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(The FASEB Journal. 2004;18:227-237.)
© 2004 FASEB

Tumor necrosis factor-alpha inhibits myogenic differentiation through MyoD protein destabilization

RAMON C. J. LANGEN*,1, JOS L. J. VAN DER VELDEN*, ANNEMIE M. W. J. SCHOLS*, MARCO C. J. M. KELDERS*, EMIEL F. M. WOUTERS* and YVONNE M. W. JANSSEN-HEININGER{dagger}

* Department of Respiratory Medicine, Maastricht University, Maastricht, The Netherlands; and
{dagger} Department of Pathology, University of Vermont, Burlington, Vermont, USA

1Correspondence: Department of Respiratory Medicine, PO Box 5800, 6202 AZ Maastricht, The Netherlands. E-mail: r.langen{at}pul.unimaas.nl


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Tumor necrosis factor {alpha} (TNF{alpha}) has been implicated as a mediator of muscle wasting through nuclear factor kappa B (NF-{kappa}B) -dependent inhibition of myogenic differentiation. The aim of the present study was to identify the regulatory molecule(s) of myogenesis targeted by TNF{alpha}/NF-{kappa}B signaling. TNF{alpha} interfered with cell cycle exit and repressed the accumulation of transcripts encoding muscle-specific genes in differentiating C2C12 myoblasts. Overexpression of a p65 (RelA) mutant lacking the transcriptional activation domain attenuated the TNF{alpha}-mediated inhibition of muscle-specific gene transcription. The ability of muscle regulatory factor MyoD to induce muscle-specific transcription in 10T1/2 fibroblasts was also disrupted by wild-type p65, demonstrating that NF-{kappa}B transcriptional activity interferes with the function of MyoD. Inhibition of muscle-specific gene expression by TNF{alpha} was restored by overexpression of MyoD, whereas endogenous MyoD protein abundance and stability were reduced by TNF{alpha} through increased proteolysis of MyoD by the ubiquitin proteasome pathway. Last, the inhibitory effects of TNF{alpha} on myogenic differentiation were demonstrated in a mouse model of skeletal muscle regeneration, in which TNF{alpha} caused a delay in myoblast cell cycle exit. These results implicate that TNF{alpha} inhibits myogenic differentiation through destabilizing MyoD protein in a NF-{kappa}B-dependent manner, which interferes with skeletal muscle regeneration and may contribute to muscle wasting.—Langen, R. C. J., van der Velden, J. L. J., Schols, A. M. W. J., Kelders, M. C. J. M., Wouters, E. F. M., Janssen-Heininger, Y. M. W. Tumor necrosis factor-alpha inhibits myogenic differentiation through MyoD protein destabilization.


Key Words: MyoD • nuclear factor kappa B • protein stability • muscle regeneration


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
MUSCLE WASTING, defined as the loss of skeletal muscle tissue, commonly occurs in chronic wasting syndromes such as acquired immune deficiency syndrome (AIDS) (1) , cancer (2) , chronic heart failure (3) , and chronic obstructive pulmonary disease (COPD) (4) and is an independent predictor of mortality. Remarkably, in many of these chronic conditions muscle wasting is associated with elevations in circulating inflammatory cytokines, particularly tumor necrosis factor {alpha} (TNF{alpha}) (5 6 7 8) . Chronic administration of TNF{alpha} or interleukin-1 (IL-1) induced weight loss and skeletal muscle wasting in rats (9) , further illustrating the important role these cytokines may play in muscle loss. Generally, muscle wasting is believed to result from disturbances in the energy or protein anabolism-catabolism balance. For example, imbalances in myofibrillar protein synthesis and proteolysis have been demonstrated in experimental models of cancer cachexia (10 , 11) or sepsis (12) .

Alternatively, imbalances in processes that govern the maintenance of skeletal muscle and muscle plasticity, such as skeletal muscle fiber degeneration, apoptosis, and regeneration, may be an important determinant of muscle wasting associated with chronic disease. Muscle regeneration occurs constantly during normal muscle use (13) but is increased in response to muscle damage, increased muscle load, or resumed muscle use after inactivity (14) . The recruitment of a reservoir of muscle precursor cells named satellite cells is crucial in this process. Upon activation, satellite cells are stimulated to proliferate (and are called myoblasts at this stage), followed by exit from the cell cycle to engage in the myogenic differentiation program (14) . Impaired satellite cell function as a result of interference with their ability to proliferate, differentiate, or fuse could ultimately result in loss of skeletal muscle tissue.

Myogenic differentiation involves cell cycle exit, the expression of muscle-specific genes, and myotube formation. Our group (15 , 16) and others (17 18 19 20 21) have demonstrated that TNF{alpha} interferes with all these aspects of myogenic differentiation. Inhibition of myotube formation by TNF{alpha} involves the induction of oxidative stress (15) , whereas TNF{alpha}-induced activation of NF-{kappa}B is responsible for impaired muscle-specific gene expression in differentiating myoblasts (16 , 20 , 21) . This differentiation deficient phenotype was accompanied by sustained proliferation in response to TNF{alpha}.

MyoD is the chief regulatory molecule of myogenic differentiation and belongs (together with Myf5, myogenin and MRF4) to the muscle regulatory factors (MRFs), a family of basic helix-loop-helix transcription factors. MyoD plays an important role in cell cycle exit of differentiating myoblasts (22 , 23) , muscle-specific gene expression (24 25 26) , and myotube formation (27) . Therefore, the aim of this study was to explore whether the inhibition of myogenesis observed in response to TNF{alpha} was mediated through impairment of MyoD function and to determine the role of NF-{kappa}B activation. As myogenic differentiation is essential to skeletal muscle regeneration, we evaluated the effect of TNF{alpha} on myoblast cell cycle exit in regenerating hind limb muscle.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Cell culture
The murine skeletal muscle cell line C2C12 and 10T1/2 fibroblasts were obtained from the American Type Culture Collection (ATCC #CRL1772 and #CCL-226, respectively, Manassas, VA, USA), and cultured in growth media (GM) composed of low glucose Dulbecco’s modified Eagle medium (DMEM) containing antibiotics (50U/mL penicillin and 50 µg/mL streptomycin) and 9% (v/v) fetal bovine serum (FBS) (all from Invitrogen Life Technologies, Rockville, MD, USA), or differentiation media (DM), which contained DMEM with 0.5% heat-inactivated FBS and antibiotics (DM). Both cell types were grown on Matrigel (Becton Dickinson Labware, Bedford, MA, USA) -coated (1:50 in DMEM) dishes as described previously (16) . Cells were plated at 104/cm2 and cultured in GM for 24 h before transfection or induction of differentiation. When applicable, murine TNF{alpha} (Calbiochem, La Jolla, CA, USA) was added once to the culture dishes directly after induction of differentiation. For determination of MyoD protein half-life, the protein synthesis inhibitor cycloheximide (CHX) (Calbiochem) was dissolved in HBSS and used at a final concentration of 50 µM. In separate experiments, proteasome inhibitors MG-132 (Calbiochem) or clasto-lactacystin ß-lactone (Sigma) diluted in DMSO were added to cell cultures at a final concentration of 100 and 2.5 µM, respectively.

RNase protection assay
RNA probe design
Mouse muscle or C2C12 RNA was used to amplify various fragments by RT-PCR, using the primers summarized in Table 1 . When possible, probes were designed corresponding to splice boundaries to prevent hybridization with contaminating genomic DNA. To facilitate subcloning into the pBluescript II SK (+/-) phagemid (Stratagene, La Jolla, CA, USA) in the correct orientation, forward primers were extended with a 3' EcoRI restriction site, whereas reverse primers contained a 5' HindIII and BglII site. The latter allowed us to screen for the presence of the insert by a single restriction enzyme digestion of plasmid isolated from colonies of transformed XL2-Blue MRF’ ultra-competent bacteria (Stratagene). Amplified plasmids were isolated using Endofree Plasmid Maxi Kit (Qiagen, Valencia, CA, USA) and sequence analysis was performed to verify the sequence of the inserted fragment.


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  		Table 1. Primer sequences, probe characteristics, and Genbank accession numbers

RNA isolation
C2C12 cells were washed twice with PBS and lysed using 4 mL solution-D (6.4 M guanidine thiocyanate, 40 mM Na-citrate, 0.8% sarcosyl, 100 mM ß-mercaptoethanol) per 100 mm dish. Alternatively, for soleus muscle RNA isolation, tissue was homogenized (Polytron, Kinematica, Switzerland) in 0.75 mL solution-D. RNA was extracted by adding 0.1 vol. 2 M sodium acetate pH 5.0, 1 vol. of water saturated phenol, and 0.2 vol. of chloroform:isoamyl alcohol (50:1). After 15 min incubation and centrifugation, RNA was precipitated with an equal volume of isopropanol. The pellet was resuspended in 0.5 mL solution D and precipitated again with isopropanol. After a wash step in 70% ethanol, the RNA was dissolved in nuclease-free water and stored at -80°C in small aliquots.

Probe synthesis and RNase protection assay
Panels containing multiple probes were prepared by subjecting EcoRI-linearized constructs mixed in equal concentrations (the GAPDH probe was prepared separately) to a transcription reaction using T7 RNA polymerase in the presence of excess unlabeled nucleotides and 50µCi [{alpha}-32P]UTP (800Ci/mmol), according to the manufacturer’s MaxiScript kit instructions (Ambion, Houston, TX, USA). The protection reaction was performed after 18 h hybridization at 42°C of a mix containing 10 µg RNA per sample, 8 x 104 CPM per probe with the exception for glyceraldehyde-3-phosphate dehydrogenase (GAPDH, at 3.2 x 105 CPM), after 3' of denaturation at 95°C according to the manufacturer’s RNase protection assay (RPA) III kit instructions (Ambion, Houston, TX, USA). The amount of probe used was determined to be saturating in optimization experiments performed for each individual mRNA species. The next day, unprotected fragments were removed by RNase digestion (1:100 diluted RNase A/RNase T1 cocktail) followed by centrifugation. Precipitated, protected fragments were dissolved in 5 µL loading buffer, denatured at 95°C, and separated on a 5% denaturing polyacrylamide gel. Gels were dried and exposed to film (Biomax MR-1, Kodak) or an imaging screen for quantification in a Personal Molecular Imager FX (Bio-Rad, Hercules, CA, USA); band intensity was analyzed using Quantity One software (Bio-Rad).

Transfections and plasmids
Transient transfections were performed using Lipofectamine 2000TM (Invitrogen, Carlsbad, CA, USA) according to manufacturer’s instructions. Cells were incubated with the Lipofectamine–DNA mix for 3 h (total amount of DNA 1.5 µg for C2C12 cells and 2.0 µg for 10T1/2 cells), followed by overnight (typically 16 h) recovery in GM before induction of differentiation. Troponin I (TnI) -luciferase plasmid, kindly provided by Dr. Albert Baldwin (University of North Carolina, Chapel Hill, NC, USA) was used as a reporter for the activity of muscle-specific transcription factors (0.25 µg per transfection). pSV-ß-gal (0.25 µg per transfection), was used to correct for differences in transfection efficiency (Promega, Madison, WI, USA). pEMSV-MyoD was a kind gift from Dr. Barbara Winter (University of Braunschweig, Germany). Plasmids encoding IKK-ß, IKK{alpha}, or p65, kindly provided by Dr. Michael Karin (University of California, San Diego, La Jolla, CA, USA), were used to activate NF-{kappa}B. Alternatively, NF-{kappa}B activation was inhibited by cotransfection of a plasmid encoding I{kappa}B{alpha}-SR, which was constitutively expressed under control of the SFFV-LTR (pSFFV-NEO I{kappa}B{alpha}-SR), kindly provided by Dr. Rosa Ten (Mayo Clinic, Rochester, MN, USA), or p65-{Delta}TD, a gift from Dr. Sankar Ghosh (Yale University, New Haven, CT, USA). To determine luciferase and ß-galactosidase activity, cells were lysed in 1x luciferase lysis buffer and stored at -80°C. Luciferase (Promega, Madison, WI, USA) and ß-galactosidase (Tropix, Bedford, MA, USA) were measured according to manufacturer’s instructions.

Western blot analysis
MyoD and ß-actin protein abundance was evaluated by Western blot. Cells were washed in PBS and whole cell lysates were prepared by addition of lysis buffer (40 mM Tris, 300 mM NaCl, 2% (v/v) Nonidet P-40, 1 mM DTT, 1 mM Na3VO4, 1 mM PMSF, 10 µg/mL leupeptin, and 1% (v/v) aprotinin). Lysates were incubated on ice for 30 min, followed by 30 min centrifugation at 16000 x g. A fraction of the supernatant was saved for protein determination; 2x Laemmli sample buffer (2% (w/v) SDS, 10% (v/v) glycerol, 0.1M DTT and 0.01% (w/v) Bromophenol Blue) was added, followed by boiling of the samples for 5 min and storage at -20°C. Total protein was assessed by the Bradford method (28) ; 20 µg of protein was loaded per lane and separated on a 10% polyacrylamide gel (Mini Protean System, Bio-Rad, Hercules), followed by transfer to a nitrocellulose membrane (Schleicher and Schuell, Keene, NH, USA) by semidry electroblotting. The membrane was blocked overnight for nonspecific binding in 5% (w/v) nonfat, dried milk at 4°C. Nitrocellulose blots were washed in PBS-Tween20 (0.05%), followed by 1 h incubation with a polyclonal antibody specific for MyoD (Santa Cruz, Santa Cruz, CA, USA), or ß-actin (Sigma) to evaluate for equal loading after stripping. After three wash steps of 20 min each, the blots were probed with a peroxidase-conjugated secondary antibody and visualized by chemiluminescence according to manufacturer’s instructions (KPL, Gaithersburg, MD, USA). To determine MyoD half-life, MyoD band intensity was determined using the CCD camera of a Molecular Imager system (Bio-Rad, Hercules).

Immunofluorescence
Fibroblasts grown on glass coverslips and transfected with MyoD, GFP, and p65 WT or p65-{Delta}TD were stained after 24 h of culture in DM for myogenin using a polyclonal antibody (M225, Santa Cruz), and a Alexa-568 fluorophore conjugated anti-rabbit secondary antibody (Molecular Probes, Leiden, The Netherlands). Transfected (GFP positive) fibroblasts were visualized using a camera-equipped fluorescent microscope (Nikon Eclipse E800) and assessed for myogenin reactivity by scoring 10 separate fields at 100x magnification.

C2C12 myoblasts grown on glass coverslips were fixed after 24, 48, or 72 h of culture in DM in the presence or absence of TNF{alpha} and stained for MyoD using a monoclonal antibody (BD, Bedford, MA, USA) in combination with a Alexa-488 fluorophore-conjugated anti-mouse secondary antibody (Molecular Probes, Leiden, The Netherlands). Nuclei were counterstained with propidium iodide (20 µg/mL). Images were obtained at 200x using a confocal scanning laser (Bio-Rad MRC 1024 ES) coupled to fluorescent microscope (Olympus BX 50).

Animals
Male C57/bl6 mice were purchased from Jackson Laboratories (Bar Harbor, ME, USA) and housed in a temperature controlled room on a 12:12 h light-dark cycle with food pellets and water provided ad libitum. All procedures were performed with approval of the University of Vermont’s Institutional Animal Care and Use Committee. Four-month-old mice were randomly assigned to one of two groups (n=8 for each group): 1) hind limb suspension (HS)/reloading (RL) with intramuscular (IM) PBS injections or 2) HS/RL with IM TNF{alpha} injections. The HS/RL model has been described and evokes regeneration of postural muscles, which involves myoblast proliferation and differentiation (29) . HS was accomplished using a tail suspension device consisting of a plastic-coated iron wire taped around the mouse’s tail and connected to a swivel hook to allow circular motility. The latter was attached to a Teflon-coated PVC ring, which slid over a iron rod spanning the length of the cage to allow longitudinal motility. The tail harness was placed while mice were lightly anesthetized using halothane inhalation and mice were raised so as to prevent the hind limbs from touching the cage floor or sides. In this way, four HS mice could be housed in one standard cage. After 2 wk of HS, mice were anesthetized and released from the tail harness and allowed to resume normal cage activity. At this time and every subsequent 24 h for 5 days, mice of group 1 received IM injections of PBS (50 µL) delivered to the gastrocnemius/soleus area of both hind limbs with a 30 gauge needle (Ultra-Fine II, Becton Dickinson, NJ, USA) while lightly anesthetized. Mice of group 2 received murine TNF{alpha} (40 µg/kg in 50 µL PBS) (Calbiochem) injections instead of PBS, and showed no signs of fever or loss of appetite compared with animals of group 1. After 5 days of reloading, mice were killed by halothane overdose. Soleus and gastrocnemius muscles were collected using standardized dissection methods, cleaned of excess fat and connective tissue, blotted dry, weighed on an analytical balance, snap frozen in liquid nitrogen, and stored at -80°C for RNA extraction.

Statistical analysis
Raw data were entered into SPSS (version 8.0) for statistical analysis. Values for the ratio luciferase/ß-galactosidase activity, percent of myogenin-positive transfected fibroblasts, or H3.2 mRNA signal normalized for GAPDH were subjected to 1-way ANOVA; the treatment groups were compared post hoc by a Student-Newman-Keuls test (P<0.05). Where applicable, 2-way ANOVA was used.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
TNF{alpha} inhibits accumulation of muscle-specific gene transcripts and cell cycle exit in differentiating myoblasts
To assess whether the inhibitory effect of TNF{alpha} on myogenesis was apparent at the mRNA level, differentiating C2C12 myoblast cultures were analyzed for the accumulation of various muscle-specific mRNA species by RPA (Fig. 1 A). Increasing mRNA levels of myogenin, troponin I-slow and fast (TnI-sl, and TnI-f, respectively), muscle creatine kinase, and myoglobin could be observed as myogenesis progressed after the induction of differentiation, whereas mRNA levels of housekeeping genes such as histone 3.3 (H3.3), and GAPDH remained constant. However, up-regulation of muscle-specific mRNA levels was markedly attenuated in myocytes differentiated in the presence of TNF{alpha}. MyoD mRNA levels were clearly reduced, whereas Myf5 was increased after 48 h of culture in the presence of TNF{alpha}. Data normalized to the housekeeping gene H3.3 are shown in Fig. 1B . H3.3 was chosen because its abundance remains constant during myogenic differentiation (30) . Similar responses were obtained after normalization to GAPDH as housekeeping gene (not shown). These data illustrate that the inhibitory effects of TNF{alpha} on myogenic differentiation are apparent at the level of muscle-specific gene expression.



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  		Figure 1. TNF{alpha} inhibits accumulation of muscle-specific gene transcripts and cell cycle exit in differentiating C2C12 cells. Myoblasts were induced to differentiate in the presence or absence of TNF{alpha} (10 ng/mL) for 24 or 48 h. Total RNA was isolated and subjected to RPA using a panel containing the indicated anti-sense probes. Shown is a representative example of 3 independent experiments (A). Signal intensity of the various mRNA bands shown in panel A were determined with PhosphorImager analysis, normalized to the housekeeping gene H3.3, and expressed in relative units as the mean ± SE. Differences between DM and DM+TNF{alpha} were determined at 24 and 48 h, with *P < 0.05, **P < 0.01, {dagger}P < 0.001, ns = nonsignificant. N.D., not detected (B). Cyclin D1 protein abundance was determined in protein lysates (20 µg) of proliferating or differentiating (48 h DM) myoblasts cultured in presence or absence of TNF{alpha} (10 ng/mL). Shown are representative data of 3 independent experiments in which cyclin D1 signal intensity was determined, and expressed as % (mean±SE) of the signal measured in proliferating myoblasts. *P < 0.001 for both DM and DM+TNF{alpha} compared with GM; {dagger}P < 0.01 for DM+TNF{alpha} compared with DM (C) Statistically significant differences were determined by 1-way ANOVA.

As myogenic differentiation requires irreversible exit from the cell cycle, some proliferation markers were assessed. First, histone 3.2 (H3.2) mRNA abundance was evaluated. A threefold decrease in H3.2 mRNA levels could be observed 24 h after induction of differentiation in control cultures compared with proliferation supportive conditions, which was absent in the TNF{alpha}-treated cultures (Fig. 1A, B ). Evaluation of cyclin D1, a protein involved in progression from the G1- to S-phase of the cell cycle, also suggested impaired cell cycle exit in response to TNF{alpha}, as the reduction in cyclin D1 abundance observed with differentiation was incomplete in TNF{alpha}-treated myoblasts (Fig. 1C ). Last, the number of cells recovered from dishes incubated with TNF{alpha} in DM for 48 h was significantly higher than in control dishes after 48 h in DM (126.7±8.6*104/dish in control vs. 177.8±11.1*104/dish in TNF{alpha}; P<0.01), indicating that impaired cell cycle exit was associated with sustained cell proliferation.

Inhibition of myogenic differentiation by TNF{alpha} requires transcriptional activation of NF-{kappa}B
NF-{kappa}B activity is inversely related to myogenesis but required for myoblast proliferation (16 , 20) . To test the effects of NF-{kappa}B activation on differentiation, C2C12 cells were transiently transfected with a Troponin I promoter-reporter construct (TnI-luc) as a marker of myogenesis (31) . TnI promoter transactivation in differentiating C2C12 cells was markedly inhibited when NF-{kappa}B was activated by either TNF{alpha} or overexpression of IKKß, IKK{alpha}, or p65 (Fig. 2 A). In contrast to wild-type (WT) p65, expression of a p65 mutant lacking both transcriptional domains (p65-{Delta}TD) in C2C12 myocytes resulted in a dose-dependent increase in TnI promoter transactivation (Fig. 2B ). Inhibition of basal NF-{kappa}B activity by a mutant form of I{kappa}B{alpha} (I{kappa}B{alpha}-SR) stimulated TnI reporter activity (Fig. 2C ). Expression of p65-{Delta}TD in TNF{alpha}-treated cultures elicited a 10-fold increase of this myogenic reporter gene compared with TNF{alpha} alone, although it did not restore the inhibitory effect of TNF{alpha} on TnI promoter activity to control values as observed for I{kappa}B{alpha}-SR (Fig. 2C ). Collectively, these data demonstrate that basal or TNF{alpha}-induced p65 transcriptional activity is involved in the inhibition of myogenesis.



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  		Figure 2. Inhibition of myogenic differentiation by TNF{alpha} requires transcriptional activation of NF-{kappa}B. C2C12 myoblasts were transiently transfected with plasmids encoding troponin I-luciferase (0.25 µg), ß-galactosidase (0.25 µg), and the indicated NF-{kappa}B signaling components (in µg) or vector control, and cultured in DM for 48 h in presence or absence of TNF{alpha} (10 ng/mL). All treatments were found different from control: P < 0.001. *P < 0.05, and **P < 0.01 illustrate dose-dependent effects (A). Alternatively, C2C12 myoblasts were transiently transfected with Troponin I-luciferase and ß-galactosidase (0.25 µg each), together with WT or {Delta}TD-p65, or vector control, and differentiated for 48 h. *P < 0.05 compared with control (B). I{kappa}B{alpha}-SR or {Delta}TD-p65 (in µg) were coexpressed with TnI-luciferase and ß-gal (0.25 µg each) in C2C12 myoblasts, which were allowed to differentiate in presence or absence of TNF{alpha} for 48 h. All groups differed from control (P<0.05), with *P < 0.05 for TNF{alpha} + {Delta}TD-p65 compared with TNF{alpha} alone, and **P < 0.01 for TNF{alpha} + I{kappa}B{alpha}-SR compared with TNF{alpha} alone (C). TnI-luciferase and ß-gal activity were determined in cell lysates, and expressed as a ratio to normalize for transfection efficiency. Shown are representative data of 3 independent experiments (n=3±SE). Statistically significant differences were determined by 1-way ANOVA.

MyoD-dependent myogenesis is modulated by NF-{kappa}B activity
To elucidate the identity of the regulatory molecules affected by NF-{kappa}B activation, MyoD function was evaluated by measuring transactivation of the TnI promoter in 10T1/2 fibroblasts. Transcriptional activation of muscle-specific promoters occurs in these cells only if MyoD (or another MRF) is overexpressed (Fig. 3 A, compare first and second bar) (25) . Activation of the NF-{kappa}B pathway by coexpression of IKK{alpha}, IKKß, or p65 inhibited MyoD-dependent TnI promoter transactivation in a dose-dependent fashion (Fig. 3A ). The integrity of the transactivation domain of p65 was critical for inhibition of MyoD function, as coexpression of p65-{Delta}TD actually enhanced MyoD-dependent transactivation, in contrast to wild-type (WT) p65 (Fig. 3B ). Identical to the observations in C2C12 cells, expression of I{kappa}B{alpha}-SR facilitated MyoD-dependent myogenic transcriptional activation (Fig. 3B ). Cotransfection of WT p65 with MyoD in 10T1/2 cells strongly inhibited myogenin expression in transfected (GFP-marked) cells, whereas expression of p65-{Delta}TD did not significantly reduce the number of transfected cells that stained positive for myogenin compared with control (MyoD only) cultures (Fig. 3C ).



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  		Figure 3. MyoD-dependent myogenesis is modulated by NF-{kappa}B activity. MyoD (0.5 µg) was expressed in 10T1/2 fibroblasts together with plasmids encoding IKK{alpha}, IKKß, p65, or vector control, and TnI-luciferase and ß-gal (0.25 µg each). After incubation in DM for 48 h, cells were lysed to measure luciferase and ß-gal activity. All treatments were found different from control: P < 0.05, with the exception of 0.25 µg IKK{alpha}. *P < 0.05, and **P < 0.01 illustrate dose-dependent effects (A). Alternatively, plasmids encoding WT p65, or transcriptionally inactive p65 ({Delta}TD-p65), I{kappa}B{alpha}-SR, or vector control were cotransfected with MyoD, TnI-luc and ß-gal (0.25 µg each), followed by incubation in DM for 48 h, after which cells were harvested to measure luciferase and ß-gal activity. *P < 0.05, **P < 0.01, and ***P < 0.001 compared with control; {dagger}P < 0.02 illustrates a dose-dependent effect of {Delta}TD-p65 (B). Shown are representative data of 3 independent experiments (n=3±SE). Statistically significant differences were determined by 1-way ANOVA. Fibroblasts grown on coverslips were transfected with MyoD (0.5 µg) and GFP (0.25 µg) encoding plasmids, together with empty vector, WT p65, or p65-{Delta}TD (1.0 µg each). After 24 h in DM cells were fixed, and transfected cells (GFP-positive) were evaluated for myogenin expression. Transfected fibroblasts were scored for myogenin expression at 100x, 10 separate fields per sample (n=3), and the results expressed as the percentage myogenin positive nuclei of the total number of GFP-positive nuclei. Values were tested for significant differences relative to control (*P<0.001) using 1-way ANOVA (C).

TNF{alpha} decreases MyoD abundance in differentiating myoblasts
As our results implied that basal and TNF{alpha}-induced NF-{kappa}B transactivation interfered with MyoD function, MyoD protein expression levels were assessed during differentiation. MyoD abundance remained constant in control cultures after 24 h of culture in DM (Fig. 4 A), whereas myocytes differentiated in the presence of TNF{alpha} consistently demonstrated a striking reduction in MyoD abundance after 72 h (Fig. 4A ). Immunostaining confirmed the reduction of MyoD protein by TNF{alpha} and revealed that only a fraction of the TNF{alpha}-treated cells stained faintly for MyoD at 24 and 48 h, with all MyoD immunoreactivity lost after 72 h of culture in DM (Fig. 4B ).



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  		Figure 4. TNF{alpha} decreases MyoD abundance in differentiating myocytes. C2C12 cells were cultured in DM in the presence or absence of TNF{alpha}, and cell lysates were evaluated for MyoD or ß-actin protein levels. Representative data of 4 independent experiments are shown. MyoD and ß-actin band intensity was measured and expressed as a ratio (mean±SE), with *P < 0.01 between DM control and DM+TNF{alpha} at 48 h and 72 h, as determined by 1-way ANOVA (A). Confocal scanning laser microscopy was used to evaluate MyoD abundance and localization. Myocytes were grown on coverslips and subsequently differentiated in the presence (D–F) or absence (A–C) of TNF{alpha} for 24 h (A, D), 48 h (B, E), or 72 h (C, F). MyoD was visualized by immunohistochemistry (green) and the nuclei were stained using propidium iodide (red). Due to colocalization of the green and red signal, MyoD-positive nuclei show up yellow. A representative of 3 experiments is shown (B).

TNF{alpha} affects MyoD protein stability
The half-life of MyoD protein has only been calculated for ectopically expressed MyoD in proliferating fibroblasts and HeLa cells (32 , 33) . Therefore, we determined the stability of endogenous MyoD in C2C12 myocytes cultured in GM or differentiation conditions (DM), using the protein synthesis inhibitor CHX. MyoD half-life in proliferating myoblasts (in GM) was calculated to be 66 ± 15 min (not shown), and increased to 130 ± 13 min after 24 h in DM. In contrast, MyoD stabilization did not occur in cultures differentiated in the presence of TNF{alpha}: its half-life was only 86 ± 17 min (P<0.05). A representative experiment of MyoD half-life determination is shown in Fig. 5 A, with corresponding calculations in Fig. 5B . In agreement with previous reports (33 , 34) , inhibition of the ubiquitin–proteasome pathway stabilized MyoD in the absence of protein synthesis (Fig. 6 A). The reduction in MyoD protein stability resulting from TNF{alpha} was almost completely restored to control levels by inhibition of the 26S proteasome pathway by preincubation with 100 µM MG-132 (Fig. 6A ) or 2.5 µM clasto-lactacystin ß-lactone (Fig. 6B ). If insufficient MyoD protein levels due to increased degradation were the mechanism by which TNF{alpha} inhibited myogenic differentiation, increasing MyoD abundance could be expected to restore myogenesis. Indeed, overexpression of MyoD restored TnI promoter transactivation in differentiating C2C12 myoblasts in the presence of TNF{alpha} (Fig. 6C ). Collectively, these data indicate MyoD as the putative myogenic regulatory molecule targeted by TNF{alpha}, leading to impaired myogenic differentiation.



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  		Figure 5. TNF{alpha} decreases MyoD protein half-life in differentiating myoblasts. CHX (50 µM) was added to myocytes after 24 h of culture in DM in the presence or absence of TNF{alpha}. Cell lysates were prepared for SDS-PAGE and Western blot analysis of MyoD protein abundance, or ß-actin as loading control (A). Band intensity was measured and normalized to t=0 (100%). For each time point, the natural logarithm (ln) of the band intensity (in %) was plotted vs. time, and MyoD half-life (t1/2) was calculated as the time point corresponding to the ln of 50% (B). A representative calculation is shown of 3 independent experiments that culminated in an average half-live of 130' ± 13'in DM vs. 86' ± 17' in DM+TNF{alpha} (P<0.05).



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  		Figure 6. TNF{alpha} increases MyoD degradation through the ubiquitin proteasome pathway. C2C12 myoblasts were cultured in DM for 24 h in the presence or absence of TNF{alpha}, followed by incubation with CHX (50 µM) or vehicle, separate or together with MG-132 (100 µM), or clasto-lactacystin ß-lactone (2.5 µM) for 4 h. MyoD and ß-actin were determined as in Fig. 5B and band intensity was assessed and expressed as a ratio. Shown are representative blots of 3 (A), and 2 (B) independent experiments. Inhibition of myogenesis is prevented by overexpression of MyoD. C2C12 myoblasts were transfected with a MyoD (or empty) expression vector and cultured in DM in the presence or absence of TNF{alpha} (10 ng/mL). After 48 h TnI-luciferase and ß-gal activity were determined and expressed as a ratio. Shown are representative data of 3 independent experiments (n=3±SE). The influence of constitutive MyoD expression on TNF{alpha}-dependent inhibition of TnI-luc was evaluated by 2-way ANOVA, which showed a significant interaction between MyoD and TNF{alpha} (P<0.005) (C).

TNF{alpha} impairs myoblast cell cycle exit in regenerating skeletal muscle
To investigate the significance of TNF{alpha}-dependent inhibition of myogenic differentiation in intact skeletal muscle, a mouse model of muscle regeneration was adopted. In a separate experiment, 2 wk of hind limb suspension resulted in atrophy of the soleus and gastrocnemius muscles, evidenced by a reduction in muscle weight of ~40.3 ± 2.2%, and 23.4 ±1.6%, respectively (n=8 per group, P<0.001 by 1-way ANOVA). Reloading (RL) of the hind limb musculature caused muscle regeneration (complete after 14 days) as judged by muscle weight, which had returned to baseline values (not shown). Muscle regeneration in this model involves satellite cell activation, proliferation, and differentiation (29) , which prompted us to evaluate the inhibitory effect of TNF{alpha} on cell cycle exit observed in vitro. Compared with mice that had received IM injections with PBS during the 5 day RL-phase, H3.2 mRNA levels were markedly elevated in the gastrocnemius (Fig. 7 A, B) and soleus (not shown) of mice that had received IM injections with TNF{alpha} every 24 h during reloading. TNF{alpha} reduced myogenin mRNA levels in regenerating skeletal muscle when normalized to H3.3 (Fig. 7B ), although no difference could be detected after correction for GAPDH (not shown). The latter may be due to a reduction of GAPDH levels by TNF{alpha} consistently observed in vivo and in vitro (Fig. 1A ). These results confirm the inhibition of cell cycle exit observed myoblast cultures in response to TNF{alpha} and illustrate the inhibitory effects of TNF{alpha} on skeletal muscle regeneration in the mouse.



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  		Figure 7. TNF{alpha} inhibits myoblast cell cycle exit in regenerating skeletal muscle. RNA extracted from gastrocnemius muscle excised from PBS- or TNF{alpha}-injected HS/RL animals (n=8 for each group) was subjected to RPA analysis to determine mRNA abundance of MyoD and myogenin, the proliferation marker H3.2 and housekeeping genes H3.3 and GAPDH (A). MyoD, myogenin, and H3.2 mRNA signal intensity was normalized to H3.3 levels (B). n = 8 per group; *P < 0.001 or {dagger}P < 0.01 for PBS vs. TNF{alpha}, as determined by 1-way ANOVA.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
We and others have demonstrated inhibition of myogenic differentiation by inflammatory cytokines such as TNF{alpha} and IL-1 through activation of NF-{kappa}B (16 , 21) . We demonstrate in the present study that the inhibitory effects of TNF{alpha} on myogenesis involved impaired mRNA accumulation of transcripts encoding muscle-specific genes and coincided with a failure of myoblasts to exit the cell cycle. The latter was evidenced by the sustained expression of cyclin-D1 protein by TNF{alpha}, as already shown (20) . mRNA of the H3.2 variant, which is restricted to the S-phase of the cell cycle (35) and absent once myoblasts differentiate (30) , remained detectable in myoblasts differentiated in the presence of TNF{alpha} (Fig. 1A, B ). Inhibition of myogenesis by TNF{alpha} required NF-{kappa}B activation (16 , 21) and, as shown in this study, relied on impairment of MyoD function (Fig. 3) . The integrity of the transactivation domain (TD) of p65 proved critical for this response, as expression of a mutant protein lacking the TD (p65-{Delta}TD) enhanced myogenesis and reversed the inhibitory effects of TNF{alpha}, albeit incompletely. The recruitment of a residual pool of WT p65 in response to TNF{alpha}, which might not have been completely replaced with mutant p65 due to the transient nature of the transfection method, may have been responsible for this remaining inhibitory effect of TNF{alpha}. This may explain the differential effect of p65-{Delta}TD expression on MyoD-dependent TnI promoter transactivation (Fig. 3B ) and myogenin expression (Fig. 3C ), as the replacement of endogenous with mutant p65 may have been more complete at the time of analysis for the TnI reporter, which was performed 72 h post-transfection vs. 48 h for myogenin expression. Nonetheless, the myogenesis stimulatory effect of inhibition of basal NF-{kappa}B was also observed by overexpression of a mutant form of I{kappa}B{alpha} (I{kappa}B{alpha}-SR) (Fig. 2C ) and is in line with the finding that NF-{kappa}B activity declines as myogenic differentiation progresses (16 , 20) . Collectively, these findings suggest that a NF-{kappa}B regulated gene product may be responsible for impairment of MyoD function in response to TNF{alpha}.

MyoD is regarded as the key regulator of myogenic differentiation, as MyoD-deficient myoblasts are incapable of completing the differentiation program successfully (27) . An additional role for MyoD has been described in the regulation of cell cycle exit, including transcriptional activation of the cell cycle inhibitor p21 (22 , 23) . Therefore, functional inhibition of MyoD may result not only in loss of muscle-specific gene expression, it may also be responsible for the perturbation of cell cycle exit in response to TNF{alpha}. MyoD activity is regulated at multiple levels, including interaction with inhibitory molecules or chromatin remodeling proteins or the availability of dimerization partners (for review, see ref 36 ). However, ectopic expression of MyoD restored myogenesis in C2C12 cells in the presence of TNF{alpha}, suggesting that the defect in myogenesis is not the result of a limited availability of cofactors required for MyoD activity, but may be due to insufficient levels of functional MyoD protein itself. MyoD protein levels decreased after TNF{alpha} treatment, as described (18) , whereas in control cultures MyoD protein abundance increased within 24 h postinduction of differentiation. Moreover, the increase in MyoD protein abundance in control cultures observed during the first 24 h after transition from GM to DM was not accompanied by increases at the mRNA level. Apart from transcriptional regulation, MyoD protein abundance is, as for any other protein, also regulated by its stability. This study demonstrates that the half-life of endogenous MyoD protein in C2C12 myoblasts doubled when cultures were shifted from GM to DM, suggesting that MyoD abundance during differentiation is governed by protein stabilization rather than transcriptional regulation. Therefore, the reduced MyoD protein stability observed with TNF{alpha} is likely the primary contributor to the decrease in MyoD protein abundance. The reduction in MyoD transcripts after TNF{alpha} may be secondary to MyoD protein instability, as MyoD expression is controlled in part by an autoregulatory mechanism in which MyoD is involved in its own transcription (37) .

MyoD destabilization in response to TNF{alpha} is potentially regulated by a NF-{kappa}B-dependent gene product. As inhibition of the proteasome almost completely restored MyoD protein stability in the presence of TNF{alpha}, candidate genes responsible for increased proteolysis of MyoD include components of the ubiquitin proteasome pathway. TNF{alpha} has been found to induce mRNA transcripts encoding the poly-ubiquitin gene in skeletal muscle (38) . The expression of the ubiquitin-conjugating enzyme UbcH2 was induced in skeletal myotubes by TNF{alpha} in a NF-{kappa}B-dependent manner (39) . mRNA encoding the ubiquitin ligases (E3 proteins), muscle RING finger 1, and muscle atrophy F-box were strongly induced in skeletal muscle by IL-1 (40 , 41) . Proteasomal degradation of MyoD is dependent on E3 ubiquitin ligase activity (42 , 43) . These observations make it tempting to speculate that NF-{kappa}B-dependent induction of specific ubiquitin-conjugating enzymes or ligases by inflammatory cytokines may contribute to muscle wasting through the targeted proteolysis of myogenic regulators like MyoD, in addition to the more ubiquitous proteolysis of myofibrillar proteins.

The pivotal role of MyoD during in vivo myogenesis has been demonstrated in models of skeletal muscle regeneration (44) . Our data suggest that TNF{alpha} may interfere with muscle regeneration, as H3.2 mRNA after 5 days of reloading of disuse atrophied soleus and gastrocnemius muscle was still markedly elevated in animals receiving TNF{alpha} injections. This is reminiscent of continued satellite cell proliferation, as the proliferative response of satellite cells due to increased muscle activity returns to baseline after 4 days (45) . Although this corresponds with the in vitro data in which TNF{alpha} perturbed myoblast cell cycle exit, the possible contribution of cell types other than satellite cells to the increased H3.2 expression in the regenerating muscles cannot be excluded. However, in other models of muscle regeneration, proliferation markers were clearly associated with myoblasts (46 , 47) . Systemic administration of TNF{alpha} in adult mice was found to induce 5-bromo-2'-deoxyuridine incorporation specifically in satellite cells, indicative of the proliferation stimulating effect of TNF{alpha} on skeletal muscle (48) . Based on our in vitro data, one could expect the inhibitory effect of TNF{alpha} on myoblast cell cycle exit to coincide with impaired myogenic differentiation. Indeed, TNF{alpha} reduced myogenin mRNA expression in regenerating skeletal muscle, indicating that TNF{alpha} may interfere with muscle regeneration in vivo. There is precedent for the notion that TNF{alpha} signaling may result in impaired muscle regeneration, as TNF receptor interacting protein 2 (RIP-2) expression is decreased in regenerating muscle (49) . The reduction in RIP-2 expression may contribute to decreased NF-{kappa}B activity during muscle regeneration, which may be required as inhibition of NF-{kappa}B stimulated muscle regeneration after traumatic injury (50) .

In summary, our data suggests that TNF{alpha} reduces MyoD protein stability in a NF-{kappa}B-dependent manner, which results in sustained myoblast proliferation and inhibition myogenic differentiation. These findings may provide important novel insights into the mechanisms involved in inflammation-associated muscle wasting.


   ACKNOWLEDGMENTS
 
This work was sponsored by Numico Research BV, The Netherlands.

Received for publication May 9, 2003. Accepted for publication October 22, 2003.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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