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(The FASEB Journal. 2001;15:1169-1180.)
© 2001 FASEB

Inflammatory cytokines inhibit myogenic differentiation through activation of nuclear factor-{kappa}B

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

* Department of Pulmonology, Maastricht University, Maastricht, The Netherlands, and
{dagger} Department of Pathology, University of Vermont, Burlington, Vermont 05405, USA

1Correspondence: Department of Pathology, University of Vermont, Medical Alumni Building A143, Burlington VT 05405, USA. E-mail: yjanssen{at}zoo.uvm.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
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-{kappa}B (NF-{kappa}B) and interfered with the expression of muscle proteins in differentiating myoblasts. Introduction of a mutant form of inhibitory protein {kappa}B{alpha} (I{kappa}B{alpha}) restored myogenic differentiation in myoblasts treated with TNF-{alpha} or interleukin 1ß. Conversely, activation of NF-{kappa}B by overexpression of I{kappa}B kinase was sufficient to block myogenesis, illustrating the causal link between NF-{kappa}B 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-{kappa}B 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-{kappa}B activation. These results demonstrate that inflammatory cytokines may contribute to muscle wasting through the inhibition of myogenic differentiation via a NF-{kappa}B-dependent pathway.—Langen, R. C. J., Schols, A. M. W. J., Kelders, M. C. J. M., Wouters, E. F. M., Janssen-Heininger, Y. M. W. Inflammatory cytokines inhibit myogenic differentiation through activation of nuclear factor-{kappa}B.


Key Words: cachexia • muscle wasting • TNF-{alpha} • NF-{kappa}B • myogenic differentiation


   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 AIDS (1) , cancer (2) , chronic heart failure (3) , and chronic obstructive pulmonary disease (4 5 6) , and is an independent predictor of mortality. Remarkably, in many of these conditions muscle wasting is associated with chronic elevations in circulating inflammatory cytokines, in particular tumor necrosis factor {alpha} (TNF-{alpha}) (7 8 9 10) . Muscle wasting is generally believed to result from disturbances in the energy or anabolism-catabolism balance. For example, imbalances in myofibrillar protein synthesis and proteolysis have been demonstrated in experimental models of cancer cachexia (11 , 12) and sepsis (13) . Elevated levels of circulating inflammatory mediators, such as TNF-{alpha} and interleukin 1 (IL-1), were believed to trigger the events leading to muscle wasting in these models. In support of this, chronic administration of TNF-{alpha} or IL-1 induced weight loss and skeletal muscle wasting in rats (14) . Muscle wasting may alternatively result from a decrease of the number of fibers. In support of this proposal, an increased prevalence of apoptotic myonuclei has been associated with the loss of skeletal muscle tissue in muscular dystrophies and inflammatory myopathies (15 16 17) . In addition, satellite cells form a reservoir of myogenic stem cells, which are essential in the replacement of damaged or degenerated myofibers. If inflammatory mediators interfere with the function of satellite cells by decreasing or blocking their ability to fuse with or replace damaged myofibers, this action could ultimately result in the loss of skeletal muscle tissue.

Evidence for a direct role of inflammatory mediators in muscle wasting has been documented in in vitro studies. For example, administration of TNF-{alpha} to primary cultures of differentiating human myoblasts inhibited the expression of {alpha}-actin and myosin heavy chain (MyHC) (18) . Others, using the murine skeletal muscle cell line C2C12 and primary cultures of rat skeletal muscle, demonstrated the depletion of MyHC in differentiated myotubes in response to TNF-{alpha} (19) .

One well-studied pathway activated by TNF-{alpha} involves signaling to the transcription factor nuclear factor-{kappa}B (NF-{kappa}B). Under normal conditions, NF-{kappa}B is present within the cytoplasm in an inactive state, bound to its inhibitory protein {kappa}B{alpha} (I{kappa}B{alpha}). Stimulation with TNF-{alpha} initiates an intracellular signaling cascade, resulting in the phosphorylation of I{kappa}B{alpha} on serine residues 32 and 36 by I{kappa}B kinase (IKK). Phosphorylation and subsequent ubiquitination of these residues target I{kappa}B{alpha} for degradation by the 26S-proteasome complex. Once liberated from its inhibitory protein, NF-{kappa}B translocates to the nucleus, where it orchestrates the transcription of a number of cytokine and chemokine, growth regulatory, and survival genes (20 , 21) .

The goals of the present study were to evaluate the effects of the inflammatory cytokines TNF-{alpha} and IL-1ß on C2C12 myocytes and to explore the role of NF-{kappa}B activation therein. We compared the effects of TNF-{alpha} on myogenesis during and after the differentiation of myoblasts into myotubes, as studies have demonstrated that this cytokine can affect both undifferentiated myoblasts and differentiated myotubes (18 , 19 , 22) . In addition to the effects of TNF-{alpha}, the effects of IL-1ß on muscle-specific gene expression were investigated, because a role for this cytokine as a mediator of cachexia has been suggested (14 , 23) . Finally, we evaluated whether the effects of TNF-{alpha} and IL-1ß on myogenic differentiation were mediated through the activation of NF-{kappa}B.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Cell culture
The murine skeletal muscle cell line C2C12 was obtained from the American Type Culture Collection (ATCC No. CRL1772, Manassas, VA). These cells are able to undergo differentiation into spontaneously contracting myotubes on growth factor withdrawal (24) . Myoblasts were cultured in low-glucose Dulbecco’s modified Eagle’s medium (DMEM) containing antibiotics (50 U/ml penicillin and 50 µg/ml streptomycin) and 9% (v/v) FBS (all from Life Technologies, Rockville, MD), referred to as growth media (GM). For experiments, 60-mm dishes (Becton Dickinson Labware, Bedford, MA) were coated for 1 h at 37°C with diluted (1:50 in DMEM) Matrigel (Becton Dickinson Labware), which contains collagen IV, laminin, heparan sulfate proteoglycan, and entactin, which are constituents of the natural extracellular matrix of skeletal muscle fibers (25) . This plating protocol resulted in enhanced and more consistent differentiation, compared with protocols that use 2% horse serum containing media to induce differentiation (unpublished results). Cells were plated at 104/cm2 and cultured in GM for 24 h, allowing them to reach ~60% confluency. To induce differentiation, cells were washed in HBSS and further cultured in low-glucose DMEM containing antibiotics and 0.5% heat-inactivated FBS, referred to as differentiation media (DM). To study the effects on differentiation, murine TNF-{alpha} or IL-1ß (Calbiochem, La Jolla, CA) was added to the culture dishes directly after induction of differentiation. To study the susceptibility of differentiated myotubes to TNF-{alpha}, myoblasts were cultured in DM for 5 days and received fresh media every other day, prior to addition of the cytokine. Cells were examined with an inverted phase-contrast microscope (Olympus CH40, Melville, NY) equipped with a camera (Nikon N6000).

Assessment of myogenic differentiation
As a morphological parameter of differentiation, the myogenic index was defined as the fraction of nuclei residing in cells containing three or more nuclei after staining with May-Grünwald Giemsa (Sigma, St. Louis, MO). Myogenic differentiation was assessed biochemically via determination of muscle creatine kinase (CK) activity. Cells were washed twice in cold PBS, lysed in 0.5% Triton X-100, and scraped off the dish with a rubber policeman. Lysates were centrifuged for 2 min at 16,000g, and the supernatant was stored in separate aliquots at -80°C for determination of protein content or CK activity. CK activity was measured by using a spectrophotometric-based (26) kit from Sigma (Sigma Diagnostics, St. Louis, MO). Specific CK activity was calculated after correction for total protein, which was assessed by the Bradford method (27) . Expression of the fast-twitch isotype of MyHC (MyHCf) or myogenin was evaluated by Western blotting. Cells were washed in PBS, and whole-cell lysates were prepared by addition of lysis buffer, composed of 40 mM Tris, 300 mM NaCl, 2% (v/v) Nonidet P-40, 1 mM dithiothreitol (DTT), 1 mM Na3VO4, 1 mM phenylmethylsulfonyl fluoride, 10 µg/ml leupeptin, and 1% (v/v) aprotinin. Lysates were incubated on ice for 30 min, followed by 30-min centrifugation at 16,000g. A fraction of the supernatant was saved for protein determination, and 2x Laemmli sample buffer (2% [w/v] sodium dodecyl sulfate, 10% [v/v] glycerol, 0.1 M DTT, and 0.01% [w/v] bromophenol blue) was added. Samples were then boiled for 5 min and stored at -20°C. Approximately 10 µg of protein was loaded per lane and separated on a 7 or 10% polyacrylamide gel (Mini Protean System, Bio-Rad, Hercules, CA), followed by transfer to a nitrocellulose membrane (Schleicher & Schuell, Keene, NH) 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% v/v), followed by a 1-h incubation with monoclonal antibodies specific for MyHCf (MY-32, Sigma) or myogenin (M-225) (Santa Cruz, Santa Cruz, CA). After three wash steps of 20 min each, the blots were probed with a peroxidase-conjugated secondary antibody and were visualized by chemiluminescence according to the manufacturer’s instructions (KPL, Gaithersburg, MD).

Assessment of cell death
Cell death was determined by measuring lactate dehydrogenase (LDH) activity in the supernatant of the culture media, using a spectrophotometric assay according the manufacturer’s instructions (Promega, Madison, WI). Activity was expressed as a percentage of maximal acitivity, i.e., the LDH activity after total lysis of the cells of a control dish. Apoptosis was evaluated after pooling detached and adherent cells from each dish. Detached cells were collected from the culture media by centrifugation at 500g for 10 min and were resuspended in 200 µl of HBSS. Adherent cells were trypsinized, centrifuged (10 min, 500g, 4°C), and resuspended in 1 ml of HBSS. A sample of 200 µl was saved to determine adherent cell number, and the remaining 800 µl was pooled with the cells collected from the culture media. Cells were permeabilized in 0.1% v/v Triton X-100 (in PBS) and stained with PBS solution containing 50 µg/ml propidium iodide (Sigma). After a 30-min incubation on ice, cell cycle distribution was assessed to evaluate the sub-G0/G1 fraction, using flow cytometry (Coulter, EPICS Elite, Miami, FL). The total adherent cell number was determined by using a Coulter particle counter (Coulter, Miami) after dilution of a suitable volume of cells in 10 ml of Isoton and was expressed as the number of adherent cells per plate.

Electrophoretic mobility shift analysis (EMSA)
To determine DNA binding activity of NF-{kappa}B, complexes binding to an oligonucleotide containing a {kappa}B consensus sequence were analyzed (Santa Cruz). Nuclear extracts were prepared as described previously (28) . A sample of 2 µg of nuclear protein was used per binding reaction, and protein-DNA complexes were resolved on a 5% polyacrylamide gel in 0.25x Tris borate-EDTA buffer at 120 V for 2 h. Gels were dried and exposed to film (X-Omat Blue XB-1, Kodak, Rochester, NY). Shifted complexes were quantitated by phosphoimager analysis (Bio-Rad). To determine the subcomposition of the complexes, supershift reactions were performed by preincubation of the nuclear extracts with antibodies specific to the RelA or p50 subunits of NF-{kappa}B (Santa Cruz).

Transfections and plasmids
For the assessment of NF-{kappa}B transcriptional activity during differentiation, stable cell lines were created to contain either the 6{kappa}B-TK-luciferase (NF-{kappa}B reporter) plasmid or the empty TK-luciferase plasmid, both kindly provided by Dr. Patrick Baeuerle (Micromet, Germany). Cells, 8 x 105/400 µl in GM, were transfected by electroporation (Genepulser, Bio-Rad) at 240 V in the presence of 2.5 µg of the plasmid DNA of interest, and 0.7 µg of a plasmid containing the neomycin resistance gene (pSV2-Neo, Stratagene, La Jolla, CA). For selection of positive clones, cells were cultured in GM containing 850 µg/ml geneticin (Life Technologies). Transient transfections were performed by calcium phosphate precipitation according to the manufacturer’s instructions (Specialty Media, Phillipsburg, NJ). The total amount of plasmid DNA was kept constant at 25 µg. pBIISK (Stratagene) was used as carrier DNA, and pSV-ß-gal (1 µg/transfection) (Promega) was employed to correct for differences in transfection efficiency. The 6{kappa}B-TK-luciferase and the troponin I (TnI)-luciferase plasmid, the latter kindly provided by Dr. Albert Baldwin (University of North Carolina, Chapel Hill, NC), were used as reporters for transcriptional activity of NF-{kappa}B or muscle-specific transcription factors, respectively (1 µg/transfection). To inhibit NF-{kappa}B activation, cells were transiently transfected (6 µg/transfection) with a plasmid encoding I{kappa}B{alpha}-SR, which was constitutively expressed under control of the Friend spleen focus-forming virus 5'-long terminal repeat (SFFV-LTR) (pSFFV-NEO I{kappa}B{alpha}-SR), kindly provided by Dr. Rosa Ten (Mayo Clinic, Rochester, MN), or an empty vector as a control. For constitutive activation of NF-{kappa}B, cells were transiently transfected with a plasmid encoding IKK-ß, kindly provided by Dr. Michael Karin (University of California, San Diego, La Jolla, CA). To determine luciferase and ß-galactosidase acitivity, cells were lysed in 1x luciferase lysis buffer and were stored at -80°C. Luciferase (Promega) and ß-galactosidase (Tropix, Bedford, MA) were measured according to manufacturer’s instructions.

Immunofluorescence
To assess the extent of myogenic differentiation in cells transfected with I{kappa}B{alpha}-SR or control vector after treatment with TNF-{alpha}, the expression of myogenin and I{kappa}B{alpha}-super-repressor (SR)-FLAG was assessed by using immunofluorescence. Cells were grown, manipulated, and fixed on glass coverslips. A biotinylated antibody (10 µg/ml, M2, Sigma) directed against the FLAG epitope, in combination with a streptavidin-conjugated Alexa fluorophore (10 µg/ml, Alexa 488, Molecular Probes, Eugene, OR), was used to identify I{kappa}B{alpha}-SR-transfected cells. Simultaneously, an antibody recognizing myogenin (10 µg/ml, M225, Santa Cruz) combined with an anti-rabbit allophycocyanin-conjugated antibody (10 µg/ml, Molecular Probes) was applied. Immunoreactivity was evaluated with fluorescent microscopy (Olympus BX50) and was analyzed for colocalization by confocal scanning laser microscopy (Bio-Rad MRC 1024 ES).

Statistical analysis
Raw data were entered into SPSS (version 8.0) for statistical analysis. Values for CK activity, myogenic index, relative DNA binding activity, LDH activity, and luciferase activity were subjected to one-way ANOVA, and the various treatment groups were compared post hoc with a Student-Newman-Keuls test (P <0.05). Time-dependent decrements in luciferase activity for 6{kappa}B-TK-luciferase and TK-luciferase were assessed by using multiple regression analysis. A two-way ANOVA was employed to address whether myoblasts and myotubes activated NF-{kappa}B (assessed by EMSA and 6{kappa}B-TK-luciferase activity) similarly in response to TNF-{alpha}.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Assessment of myogenic differentiation
Initial experiments served to characterize the time frame of differentiation of C2C12 cells after replacement of GM with DM, by analysis of muscle-specific protein expression. The activity of CK, an enzyme expressed in mature skeletal and cardiac muscle (29) , was found to increase with differentiation (Fig. 1A ). Small increases in CK activity became apparent within 2 days after induction of differentiation, and a striking augmentation was observed between days 2 and 9. Increases in CK activity corresponded with increases in expression of MyHCf, a major myofibrillar protein (29) , and followed the expression of myogenin, a key muscle-specific transcription factor (30) (Fig. 1 B , C , respectively).



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  		Figure 1. Biochemical assessment of the differentiation state of C2C12 cells. Myoblasts were grown to ~60% confluency and allowed to differentiate by switching the culture media from GM to DM. A) CK activity was measured in cell lysates and normalized for total protein. Depicted is the average ± SE (n =2) for each time point. The expression of MyHCf (B) or myogenin (C) was determined by Western blot analysis of 10 µg of protein lysate. Data represent two or three independent experiments.

Effect of TNF-{alpha} on differentiated myotubes
To investigate the effect of the inflammatory cytokine TNF-{alpha} on the abundance of muscle-specific proteins in differentiated myotubes, cells were cultured in DM for 5 days, followed by an additional 24- or 72-h incubation in the presence or absence of TNF-{alpha}. Because catabolic effects of TNF-{alpha} are well documented (19) , we determined the total protein content per dish after exposure to the cytokine. However, the total protein content was not altered after 24 h of culture in the presence of the cytokine (data not shown). In fact, 72 h after the addition of TNF-{alpha} to the myotube cultures, a dose-dependent increase in total protein content became apparent (Table 1 ), indicating that TNF-{alpha} did not exert a catabolic effect in myotubes. CK activity was decreased 72 h after exposure to TNF-{alpha} (Fig. 2A ). In addition, slight reductions in MyHCf content were detected in lysates of myotubes treated with TNF-{alpha} for 24 h (Fig. 2B ). Overall, the effects of TNF-{alpha} on differentiated myotubes were marginal and occurred only at the highest dose (50 ng/ml) of cytokine.


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  		Table 1. Decreased myoblast and increased myotube protein content in response to TNF-{alpha}



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  		Figure 2. Assessment of CK activity and MyHCf abundance in differentiated myotubes treated with TNF-{alpha}. Cells were grown for 24 h in GM and were allowed to differentiate for 5 days by incubation in DM. TNF-{alpha} (1–50 ng/ml) was added, and cells were cultured for an additional 24 or 72 h. A) CK activity was determined in cell lysates and corrected for total protein. Depicted are the average ± SE values (n =2) for each treatment group. B) MyHCf expression was assessed by Western blot analysis of 10 µg of protein lysate. Statistically significant differences (*P <0.05) between the various treatment groups and their respective controls were determined by one-way ANOVA for each separate time point.

Effect of TNF-{alpha} on differentiating myoblasts
We next assessed the effects of TNF-{alpha} during the myogenic differentiation process. As shown in Fig. 3A , addition of TNF-{alpha} immediately after induction of differentiation markedly inhibited the formation of myotubes. This effect of the cytokine was further evidenced by a dose-dependent decrease of the myogenic index (Fig. 3B ). In addition, the total protein content per dish was lower when myoblasts were cultured in DM for 72 h in the presence of TNF-{alpha} (Table 1) . The appearance of biochemical differentiation markers was also blocked by TNF-{alpha}. For instance, CK activity (Fig. 3C ), MyHCf content (Fig. 3D ), and myogenin expression (Fig. 3E ) were suppressed in a dose-dependent manner in differentiating myocytes treated with the cytokine, even though all of these parameters were normalized for total protein. These results illustrate that TNF-{alpha} preferentially affects myogenesis in differentiating myoblasts, whereas differentiated myotubes are more refractory to the effects of TNF-{alpha} on muscle protein expression. Concentrations of TNF-{alpha} that interfere with myogenic differentiation here have been documented in the circulation of patients with cachexia (9) .



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  		Figure 3. TNF-{alpha} interferes with myogenic differentiation. Cells were induced to differentiate in the presence or absence of TNF-{alpha} (0.1–10 ng/ml) for 72 h by incubation in DM. Morphology was evaluated by phase-contrast microscopy (A) or by assessment of the myogenic index (B). Data are expressed as the average ± SE of five random fields from duplicate samples. Biochemical indicators of differentiation were determined in control and TNF-{alpha}-exposed cells. C) CK activity (average ±SE) was assessed in duplicate treated samples and corrected for total protein. Expression of MyHCf (D) and myogenin (E) was assessed by Western blot analysis of 10 µg of protein lysate. Data represent three independent experiments. Statistically significant differences (*P <0.03) between the various treatment groups and controls were determined for CK activity and myogenic index by one-way ANOVA. Linear trends over dose were also tested and demonstrated a dose-dependent effect of TNF-{alpha} for both the myogenic index and CK activity (P <0.001).

TNF-{alpha}: cytotoxicity and reversibility
To investigate whether the loss of myogenesis in differentiating myoblasts was the result of cytotoxicity by TNF-{alpha}, cell death was assessed. LDH activity in the culture media, resulting from its release by necrotic cells, was determined as a measure of cell death. Increasing LDH activity was found in control cells during the differentiation process, reflecting a population of cells that appeared to be incapable of survival under growth-restricting conditions. The presence of TNF-{alpha} increased LDH release in a dose-dependent manner during differentiation (Fig. 4A ). Analysis of cell death by flow cytometry, using the fluorimetric exclusion dye Sytox, confirmed the rise in cell death inherent to differentiation and the additional increase after TNF-{alpha} (data not shown). Further investigation revealed that the observed necrosis could be secondary to an apoptotic mechanism, as myoblasts treated with TNF-{alpha} for 48 and 72 h demonstrated a dose-dependent elevation in the sub-G0/G1 fraction, as shown by staining with propidium iodide and analyzed by flow cytometry (Fig. 4B ). Surprisingly, the total number of adherent cells increased after 48 h of exposure to TNF-{alpha}, illustrating that this cytokine causes proliferation in the first 48 h of myogenic differentiation (Fig. 4C ). After 72 h, however, cell numbers after TNF-{alpha} treatment were equal to control cell numbers again, which had remained constant, suggesting that additional cell death observed in the presence of TNF-{alpha} during this period (Fig. 4A , B ) may in fact result from apoptosis of these myocytes that are initially proliferating. We next tested whether the effect of TNF-{alpha} on differentiation was reversible, which would not expected to be the case for a nonspecific cytotoxic signal. TNF-{alpha} was washed out after 48 h of culture, and fresh DM was added to the myocytes. Although LDH activity continued to increase after removal of the cytokine (Fig. 5C ), myogenesis was restored completely and rapidly, as was demonstrated by CK activity (Fig. 5A ) and myogenin (Fig. 5B ). These results confirm that the inhibitory effects of TNF-{alpha} on myogenic differentiation are not due to a nonspecific toxic effect.



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  		Figure 4. Increased cell death during differentiation by TNF-{alpha} coincides with a proliferative response. C2C12 myoblasts were allowed to differentiate in the absence or presence of TNF-{alpha}. After 24, 48, and 72 h, cell death was determined as LDH activity in the media and expressed as a percentage of maximal activity (A). Apoptosis was assessed for the same time points as the sub-G0/G1 fraction of the total (adherent and detached) cell population (B). In addition, the adherent cells were counted and expressed as numbers per dish (C). Data (average ±SE) represent two independent experiments. Statistically significant differences (*P <0.05) between the TNF-{alpha} treated groups and controls were determined for each separate time point by one-way ANOVA. Linear trends were tested over time and dose. Control cells demonstrated a time-dependent increase in LDH activity and sub-G0/G1 fraction (P <0.002). Dose-dependent increases were found for LDH activity at all time points (P <0.005) and for the percentage of cells in sub-G0/G1 fraction at 48 and 72 h (P <0.005) following exposure to TNF-{alpha}.



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  		Figure 5. Inhibition of myogenic differentiation by TNF-{alpha} is reversible. Myoblasts were induced to differentiate in the absence or presence of TNF-{alpha} (10 ng/ml). After 48 h, TNF-{alpha} was washed out and the cells were cultured for an additional 24 h (72 h total) or 48 h (96 h total) in DM. CK activity (A) and myogenin expression (B) were determined in cell extracts, and LDH activity (C) was determined in the media. The CK activity of control (72 h) and washed (96 h) myocytes following TNF-{alpha} was compared by two-way ANOVA. Removal of TNF-{alpha} after 48 h significantly (*P <0.01) restored CK activity. For LDH activity, statistically significant differences (*P <0.001) between the TNF-{alpha}-treated groups and controls were determined for each separate time point by one-way ANOVA.

NF-{kappa}B in myoblasts and myotubes
To investigate potential mechanisms responsible for the observed inhibition of myogenic differentiation, we focused on NF-{kappa}B, a transcription factor that has a central role in inflammatory responses and is known to be activated by inflammatory cytokines such as TNF-{alpha} and IL-1ß (21) . Exposure to TNF-{alpha} caused a marked and sustained activation of NF-{kappa}B DNA binding activity (Fig. 6A ) and transactivation (Fig. 6B ) in response to TNF-{alpha}. Analysis of the subcomposition of the retarded complexes by supershift assay, using antibodies directed against the RelA and p50 subunits of NF-{kappa}B, revealed that the major complex induced by TNF-{alpha} (upper band) was composed of a RelA-p50 heterodimer, and possibly another RelA hetero- or homodimer, because preincubation with the p50 antibody did not result in a complete shift of this band (Fig. 6A ). As expected, NF-{kappa}B DNA binding activity and transactivation in response to TNF-{alpha} were preceded by rapid degradation of I{kappa}B{alpha} (Fig. 6C ), as determined by Western blotting using an antibody directed against I{kappa}B{alpha} (sc371, Santa Cruz).



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  		Figure 6. Activation of NF-{kappa}B by TNF-{alpha}. C2C12 cells were allowed to differentiate for 2–24 h in the presence or absence of TNF-{alpha} (10 ng/ml). Nuclear extracts were prepared at the indicated times and assessed for DNA binding activity to a consensus NF-{kappa}B oligonucleotide by EMSA. Relative DNA binding activity was determined by phosphoimager analysis and expressed as "fold" increase over control for each time point. Complex subcomposition was assessed in an extract of TNF-{alpha}-exposed cells (10 ng/ml, 4 h), by supershift analysis using antibodies against RelA or p50 (A). Transcriptional activation of NF-{kappa}B was determined in C2C12 cells stably expressing the 6{kappa}B-TK-luciferase construct. Lysates from untreated and treated cells were prepared 2, 4, 8, and 24 h after induction of differentiation for assessment of luciferase activity (B). NF-{kappa}B activation by TNF-{alpha} is preceded by degradation of I{kappa}B{alpha}. Cells treated with TNF-{alpha} (10 ng/ml) were harvested at the indicated exposure times, and 10 µg of protein lysate was analyzed by Western blotting (C). Data represent two or three independent experiments. Statistically significant differences (*P <0.05) between treated and control groups were determined for each individual time point by one-way ANOVA.

We next determined how NF-{kappa}B activity correlated with myogenic differentiation. Results in Fig. 7 demonstrate baseline DNA binding activity of NF-{kappa}B complexes composed of both RelA-p50 and p50 homodimers in unstimulated, undifferentiated cells. Basal levels of NF-{kappa}B DNA binding activity decreased during myogenic differentiation, after an initial increase in binding by the p50 homodimer (Fig. 7A ). The use of C2C12 cells that were stably transfected with 6{kappa}B-TK-luciferase revealed that basal transcriptional activity of NF-{kappa}B also decreased during muscle differentiation. In contrast, no changes were observed in a cell line containing the (control) TK-luciferase construct (Fig. 7B ). The decrease in NF-{kappa}B activity was accompanied by increasing levels of I{kappa}Bß, whereas I{kappa}B{alpha} abundance remained constant during differentiation (Fig. 7C ). However, the inducibility of NF-{kappa}B was the same during or after differentiation, as exposure to TNF-{alpha} increased NF-{kappa}B DNA binding activity (Fig. 7D , upper panel) and transcriptional activity (Fig. 7D , lower panel) to a similar extent in myoblasts and differentiated myotubes. These results demonstrated an inverse relation between basal NF-{kappa}B activity and myogenic differentiation. Because IL-1, like TNF-{alpha}, is known to activate NF-{kappa}B, differentiating myoblasts were treated with this cytokine (IL-1ß, 0.1–10 ng/ml). As observed for TNF-{alpha}, IL-1ß inhibited myogenic differentiation in a dose-dependent manner, as was revealed by measurement of CK activity and assessment of MyHCf expression (data not shown).



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  		Figure 7. NF-{kappa}B DNA binding and transcriptional activity decreases during differentiation. A) C2C12 cells were induced to differentiate by incubation in DM, and nuclear extracts were prepared at subsequent days of incubation in DM for analysis of binding activity to a consensus NF-{kappa}B oligonucleotide by EMSA. Indicated are the subcompositions of the various retarded complexes; n.s.= nonspecific. B) Assessment of NF-{kappa}B transactivation in a line of C2C12 cells stably expressing 6{kappa}B-TK-luciferase. Cells were harvested after the indicated times of culture in DM (n =2) for the determination of luciferase activity (average ±SE). As a control, C2C12 cells were stably transfected with an empty TK-luciferase construct (TK-luc), which does not respond to NF-{kappa}B activation. Multiple regression analysis demonstrated a significant difference in the slopes of 6{kappa}B-TK-luciferase and TK-luciferase (P =0.002). Moreover, luciferase activity decreased significantly over time for 6{kappa}B-TK-luciferase (P <0.0001), whereas no significant decrease was found for TK-luciferase (P =0.08). C) I{kappa}B{alpha} abundance remained stable and levels of I{kappa}Bß increased during differentiation. Cell extracts were prepared at the indicated times after induction of differentiation and assessed for I{kappa}B{alpha} and I{kappa}Bß by Western blot analysis. D) Similar activation of NF-{kappa}B by TNF-{alpha} in myoblasts and myotubes. NF-{kappa}B DNA binding activity was determined by EMSA in nuclear extracts of myoblasts and myotubes that had been exposed to TNF-{alpha} (10 ng/ml) for 4 h. Relative binding activity of the RelA-p50 dimer, quantified by phosphoimaging analysis, was expressed as a "fold" increase over untreated controls and indicated above each set of lanes (upper panel). NF-{kappa}B transactivation was assessed in myoblasts or myotubes of the C2C12 6{kappa}B-TK-luciferase cell line after 6 h of TNF-{alpha} treatment. Luciferase units were normalized to protein content, and data were expressed as a fold increase over sham (lower panel). The response of myoblasts and myotubes to TNF-{alpha} was compared and found to be identical by a two-way ANOVA, which showed no interaction between differentiation status and treatment.

NF-{kappa}B inhibits myogenic differentiation
To determine whether NF-{kappa}B activation was causally involved in the inhibition of myogenic differentiation in response to TNF-{alpha} or IL-1ß, cells were transiently transfected with I{kappa}B{alpha}-SR, a nondegradable mutant of I{kappa}B{alpha}, and a troponin promoter-luciferase reporter plasmid (TnI-luciferase), which is expressed only in differentiated muscle. As expected, expression of I{kappa}B{alpha}-SR reduced NF-{kappa}B transactivation by TNF-{alpha} (Fig. 8A ) and IL-1ß (Fig. 8B ). It is important to note that overexpression of I{kappa}B{alpha}-SR enhanced myogenic differentiation (Fig. 9A ) and restored the loss of TnI-luciferase expression (Fig. 9B ) in response to exposure to these cytokines. The causal role of NF-{kappa}B in the TNF-{alpha}-induced inhibition of myogenic differentiation was also illustrated by immunofluorescence, which revealed that only I{kappa}B{alpha}-SR-positive cells expressed myogenin when cultured in DM in the presence of TNF-{alpha} (Fig. 10 ). Finally, overexpression of IKKß, which resulted in constitutive activation of NF-{kappa}B (Fig. 8B ), dose dependently blocked TnI-luciferase expression (Fig. 9C ), further emphasizing the causal role of NF-{kappa}B activation in the inhibition of myogenic differentiation.



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  		Figure 8. Expression of I{kappa}B{alpha}-SR inhibits, whereas expression of IKKß induces NF-{kappa}B transactivation. C2C12 cells were transiently transfected with I{kappa}B{alpha}-SR, control vector, or IKKß in the presence of 6{kappa}B-TK-luciferase and pSV-ß-Gal. At 16 h after transfection, cells were allowed to recover for 12 h in GM and were subsequently induced to differentiate in the presence or absence of TNF-{alpha} (A) or IL-1ß (B), to evaluate whether I{kappa}B{alpha}-SR expression could block NF-{kappa}B transactivation. IKKß was overexpressed for the constitutive activation of NF-{kappa}B (B). Cells were harvested 6 h after treatment (in DM) for evaluation of luciferase and ß-galactosidase activity, the latter of which served to correct for differences in transfection efficiency. Statistically significant differences were determined by one-way ANOVA. *P < 0.05 compared with untreated cells that did not contain I{kappa}B{alpha}-SR; {dagger}P < 0.05 compared with cells treated with TNF-{alpha} (A) or IL-1ß (B) that did not contain I{kappa}B{alpha}-SR.



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  		Figure 9. Blockade of NF-{kappa}B activation reverses the inhibitory effects of TNF-{alpha} or IL-1ß on myogenic differentiation. Cells were cotransfected with I{kappa}B{alpha}-SR or control vector and TnI-luciferase to assess whether blocking NF-{kappa}B activation reversed inhibition of differentiation by TNF-{alpha} or IL-1ß. At 16 h after transfection, cells were allowed to recover for 12 h in GM and were subsequently induced to differentiate in the presence or absence of TNF-{alpha} (A) or IL-1ß (B). After 48 h in DM, cell lysates were prepared for evaluation of luciferase and ß-galactosidase activity. Alternatively, cells were cotransfected with IKKß and TnI-luciferase for 16 h, allowed to recover for 12 h, and harvested after 48 h of subsequent incubation in DM (C). Luciferase activity was expressed relative to ß-galactosidase activity to account for differences in transfection efficiency. Shown are the representative data of two or three independent experiments. Statistically significant differences were determined by one-way ANOVA. *P < 0.05 compared with untreated cells (48 h in DM) that did not contain I{kappa}B{alpha}-SR; {dagger}P < 0.05 compared with cells treated with TNF-{alpha} (A) or IL-1ß (B) that did not contain I{kappa}B{alpha}-SR. Linear trends over dose were also tested and demonstrated a dose-dependent effect of IKKß for TnI-luciferase (P <0.001).



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  		Figure 10. Inhibition of NF-{kappa}B restores myogenin expression in differentiating myoblasts exposed to TNF-{alpha}. Cells were grown on glass coverslips, were transfected with FLAG-tagged I{kappa}B{alpha}-SR or control vector, and were allowed to differentiate in the presence or absence of TNF-{alpha}. After 48 h in DM, cells were fixed, stained, and analyzed for myogenin (A) and I{kappa}B{alpha}-SR (FLAG) (B) by immunofluoresence using confocal scanning laser microscopy. Positive cells are indicated by arrowheads.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Cachexia has been associated in many chronic wasting diseases with elevated levels of circulating inflammatory mediators, including TNF-{alpha} (7 8 9 10 , 31) . Furthermore, chronic administration of TNF-{alpha} or IL-1ß (14) , or the implantation of a TNF-{alpha}-producing tumor (12) , results in muscle wasting. Because the involvement of catabolic hormones or other mediators could not be ruled out in those studies, it is not clear whether muscle wasting is mediated through a direct effect of these inflammatory cytokines on skeletal muscle. In the present study, the effects of TNF-{alpha} on C2C12 cells were analyzed during and after their differentiation. Undifferentiated C2C12 cells, or myoblasts, are the in vitro equivalent of satellite cells, which constitute the myogenic stem cell population in postnatal skeletal muscle (32) . Although present as a low percentage of total myonuclei in adult skeletal muscle, satellite cells are indispensable in the maintenance of functional muscle because they possess the capacity to proliferate and differentiate to replace lost or injured muscle fibers. Our findings demonstrate dramatically different responses to TNF-{alpha} in differentiating myoblasts vs. fully differentiated myotubes. First, when cultured in the presence of TNF-{alpha}, differentiating myoblasts demonstrated a decreased protein content per dish compared with controls, whereas differentiated myotubes contained more protein per dish. The latter result is in contrast to that from a study in which a catabolic effect of TNF-{alpha} on total protein content of differentiated myotubes was reported (19) . Second, although some loss in MyHCf content and CK activity was observed after exposure of myotubes to TNF-{alpha}, these decreases were marginal. In contrast, addition of TNF-{alpha} to differentiating myoblasts completely inhibited myogenic differentiation at concentrations of 1–10 ng/ml, which have been reported in the circulation in patients with chronic inflammatory diseases (31 , 33 , 34) . In support of our findings, others have also reported marked inhibitory effects of TNF-{alpha} on myogenic differentiation (18 , 22 , 35 , 36) and have failed to demonstrate effects in differentiated myotubes (18 , 35) . A recent study also demonstrated this differential effect of TNF-{alpha} but showed that myotubes became susceptible to TNF-{alpha} once interferon-{gamma} was administered in addition to TNF-{alpha} (37) . This result is a strong indication that a secondary signaling event is required in the sensitization of myotubes to TNF-{alpha} and could involve the activation of STAT1 (38) .

TNF-{alpha} has been shown to evoke toxic effects in a variety of cell types (39 , 40) and proliferative responses in other cell types (41) . Our study also demonstrates that in the presence of TNF-{alpha}, a subpopulation of myoblasts undergoes cell death when cultured in DM, which is evidenced by increased LDH levels in the medium. However, the inhibition of myogenesis was completely reversible after removal of the cytokine, indicative of a specific inhibitory effect of TNF-{alpha}. We speculate that the observed cell death here may involve apoptosis of the fraction of myoblasts, which initially responded to TNF-{alpha} by inappropriate proliferation.

Similar to treatment with TNF-{alpha}, exposure of differentiating myocytes to IL-1ß (0.1–10 ng/ml) impaired myogenesis, as indicated by decreased CK activity and MyHCf expression (data not shown). We therefore propose that the inhibition of myogenic differentiation of satellite cells, resulting from a sustained elevation in the levels of circulating inflammatory cytokines, may constitute an important mechanism leading to muscle wasting in chronic inflammatory conditions, through impairment of the regeneration process.

Binding of TNF-{alpha} and IL-1 to their respective receptors can trigger multiple pathways that, depending on the adaptor complexes that are associated with the cytoplasmic domain, can converge at the level of the I{kappa}B-kinase (IKK) complex, resulting in the activation of NF-{kappa}B (20) . We tested here whether TNF-{alpha}- or IL-1-induced activation of NF-{kappa}B was causally involved in the inhibition of myogenic differentiation. We first demonstrated that baseline DNA binding and transcriptional activity of NF-{kappa}B markedly decreased during differentiation, in agreement with findings of others (42 , 43) . More detailed analysis revealed that the overall decrease in DNA binding activity was preceded by an initial increase in p50 homodimer binding acitivity. Such a transient rise in NF-{kappa}B DNA binding activity during the initial phase of myogenic differentiation has been described (44) . In that study it was concluded, on the basis of additional data demonstrating a loss of myogenesis following the addition of nonspecific NF-{kappa}B inhibitors, that NF-{kappa}B activation is required for myogenic differentiation to occur. However, NF-{kappa}B transactivation was not assessed during any phase of differentiation, nor was the composition of the NF-{kappa}B complex contributing to the increased DNA binding activity identified. Because the DNA binding activity of the p50 homodimer was selectively increased in the initial phase of differentiation in our system, we believe that it may in fact function as a repressor of NF-{kappa}B-dependent transcription, as the p50 subunit does not contain a transactivation domain but can still occupy NF-{kappa}B binding sites (45) . This idea is further supported by the observation that NF-{kappa}B transactivation decreased during differentiation (Fig. 7B ). In addition, I{kappa}Bß levels increased during differentiation, whereas I{kappa}B{alpha} abundance remained constant. The decrease in NF-{kappa}B DNA binding and transcriptional activity during differentiation coincided with increasing expression of muscle-specific proteins and fusion of cells into myotubes. Others have reported that reduction of basal NF-{kappa}B activity, via retroviral transfer of I{kappa}B{alpha}-SR or by addition of the NF-{kappa}B inhibitor curcumin, enhanced myogenic differentiation (43 , 46) .

These observations led us to further investigate the link between TNF-{alpha}- or IL-1ß-induced NF-{kappa}B activation and the observed inhibition of myogenic differentiation. We report here that expression of I{kappa}B{alpha}-SR in differentiating myoblasts abrogated TNF-{alpha}- or IL-1ß-induced NF-{kappa}B transactivation and, more important, restored myogenic differentiation in the presence of these cytokines, demonstrating that TNF-{alpha} and IL-1ß block myogenic differentiation through the activation of NF-{kappa}B. Despite equivalent activation of NF-{kappa}B in myoblasts and myotubes (Fig. 7D ), only myoblasts appear to be adversely affected by TNF-{alpha}, illustrating that the cells can respond to TNF-{alpha} in both stages and that the lack of effects of TNF-{alpha} in myotubes is not the result of a loss of TNF-{alpha} signaling. These observations contrast with a report that demonstrated enhanced activation of NF-{kappa}B in C2C12 myotubes compared with myoblasts, presumably because of the higher expression of TNF receptor-associated factor 2 (TRAF-2) in the myotubes (47) . Possibly, these discrepancies can be attributed to the different protocols used to allow differentiation of the C2C12 cells, or to differences in the transfection procedures that were used.

An explanation of why myocytes are uniquely susceptible to TNF-{alpha} during differentiation may be linked to the cell cycle machinery. For example, it has been demonstrated that cyclin D1 is transcriptionally regulated by NF-{kappa}B (43) . Initiation of myogenic differentiation is preceded by exit from the cell cycle, an event that requires down-regulation of cyclin D1. Therefore, activation of NF-{kappa}B by TNF-{alpha} and subsequent expression of cyclin D1 may inhibit myogenesis in myoblasts by preventing cell cycle exit, whereas expression of cyclin D1 is irrelevant in myotubes, because they irreversibly reside in G0/G1 (48) . In support of this, we observed an increase in cell number after 24 and 48 h of culture in DM when TNF-{alpha} was present (Fig. 4D ), which coincided with an increased fraction of cells residing in the S and G2/M phases of the cell cycle (data not shown). Alternatively, the activity of the muscle-specific basic-helix-loop-helix transcription factors MyoD and Myf-5, which have proved to be indispensable for myogenic differentiation (49) , could be repressed as a direct consequence of NF-{kappa}B activation. In support of the latter alternative, it has been shown that the catalytic subunit of protein kinase A (PKAc) is associated with active NF-{kappa}B (50) , whereas another study demonstrated that PKA can inactivate MyoD and Myf-5 (51) . Another mechanism by which MyoD may be inhibited by NF-{kappa}B has been described very recently. This study demonstrated that the MyoD mRNA half-life decreased in differentiating myoblasts following TNF-{alpha} exposure and that this depended on NF-{kappa}B-regulated gene expression (37) .

In summary, we have demonstrated that the inflammatory cytokines TNF-{alpha} and IL-1ß interfere with myogenic differentiation. Once skeletal muscle cells have differentiated, they appear to be become relatively resistant to these cytokines. Therefore, it is possible that the loss of skeletal muscle associated with chronic inflammatory disease states is the result of an inability of satellite cells to differentiate into functional fibers after damage or degeneration. Although therapeutic interventions for skeletal muscle wasting are usually designed to restore imbalances in protein anabolism and catabolism, our results suggest that intervention strategies should also focus on stimulation or restoration of satellite cell differentiation, because this process may be compromised under conditions of chronic inflammation. Besides the administration of agents that have been demonstrated to increase muscle mass in healthy individuals, such as specific nutritional supplements, hormonal treatment, and insulin-like growth factor I, specific intracellular pathways such as the activation of NF-{kappa}B may have to be modulated in cachectic patients for these agents to have an effect.


   ACKNOWLEDGMENTS
 
We thank Dr. Pam Vacek for assistance with the statistical analysis. We also thank Dries Testelmans for excellent technical support and Colette Charland (Flow Cytometry Facility, University of Vermont) for technical assistance with the flow cytometry analysis. This work was supported by a grant from Numico Research BV, The Netherlands.

Received for publication August 9, 2000. Revision received November 20, 2000.
   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
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