed with p75(NTR). In vitro, proliferating C2C12 and primary myoblasts co-expressed the p75(NTR) and MyoD, but immunofluorescent analysis of primary myoblasts and RT-PCR analysis of C2C12 mRNA revealed that myoblasts were devoid of trk A. In contrast to the cell death functions that characterize the p75(NTR) in neurons, p75(NTR)-positive primary and C2C12 myoblasts did not differentiate or undergo apoptosis in response to neurotrophins. Rather, myoblasts survived and even proliferated when grown at subconfluent densities in the presence of the neurotrophins. Furthermore, when myoblasts treated with NGF were lysed and immunoprecipitated with antibodies against phosphorylated I-kappaB and AKT, the cells contained increased levels of both phospho-proteins, both of which promote cell survival. By contrast, neurotrophin-treated myoblasts did not induce phosphorylation of Map Kinase p42/44 or p38, indicating the survival was not mediated by the trk A receptor. Taken together, the data indicate that the p75(NTR) mediates survival of myoblasts prior to differentiation and that the activity of this receptor during myogenesis is important for developing muscle.

Nuclear factor kappaB and activating protein 1 are involved in differentiation-related resistance to oxidative stress in skeletal muscle cells

Skeletal muscle cells are continuously exposed to oxidative stress. Thus, they compensate environmental challenges by increasing adaptive responses, characterized by activating protein 1 (AP-1)- and nuclear factor kappaB (NF-kappaB)-mediated transcriptional upregulation of endogenous enzymatic and nonenzymatic antioxidants. We investigated the crosstalk of molecules involved in redox signaling in muscle cells, by using the rat L6C5 and mouse C2C12 cell lines, which represent a useful experimental model for studying muscle metabolism. We analyzed specific antioxidant systems, including glutathione, thioredoxin reductase, and antioxidant enzymes, and the redox-sensitive transcription factors AP-1 and NF-kappaB, in both myoblasts and myotubes. We found that the high levels of NF-kappaB DNA binding activity and thioredoxin reductase, together with inhibitory AP-1 complexes, allowed increased expression of antioxidant enzymes and survival of C2C12 cells after oxidant exposure. On the contrary, L6C5 myoblasts had a sensitive phenotype, correlated with lower levels of thioredoxin reductase, catalase, and NF-kappaB activity and higher levels of GSSG and activating AP-1 complexes. Interestingly, this cell line acquired an apoptosis-resistant phenotype, accompanied by drastic changes in the oxidant/antioxidant balance, when induced to differentiate. In conclusion, the two cell lines, although similar in terms of growth and differentiation, displayed significant heterogeneity in terms of redox homeostasis.

Glutathione depletion impairs myogenic differentiation of murine skeletal muscle C2C12 cells through sustained NF-kappaB activation

Skeletal muscle differentation is a complex process regulated at multiple levels. This study addressed the effect of glutathione (GSH) depletion on the transition of murine skeletal muscle C2C12 myoblasts into myocytes induced by growth factor inactivation. Cellular GSH levels increased within 24 hours on myogenic stimulation of myoblasts due to enhanced GSH synthetic rate accounted for by stimulated glutamate-L-cysteine ligase (also known as gamma-glutamylcysteine synthetase) activity. In contrast, the synthesis rate of GSH using gamma-glutamylcysteine and glutamate as precursors, which reflects the activity of the GSH synthetase, did not change during differentiation. The stimulation of GSH stores preceded the myogenic differentiation of C2C12 myoblasts monitored by expression of muscle-specific genes, creatine kinase (CK), myosin heavy chain (MyHC), and MyoD. The pattern of DNA binding activity of NF-kappaB and AP-1 in differentiating cells was similar both displaying an activation peak at 24 hours after myogenic stimulation. Depletion of cellular GSH levels 24 hours after stimulation of differentiation abrogated myogenesis as reflected by lower CK activity, MyHC levels, MyoD expression, and myotubes formation, effects that were reversible on GSH replenishment by GSH ethyl ester (GHSEE). Moreover, GSH depletion led to sustained activation of NF-kappaB, while GSHEE prevented it. Furthermore, inhibition of NF-kappaB activation restored myogenesis despite GSH depletion. Thus, GSH contributes to the formation of myotubes from satellite myoblasts by ensuring inactivation of NF-kappaB, and hence maintaining optimal GSH levels may be beneficial in restoring muscle mass in chronic inflammatory disorders.

Human fast skeletal myosin light chain 2 cDNA: isolation, tissue specific expression of the single copy gene, comparative sequence analysis of isoforms and evolutionary relationships

A cDNA clone encoding human fast skeletal myosin regulatory light chain (HSRLC) has been isolated and characterized from a fetal muscle cDNA library. The cDNA contains the coding sequence of 170 amino acids (aa) and 58 and 91 nucleotides in the 5' and 3' untranslated regions (UTRs), respectively. HSRLC is encoded by a single copy gene in the human genome and shows a tissue-specific pattern of expression in skeletal muscle. Comparison of derived amino acid sequence of HSRLC with database sequences reveals highly conserved 12 amino acid residues in a putative calcium-binding region. HSRLC is unique among all RLC sequences in having three consecutive potential phosphorylatable serine residues. The Cys-129 of HSRLC corresponds to the critical Gly-117 of scallop RLC that is essential for its regulatory function. The clusters of hydrophobic residues that are believed to stabilize the binding of NH2-terminal of RLC with myosin heavy chain show high sequence conservation in RLCs. Besides identifying specific targets for functional studies of HSRLC by mutagenesis, the results support the concept of an ancestral gene from which the RLC genes have evolved.

Okadaic acid augments utrophin in myogenic cells

Duchenne muscular dystrophy is a fatal childhood disease caused by mutations that abolish the expression of dystrophin in muscle. Utrophin is a paralogue of dystrophin and can functionally replace it in skeletal muscle. A potential therapeutic approach is to increase utrophin levels in muscle. One way to achieve this aim is to increase the expression of the utrophin gene at a transcriptional level via promoter activation. In this study, we have shown that utrophin A mRNA levels can be induced by okadaic acid in murine myogenic C2C12 cells. We have found that a utrophin A promoter reporter can be induced by Sp1 in C2C12 myoblasts, but not in myotubes. This activation can be enhanced by okadaic acid treatment. Our data suggest that this induction is due to Sp1 phosphorylation during myogenesis and thus, utrophin expression in muscle could be regulated by treatment with phosphatase inhibitors. Control of utrophin promoter activation could then be used to increase the expression of utrophin, and thus ameliorate the symptoms of Duchenne muscular dystrophy.

Inhibition of MAP/ERK kinase prevents IGF-I-induced hypertrophy in rat muscles

Insulin-like growth factor-I (IGF-I) has been shown to stimulate a hypertrophy response in skeletal muscles in vivo. In vitro studies have delineated two primary intracellular pathways that appear to mediate the effects of IGF-I in skeletal muscle: the Ras-ERK pathway and the phosphoinositide-3 kinase pathway. In vitro, the Ras pathway appears to regulate the mitogenic effects of IGF-I signaling, whereas the phosphoinositide-3 kinase pathway is associated with cellular differentiation. On the basis of the results from in vitro studies, we hypothesized that the coinfusion of both IGF-I and an inhibitor of the Ras pathway would result in some increase in muscle protein but an inhibition of cell proliferation. Our results show that 14 days of coinfusion of MAPK/ERK kinase inhibitor PD-098059 (PD) limited the phosphorylation of ERK and prevented IGF-I induced increases in protein (18%, P < 0.05 vs. 7%, not significant) or myofibrillar protein (23%, P < 0.01 vs. 5%, not significant). However, there were similar increases in indicators of cell proliferation (e.g., total DNA, 50 and 52%, P < 0.001) in both the IGF- and IGF+PD-infused muscles. The most notable impact on IGF-I signaling was a significant blunting of IGF-I induced increase in S6K1 phosphorylation by PD-98059 coinfusion ( approximately 5-fold, P < 0.001 vs. 3-fold, P < 0.01). These results suggest that there are interactions between the various pathways down stream of the IGF-I receptor that may behave differently in vivo than in myogenic cell lines in vitro.

Induction of terminal differentiation by the c-Jun dimerization protein JDP2 in C2 myoblasts and rhabdomyosarcoma cells

Muscle cell differentiation is a result of a complex interplay between transcription factors and cell signaling proteins. Proliferating myoblasts must exit from the cell cycle prior to their differentiation. The muscle regulatory factor and myocyte enhancer factor-2 protein families play a major role in promoting muscle cell differentiation. Conversely, members of the AP-1 family of transcription factors that promote cell proliferation antagonize muscle cell differentiation. Here we tested the role of the c-Jun dimerization protein JDP2 in muscle cell differentiation. Endogenous expression of JDP2 was induced in both C2C12 myoblast and rhabdomyosarcoma (RD) cells programmed to differentiate. Ectopic expression of JDP2 in C2C12 myoblast cells inhibited cell cycle progression and induced spontaneous muscle cell differentiation. Likewise, constitutive expression of JDP2 in RD cells reduced their tumorigenic characteristics and restored their ability to differentiate into myotubes. JDP2 potentiated and synergized with 12-O-tetradecanoylphorbol-13-acetate to induce muscle cell differentiation of RD cells. In addition, JDP2 induced p38 activity in both C2 and RD cells programmed to differentiate. This is the first demonstration of a single transcription factor that rescues the myogenic program in an otherwise non-differentiating cancer cell line. Our results indicate that the JDP2 protein plays a major role in promoting skeletal muscle differentiation via its involvement in cell cycle arrest and activation of the myogenic program.

Selective changes in DNA binding activity of transcription factors in UM-X7.1 cardiomyopathic hamsters

UM-X7.1 hamsters (CH) are considered a representative model for human cardiomyopathy. CH display the loss of the cytoskeletal delta-sarcoglycan protein, associated with myocardium remodeling and fatal reduction of heart functional efficiency. Even though altered redox balance and calcium homeostasis have already been reported to affect cardiomyocyte function, the molecular mechanisms underlying this pathology are largely unknown. We found no significant differences in DNA binding activity of redox-related (NF-kappaB, Sp1, AP-1 and AP-2) transcription factors in heart ventricles of 90 day-old CH, compared to normal animals. On the other hand, DNA binding activity of calcium-dependent transcription factors NF-AT3 and CREB were increased and decreased respectively in CH vs. normal ventricles. Western blot experiments confirmed the down regulation of CREB levels and suggest a novel regulation mechanism for this transcription factor in the heart. Our results are consistent with recent studies on NF-AT3, GATA4 and CREB transgenic mice, and provide clues for the comprehension of pathogenetic mechanisms of hamster hereditary cardiomyopathy.

Indications of associations of the porcine FOS proto-oncogene with skeletal muscle fibre traits

Skeletal muscle fibre characteristics are key determinants of meat quality. High fibre diameters and shifting towards higher white fibre proportions lead to increasing R-values (degree of desamination of adenosine) and lactate-production, resulting in high incidences of pale, soft, exudative (PSE) meat and stress susceptibility in European and American pig breeds. Development of muscle fibres including their enzymes, is regulated by the MyoD-gene family together with transcription factors like FOS. We report on the associations between the chromosomal region of FOS with skeletal muscle fibre and metabolism traits. The BB genotype representing the European Pietrain breed had 10.9% more white fibres with fibre diameters decreased by 6.1%, with 3.9% higher R-values and 8.5% higher lactate levels than the AA genotype of the Chinese Meishan. Lactate levels and R-values per microm of fibre diameter were increased to 18.4 and 11.6% in the BB genotype. The contrast between the two quantitative trait loci (QTL) alleles associated with a polymorphism in the FOS gene explained up to 5.13% of the total variance. A new TaiI-restriction fragment length polymorphism (RFLP) connected to a Asn258/Ser mutation, located in a transcription activator region, was used to map FOS between markers S0115 and Sw581 on SSC7. The QTLs for skeletal muscle fibre and metabolism traits have been mapped to the marker interval around FOS. The present data suggest that variability in FOS gene may underlie phenotypic variation in skeletal muscle fibre and metabolism traits in the pig.

Peroxisome proliferator-activated receptor gamma ligands differentially modulate muscle cell differentiation and MyoD gene expression via peroxisome proliferator-activated receptor gamma -dependent and -independent pathways

The effects of distinct classes of peroxisome proliferator-activated receptor gamma (PPARgamma) ligands on myogenesis and MyoD gene expression were examined in mouse skeletal muscle C2C12 myoblasts. Treatment of C2C12 cells with the PPARgamma ligand, 15-deoxy-Delta12,14-prostaglandin J2 (15d-PGJ2), repressed morphologically defined myogenesis and reduced endogenous mRNA levels of the myogenic differentiation markers MyoD, myogenin, and alpha-actin. In contrast, two synthetic PPARgamma ligands, L-805645 and ciglitazone, exhibited no effects. In transient transfection assays, 15d-PGJ2 specifically inhibited the expression of a MyoD promoter-luciferase reporter gene (MyoDLuc) in a cell type- and promoter-specific manner, indicating that 15d-PGJ2 functions in part by repressing MyoD gene transcription. The inhibition of MyoD gene expression by 15d-PGJ2 is mediated by the distal region of the MyoD gene promoter. PPARgamma on its own also inhibited MyoDLuc expression and further augmented the 15d-PGJ2 response. In contrast, L-805645 and ciglitazone did not inhibit MyoDLuc expression on their own but did so in the presence of ectopically expressed PPARgamma. Interestingly, a transdominant inhibitor of PPARgamma (hPPARgamma2Delta500) had no effect on the 15d-PGJ2-dependent repression of MyoDLuc expression but overcame L-805645/PPARgamma-dependent repression. Finally, saturating concentrations of L-805645, which did not affect myogenesis, failed to ablate 15d-PGJ2-mediated repression of the myogenic program. Thus, distinct PPARgamma ligands may repress MyoD gene expression through PPARgamma-dependent and -independent pathways, and 15d-PGJ2 can inhibit the myogenic program independent of its cognate receptor, PPARgamma.

Nuclear factor kappa B-inducing kinase and Ikappa B kinase-alpha signal skeletal muscle cell differentiation

Nuclear factor kappaB (NF-kappaB)-inducing kinase (NIK), IkappaB kinase (IKK)-alpha and -beta, and IkappaBalpha are common elements that signal NF-kappaB activation in response to diverse stimuli. In this study, we analyzed the role of this pathway during insulin-like growth factor II (IGF-II)-induced myoblast differentiation. L6E9 myoblasts differentiated with IGF-II showed an induction of NF-kappaB DNA-binding activity that correlated in time with the activation of IKKalpha, IKKbeta, and NIK. Moreover, the activation of IKKalpha, IKKbeta, and NIK by IGF-II was dependent on phosphatidylinositol 3-kinase, a key regulator of myogenesis. Adenoviral transduction with the IkappaBalpha(S32A/S36A) mutant severely impaired both IGF-II-dependent NF-kappaB activation and myoblast differentiation, indicating that phosphorylation of IkappaBalpha at Ser-32 and Ser-36 is an essential myogenic step. Adenoviral transfer of wild-type or kinase-deficient forms of IKKalpha or IKKbeta revealed that IKKalpha is required for IGF-II-dependent myoblast differentiation, whereas IKKbeta is not essential for this process. Finally, overexpression of kinase-proficient wild-type NIK showed that the activation of NIK is sufficient to generate signals that trigger myogenin expression and multinucleated myotube formation in the absence of IGF-II.

Inflammatory cytokines inhibit myogenic differentiation through activation of nuclear factor-kappaB

Muscle wasting is often associated with chronic inflammation. Because tumor necrosis factor alpha (TNF-alpha) has been implicated as a major mediator of cachexia, its effects on C2C12 myocytes were examined. TNF-alpha activated nuclear factor-kappaB (NF-kappaB) and interfered with the expression of muscle proteins in differentiating myoblasts. Introduction of a mutant form of inhibitory protein kappaBalpha (IkappaBalpha) restored myogenic differentiation in myoblasts treated with TNF-alpha or interleukin 1beta. Conversely, activation of NF-kappaB by overexpression of IkappaB kinase was sufficient to block myogenesis, illustrating the causal link between NF-kappaB activation and inhibition of myogenic differentiation. The inhibitory effects of TNF-alpha on myogenic differentiation were reversible, indicating that the effects of the cytokine were not due to nonspecific toxicity. Treatment of differentiated myotubes with TNF-alpha did not result in a striking loss of muscle-specific proteins, which shows that myogenesis was selectively affected in the myoblast stage by TNF-alpha. An important finding was that NF-kappaB was activated to the same extent in differentiating and differentiated cells, illustrating that once myocytes have differentiated they become refractory to the effects of NF-kappaB activation. These results demonstrate that inflammatory cytokines may contribute to muscle wasting through the inhibition of myogenic differentiation via a NF-kappaB-dependent pathway.

The transcription factors NF-kappab and AP-1 are differentially regulated in skeletal muscle during sepsis

Sepsis is associated with increased muscle proteolysis and upregulated transcription of several genes in the ubiquitin-proteasome proteolytic pathway. Glucocorticoids are the most important mediator of sepsis-induced muscle cachexia. Here, we examined the influence of sepsis in rats on the transcription factors NF-kappaB and AP-1 in skeletal muscle and the potential role of glucocorticoids in the regulation of these transcription factors. Sepsis was induced by cecal ligation and puncture (CLP). Control rats were sham-operated. NF-kappaB and AP-1 DNA binding activity was determined by electrophoretic mobility shift assay (EMSA) in extensor digitorum longus muscles at different time points up to 16 h after sham-operation or CLP. Sepsis resulted in an early (4 h) upregulation of NF-kappaB activity followed by inhibited NF-kappaB activity at 16 h. AP-1 binding activity was increased at all time points studied during the septic course. When rats were treated with the glucocorticoid receptor antagonist RU38486, NF-kappaB activity increased, whereas AP-1 activity was not influenced by RU38486. The results suggest that NF-kappaB and AP-1 are differentially regulated in skeletal muscle during sepsis and that glucocorticoids may regulate some but not all transcription factors in septic muscle.

X-linked vacuolated myopathy : TNF-alpha and IFN-gamma expression in muscle fibers with MHC class I on sarcolemma

The presence and the distribution of tumor necrosis factor-alpha, interferon-gamma, and p65 subunit of nuclear factor-kappaB, molecules known to induce synergistically and to mediate major histocompatibility complex (MHC) class I expression, were determined in muscle sections from control and X-linked vacuolated myopathy patients. MHC class I colocalized with tumor necrosis factor-alpha and interferon-gamma, as well as with p65, in most of the membrane attack complex- and/or calcium-positive muscle fibers in X-linked vacuolated myopathy. These results suggest that the expression of MHC class I in X-linked vacuolated myopathy could be induced by tumor necrosis factor-alpha and interferon-gamma and partly mediated by nuclear factor-kappaB.

Regulation of muscle regulatory factors by DNA-binding, interacting proteins, and post-transcriptional modifications

Skeletal muscle differentiation is influenced by multiple pathways, which regulate the activity of myogenic regulatory factors (MRFs)-the myogenic basic helix-loop-helix proteins and the MEF2-family members-in positive or negative ways. Here we will review and discuss the network of signals that regulate MRF function during myocyte proliferation, differentiation, and post-mitotic growth. Elucidating the mechanisms governing muscle-specific transcription will provide important insight in better understanding the embryonic development of muscle at the molecular level and will have important implications in setting out strategies aimed at muscle regeneration. Since the activity of MRFs are compromised in tumors of myogenic derivation-the rhabdomyosarcomas-the studies summarized in this review can provide a useful tool to uncover the molecular basis underlying the formation of these tumors.

Ceramide initiates NFkappaB-mediated caspase activation in neuronal apoptosis

The objective of the present study was to evaluate the role of ceramide in mediating apoptosis of dorsal root ganglion neurons induced by either nerve growth factor withdrawal or treatment with the chemotherapeutic agents suramin and cisplatin. Measurement of ceramide accumulation by mass spectrometry and the diacylglycerol kinase assay revealed elevation of intracellular ceramide only in suramin treated cultures. Ceramide-mediated neuronal cell death was inhibited by the caspase inhibitor zVAD.fmk. In these experimental models, ceramide accumulation mediated activation and nuclear translocation of the transcription factor NFkappaB and cyclin D1 protein expression. Specific inhibition of NFkappaB using a molecular decoy strategy resulted in increased cell viability accompanied by diminished caspase activity and cyclin D1 expression. Inhibition of NFkappaB did not alter intracellular ceramide levels. Our study suggests that ceramide generation occurs upstream of NFkappaB activation, cell cycle reentry, and caspase activation in the neuronal death pathway.

Down-regulation of AP1 activities after polarization of vas deferens epithelial cells correlates with androgen-induced gene expression

Vas deferens epithelial cell subcultures were used to study the sequential regulation of jun/fos proto-oncogene expression and AP1 activities during cell proliferation, polarization and androgen-induced expression of a terminal differentiation marker, i. e. the mvdp gene. Proliferation of epithelial cells is associated with a high expression in the nucleus of most Jun and Fos oncoproteins. After cell seeding on an extracellular matrix which allows polarization and expression of the mvdp gene in response to androgens, AP1 protein accumulation is greatly altered and consists in a loss of JunB, Fra1, FosB and a decrease in c-Fos, c-Jun and Fra2, while JunD remained at the same level. This was correlated with a drop in AP1 binding activity as evaluated by gel shift assay using either AP1 consensus sequence or AP1 binding sites of the mvdp gene promoter region, and in AP1 transactivating activity, as estimated by stable transfection experiments using an AP1 responsive promoter (TRE-TK-luc). Androgens did not significantly influence AP1 activities. On the contrary, stimulation of AP1 proteins by the tumor-promoting phorbol ester caused a decrease in androgen-induced mvdp mRNA accumulation, and this effect was reversed by staurosporine, a potent inhibitor of PKC. Our data suggest that a down-regulation of AP1 activities induced by epithelial cell differentiation is a prerequisite to androgen-induced mvdp gene expression. The high AP1 activities observed during proliferative state or induced in TPA-treated polarized cells, exert a repressive effect on androgen action.

Regulation of antioxidant enzyme gene expression in response to oxidative stress and during differentiation of mouse skeletal muscle

Various properties of skeletal muscle, including high metabolic activity and high levels of heme-containing proteins, render it particularly susceptible to free radical injury. Indeed, cellular injury from reactive oxygen species (ROS) has been implicated in many muscle disorders. Thus muscle cell survival is critically dependent on the ability of the cell to respond to periods of oxidative stress. To investigate this important homeostatic response, we studied the effect of oxidative challenges on the expression of genes encoding the antioxidant enzymes Cu,Zn-superoxide dismutase (CuZnSOD), Mn-superoxide dismutase (MnSOD), glutathione peroxidase (GPx), and catalase (CAT) in myotube cultures. Using Northern blot analysis, we found that treatment with the pro-oxidant paraquat resulted in time- and dose-dependent increases of transcript levels that were greatest for GPx and CAT (approximately 4-5 fold). CuZnSOD and MnSOD transcripts were also increased, albeit more modestly (approximately 2-3 fold). Transcript levels were also induced by treatment of the cells with two other pro-oxidants, menadione and H2O2, and correlated with the level of oxidative injury to the cells, measured as protein carbonyl group formation. Activities of all of the enzymes increased in response to the oxidative challenges, although the magnitudes of the increases were less robust than the increases of the respective transcript levels. In studying the effect of cellular differentiation on antioxidant gene expression and susceptibility to oxidative stress, we found that pro-oxidant treatment resulted in greater oxidative injury to differentiated myotubes than to undifferentiated myoblasts. Furthermore, the increased susceptibility of myotubes correlated with decreased antioxidant defenses-as muscle cells differentiated, both transcript and activity levels of antioxidant enzymes decreased. These data suggest that muscle cells regulate antioxidant defenses in response to oxidative stress and cellular differentiation.

Systemic administration of the NF-kappaB inhibitor curcumin stimulates muscle regeneration after traumatic injury

Skeletal muscle is often the site of tissue injury due to trauma, disease, developmental defects or surgery. Yet, to date, no effective treatment is available to stimulate the repair of skeletal muscle. We show that the kinetics and extent of muscle regeneration in vivo after trauma are greatly enhanced following systemic administration of curcumin, a pharmacological inhibitor of the transcription factor NF-kappaB. Biochemical and histological analyses indicate an effect of curcumin after only 4 days of daily intraperitoneal injection compared with controls that require >2 wk to restore normal tissue architecture. Curcumin can act directly on cultured muscle precursor cells to stimulate both cell proliferation and differentiation under appropriate conditions. Other pharmacological and genetic inhibitors of NF-kappaB also stimulate muscle differentiation in vitro. Inhibition of NF-kappaB-mediated transcription was confirmed using reporter gene assays. We conclude that NF-kappaB exerts a role in regulating myogenesis and that modulation of NF-kappaB activity within muscle tissue is beneficial for muscle repair. The striking effects of curcumin on myogenesis suggest therapeutic applications for treating muscle injuries.

Insulin-like growth factor-II, phosphatidylinositol 3-kinase, nuclear factor-kappaB and inducible nitric-oxide synthase define a common myogenic signaling pathway

Insulin-like growth factors (IGFs) are potent inducers of skeletal muscle differentiation and phosphatidylinositol (PI) 3-kinase activity is essential for this process. Here we show that IGF-II induces nuclear factor-kappaB (NF-kappaB) and nitric-oxide synthase (NOS) activities downstream from PI 3-kinase and that these events are critical for myogenesis. Differentiation of rat L6E9 myoblasts with IGF-II transiently induced NF-kappaB DNA binding activity, inducible nitric-oxide synthase (iNOS) expression, and nitric oxide (NO) production. IGF-II-induced iNOS expression and NO production were blocked by NF-kappaB inhibition. Both NF-kappaB and NOS activities were essential for IGF-II-induced terminal differentiation (myotube formation and expression of skeletal muscle proteins: myosin heavy chain, GLUT 4, and caveolin 3), which was totally blocked by NF-kappaB or NOS inhibitors in rat and human myoblasts. Moreover, the NOS substrate L-Arg induced myogenesis in the absence of IGFs in both rat and human myoblasts, and this effect was blocked by NOS inhibition. Regarding the mechanisms involved in IGF-II activation of NF-kappaB, PI 3-kinase inhibition prevented NF-kappaB activation, iNOS expression, and NO production. Moreover, IGF-II induced, through a PI 3-kinase-dependent pathway, a decrease in IkappaB-alpha protein content that correlated with a decrease in the amount of IkappaB-alpha associated with p65 NF-kappaB.

BTG1: a triiodothyronine target involved in the myogenic influence of the hormone

The product of the B-cell translocation gene 1 (BTG1), a member of an antiproliferative protein family including Tis-21/PC3 and Tob, is thought to play an important role in the regulation of cell cycle progression. We have shown in a previous work that triiodothyronine (T3) stimulates quail myoblast differentiation, partly through a cAMP-dependent mechanism involved in the stimulation of cell cycle withdrawal. Furthermore, we found that T3 or 8-Br-cAMP increases BTG1 nuclear accumulation in confluent myoblast cultures. In this study, we report that BTG1 is essentially expressed at cell confluence and in differentiated myotubes. Whereas neither T3 nor cAMP exerted a direct transcriptional control upon BTG1 expression, we found that AP-1 activity, a crucial target involved in the triiodothyronine myogenic influence, repressed BTG1 expression, thus probably explaining the low BTG1 expression level in proliferating myoblasts. In transient transfection studies, we demonstrated that an AP-1-like sequence located in the BTG1 promoter was involved in this negative regulation. Our present data also bring evidence that the stimulation of BTG1 nuclear accumulation by T3 or 8-Br-cAMP probably results from an increased nuclear import or retention in the nucleus. Lastly, BTG1 overexpression in quail myoblasts mimicked the T3 or 8-Br-cAMP myogenic influence: (i) inhibition of myoblast proliferation due to an increased rate of myoblast withdrawal from the cell cycle; and (ii) stimulation of terminal differentiation. These data suggest that BTG1 is probably involved in T3 and cAMP myogenic influences. In conclusion, BTG1 is a T3 target involved in the regulation of myoblast differentiation.