IL-1beta stimulates IL-6 production in cultured skeletal muscle cells through activation of MAP kinase signaling pathway and NF-kappa B

Inter-relation between interleukin (IL)-1, IL-6 and body fat regulating circuits of the hypothalamic arcuate nucleus

Interleukin (IL)-1 and IL-6 are immune modulating cytokines that also affect metabolic function because both IL-1 receptor I deficient (IL-1RI⁻/⁻) and IL-6 deficient (IL-6⁻/⁻) mice develop late-onset obesity and leptin resistance. Both IL-1 and IL-6 appear to target the central nervous system (CNS) to increase energy expenditure. The hypothalamic arcuate nucleus (ARC) is a major relay between the periphery and CNS in body fat regulation (e.g. by being a target of leptin). The present study aimed to investigate the possible mechanisms responsible for the effects exerted by endogenous IL-1 and IL-6 on body fat at the level of the ARC, as well as possible interactions between IL-1 and IL-6. Therefore, we measured the gene expression of neuropeptides of the ARC involved in energy balance in IL-1RI⁻/⁻ and IL-6⁻/⁻ mice. We also investigated the interactions between expression of IL-1 and IL-6 in these mice, and mapped IL-6 receptor α (IL-6Rα) in the ARC. The expression of the obesity promoting peptide neuropeptide Y (NPY), found in the ARC, was increased in IL-1RI⁻/⁻ mice. The expression of NPY and agouti-related peptide (AgRP), known to be co-expressed with NPY in ARC neurones, was increased in cold exposed IL-6⁻/⁻ mice. IL-6Rα immunoreactivity was densely localised in the ARC, especially in the medial part, and was partly found in NPY positive cell bodies and also α-melanocyte-stimulating hormone positive cell bodies. The expression of hypothalamic IL-6 was decreased in IL-1RI⁻/⁻ mice, whereas IL-1ß expression was increased in IL-6⁻/⁻ mice. The results of the present study indicate that depletion of the activity of the fat suppressing cytokines IL-1 and IL-6 in knockout mice can increase the expression of the obesity promoting neuropeptide NPY in the ARC. Depletion of IL-1 activity suppresses IL-6 expression, and IL-6Rα-like immunoreactivity is present in neurones in the medial ARC, including neurones containing NPY. Therefore, IL-6, IL-1 and NPY/AgRP could interact at the level of the hypothalamic ARC in the regulation of body fat.

Gli1 deletion prevents Helicobacter-induced gastric metaplasia and expansion of myeloid cell subsets

Chronic inflammation in the stomach induces metaplasia, the pre-cancerous lesion that precedes inflammation-driven neoplastic transformation. While Hedgehog signaling contributes to the initiation of some cancers, its role in gastric transformation remains poorly defined. We found that Helicobacter-infected C57BL/6 mice develop extensive mucous cell metaplasia at 6 month but not at 2 months post-infection. Gastric metaplasia coincided with the appearance of CD45⁺MHCII⁺CD11b⁺CD11c⁺ myeloid cells that were normally not present in the chronic gastritis at 2 months. The myeloid regulatory gene Schlafen-4 was identified in a microarray analysis comparing infected WT versus Gli1 null mice and was expressed in the CD11b⁺CD11c⁺ myeloid population. Moreover this same population expressed IL-1β and TNFα pro-inflammatory cytokines. By 6 months, the mucous neck cell metaplasia (SPEM) expressed IL-6, phosphorylated STAT3 and the proliferative marker Ki67. Expression was not observed in Gli1 mutant mice consistent with the requirement of Gli1 to induce this pre-neoplastic phenotype. Ectopic Shh ligand expression alone was not sufficient to induce SPEM, but with Helicobacter infection synergistically increased the histologic severity observed with the inflammation. Therefore Hedgehog signaling is required, but is not sufficient to generate pre-neoplastic changes during chronic gastritis. Gli1-dependent myeloid cell differentiation plays a pivotal role in the appearance of myeloid cell subtypes ostensibly required for SPEM development. Moreover, it suggests that therapies capable of targeting this phenotypic switch might prevent progression to metaplasia, the pre-neoplastic change that develops prior to dysplasia and gastric cancer, which also occurs in other epithelial-derived neoplasias initiated by chronic inflammation.

Cytokine response of cultured skeletal muscle cells stimulated with proinflammatory factors depends on differentiation stage

Myoblast proliferation and myotube formation are critical early events in skeletal muscle regeneration. The attending inflammation and cytokine signaling are involved in regulation of skeletal muscle cell proliferation and differentiation. Secretion of muscle-derived cytokines upon exposure to inflammatory factors may depend on the differentiation stage of regenerating muscle cells. Cultured human myoblasts and myotubes were exposed to 24-hour treatment with tumor necrosis factor (TNF)-α or lipopolysaccharide (LPS). Secretion of interleukin 6 (IL-6), a major muscle-derived cytokine, and interleukin 1 (IL-1), an important regulator of inflammatory response, was measured 24 hours after termination of TNF-α or LPS treatment. Myoblasts pretreated with TNF-α or LPS displayed robustly increased IL-6 secretion during the 24-hour period after removal of treatments, while IL-1 secretion remained unaltered. IL-6 secretion was also increased in myotubes, but the response was less pronounced compared with myoblasts. In contrast to myoblasts, IL-1 secretion was markedly stimulated in LPS-pretreated myotubes. We demonstrate that preceding exposure to inflammatory factors stimulates a prolonged upregulation of muscle-derived IL-6 and/or IL-1 in cultured skeletal muscle cells. Our findings also indicate that cytokine response to inflammatory factors in regenerating skeletal muscle partially depends on the differentiation stage of myogenic cells.

Characterization of contraction-induced IL-6 up-regulation using contractile C2C12 myotubes

Muscle contractile activity functions as a potent stimulus for acute interleukin (IL)-6 expression in working skeletal muscles. Recently, we established an "in vitro contraction model" using highly-developed contractile C2C12 myotubes by applying electric pulse stimulation (EPS). Herein, we characterize the effects of EPS-evoked contraction on IL-6 expression in contractile C2C12 myotubes. Both secretion and mRNA expression of IL-6 were significantly up-regulated by EPS in a frequency-dependent manner in contracting myotubes during a 24-h period, and the response was blunted by cyclosporine A, a calcineurin inhibitor. Longer time (~12h) was required for the induction of IL-6 after the initiation of EPS as compared to that of other contraction-inducible CXC chemokines such as CXCL1/KC, which were induced in less than 3 hours. Furthermore, these acute inducible CXC chemokines exhibited no autocrine effect on IL-6 expression. Importantly, contraction-dependent IL-6 up-regulation was markedly suppressed in the presence of high levels of glucose along with increased glycogen accumulations. Experimental manipulation of intracellular glycogen contents by modulating available glucose or pyruvate during a certain EPS period further established the suppressive effect of glycogen accumulations on contraction-induced IL-6 up-regulation, which appeared to be independent of calcineurin activity. We also document that EPS-evoked contractile activity improved insulin-responsiveness in terms of intracellular glycogen accumulations. Taken together, these data provide important insights into the regulation of IL-6 expression in response to contractile activity of muscle cells, which is difficult to examine using in vivo experimental techniques. Our present results thus expand the usefulness of our "in vitro contraction model".

Upregulation of amino acid transporter expression induced by L-leucine availability in L6 myotubes is associated with ATF4 signaling through mTORC1-dependent mechanism

OBJECTIVE

Essential amino acids, especially l-leucine, initiate the signaling of the mammalian target of rapamycin complex-1 (mTORC1) and protein synthesis in skeletal muscle. Current information on the relation between amino acid transporter mechanisms and mTORC1 signaling is sparse. The objectives of this study were to determine whether an increase in leucine availability upregulates the gene transcription and translation of amino acid transporters and other amino acid members in an mTORC1-dependent pathway that control amino acid use (general control non-repressed-2 and activating transcription factor-4) and to measure the factors related to protein synthesis and proteolysis.

METHODS

L6 skeletal muscle cells that had been treated with l-leucine (0.105 g/L) were incubated for 30 min to stimulate the transcription of L-type amino acid transporter-1, CD98, and sodium-coupled neutral amino acid transporter-2 and increase activating transcription factor-4 protein, which is dependent on the mTORC1 signaling pathway.

RESULTS

A rapid, high level of p70 S6 kinase-1 phosphorylation was detected but was suppressed by rapamycin (P < 0.05). The addition of leucine decreased the atrogin-1 transcription abundance in an insulin-involved manner (P < 0.05), which could not be completely blocked by rapamycin (P = 0.055).

CONCLUSIONS

Our findings indicate that the mTOR is a component of the nutrient signaling pathway, which regulates system A and L amino acid transporters, the initiation factors involved in mRNA translation, and is downstream of forkhead box-O in L6 myotubes.

Hyperthermia increases interleukin-6 in mouse skeletal muscle

Skeletal muscles produce and contribute to circulating levels of IL-6 during exercise. However, when core temperature is reduced, the response is attenuated. Therefore, we hypothesized that hyperthermia may be an important and independent stimulus for muscle IL-6. In cultured C2C12 myotubes, hyperthermia (42°C) increased IL-6 gene expression 14-fold after 1 h and 35-fold after 5 h of 37°C recovery; whereas exposure to 41°C resulted in a 2.6-fold elevation at 1 h. IL-6 protein was secreted and significantly elevated in the cell supernatant. Similar but reduced responses to heat were seen in C2C12 myoblasts. Isolated soleus muscles from mice, exposed ex vivo to 41°C for 1 h, yielded similar IL-6 gene responses (>3-fold) but without a significant effect on protein release. When whole animals were exposed to passive hyperthermia, such that core temperature increased to 42.4°C, IL-6 mRNA in soleus increased 5.4-fold compared with time matched controls. Interestingly, TNF-α gene expression was routinely suppressed at all levels of hyperthermia (40.5-42°C) in the isolated models, but TNF-α was elevated (4.2-fold) in the soleus taken from intact mice exposed, in vivo, to hyperthermia. Muscle HSP72 mRNA increased as a function of the level of hyperthermia, and IL-6 mRNA responses increased proportionally with HSP72. In cultured C2C12 myotubes, when heat shock factor was pharmacologically blocked with KNK437, both HSP72 and IL-6 mRNA elevations, induced by heat, were suppressed. These findings implicate skeletal muscle as a "heat stress sensor" at physiologically relevant hyperthermia, responding with a programmed cytokine expression pattern characterized by elevated IL-6.

Rel A/p65 is required for cytokine-induced myotube atrophy

Muscle atrophy can be triggered by systemic illnesses that are associated with elevated proinflammatory/catabolic cytokines, which, in turn, are thought to contribute to muscle wasting. In this study, we found that the prototypical NF-κB transcription factor, Rel A (p65), is required for NF-κB activation in C2C12 and L6 myotubes due to treatment with exogenous TNF-α, IL-1α, IL-1β, TNF-related weak inducer of apoptosis, but not IL-6. All five cytokines induced atrophy in C2C12 myotubes, and inhibition of p65 reversed atrophy due to TNF-α, IL-1α, IL-1β, TNF-related weak inducer of apoptosis, but not IL-6 treatment. p65 was also required for TNF-α-induced increase in atrophy and inflammatory gene expression. TNF-α- and IL-1β-treated myotubes increased IL-6 protein expression, but use of an IL-6 blocking antibody showed that the IL-6 production did not contribute to atrophy. These data show that p65 is a required transcription factor mediating the catabolic effects of four different cytokines in cultured myotubes, but IL-6 works by a different mechanism.

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

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

Time-course analysis of injured skeletal muscle suggests a critical involvement of ERK1/2 signaling in the acute inflammatory response

INTRODUCTION

The coupling and timing of pro- and anti-inflammatory processes in skeletal muscle injury is poorly understood. We investigated the temporal response and regulated processes of extracellular signal-regulated kinases 1 and 2 (ERK1/2), p38, and IkappaB kinase (IKK) α/β signaling pathways after traumatic injury.

METHODS

Traumatic freeze injury was delivered to the tibialis anterior (TA) muscle in C57BL/6J mice, and injured and uninjured TA muscles were analyzed 3-72 h into the recovery period.

RESULTS

Significant increases in pro-inflammatory cytokine transcription accompanied IKKβ phosphorylation, robust ERK pathway activation, and reduced heat shock protein (Hsp) protein expression at 3-24 h. At 24 h, ERK activation was abolished concomitantly with a significant increase in mitogen-activated protein kinase phosphatase-1 (MKP-1). After 24 h, cytokine transcription along with ERK1/2 and IKKβ phosphorylation remained suppressed, whereas Hsp protein expression rose to significant levels by 72 h and associated with IKKβ.

CONCLUSIONS

Results indicate a bimodal regulation of ERK1/2 in acute inflammation in which it is supportive from 3 to 24 h, and suppressive from 24 to 72 h.

Lipoteichoic acid of Staphylococcus aureus enhances IL-6 expression in activated human basophils

At allergic inflammation, cross-linking of FcɛRI with multivalent antigen-bound IgE triggers the signaling pathways via activation of protein kinase C (PKC) and mobilization of intracellular Ca(2+) leading to the production of various mediators such as interleukin-6 (IL-6). Accumulating reports demonstrated that interaction of Toll-like receptor 2 (TLR2) expressed on basophils with a TLR2 ligand, lipoteichoic acid (LTA) of Staphylococcus aureus, exacerbated allergic inflammation. Here, we showed that staphylococcal LTA (Sa.LTA) substantially enhanced IL-6 expression at both the protein and the mRNA levels in the human basophil line, KU812, in the presence of a PKC activator (phorbol 12-myristrate 13-acetate; PMA), and a calcium ionophore (A23187), whereas Sa.LTA alone could not induce IL-6 expression. PMA/A23187 augmented the expression of CD14 and TLR2 on the surface of KU812 cells and concomitantly increased the binding of fluorochrome-labeled Sa.LTA to the cells. Sa.LTA enhanced the phosphorylation of mitogen-activated protein (MAP) kinases in PMA/A23187-stimulated KU812 cells. Notably, Sa.LTA could not enhance PMA/A23187-induced IL-6 expression in the presence of inhibitors of MAP kinases, reactive oxygen species, or protein kinase C. Furthermore, Sa.LTA enhanced the PMA/A23187-increased DNA-binding activities of the transcription factors NF-κB and AP-1. Experiments using human peripheral blood mononuclear cells demonstrated that not only PMA/A23187 but also Sa.LTA increased the intracellular IL-6 expression in the basophil population and Sa.LTA plus PMA/A23187 further enhanced the IL-6 expression. Collectively, these results suggest that Sa.LTA exacerbates allergic inflammation by potentiating IL-6 production from activated basophils.

Stimulation of inflammatory gene expression in human preadipocytes by macrophage-conditioned medium: upregulation of IL-6 production by macrophage-derived IL-1β

The aim of this study was to examine the effects of macrophage secretions on global gene expression in human preadipocytes using microarrays. Preadipocytes were cultured with unconditioned or conditioned medium from U937 macrophages, and gene expression examined with Agilent arrays (43,000 probes). 472 transcripts were differentially regulated (>2-fold difference; P<0.05) between preadipocytes in the conditioned medium compared to the unconditioned; 401 were upregulated and 71 downregulated. The upregulated transcripts were particularly linked to inflammation, including IL-1β, IL-6, and CCL20 (16.8-, 10.0-, and 8.9-fold increases, respectively) together with matrix metalloproteinases (MMP3, MMP9 and MMP12). Major pathways regulated by the conditioned medium were linked to inflammation, macrophage infiltration and lipid accumulation. Network analysis identified NFkB and IL-1β as central nodes in the upregulation of multiple inflammation-related genes. Treatment with an IL-1β neutralising antibody abolished the stimulation of IL-6 secretion by conditioned medium, indicating that IL-1β is a key regulator of preadipocyte IL-6 production. Macrophages evoke extensive changes in preadipocyte gene expression.

Plasma cytokine response, lipid peroxidation and NF-kB activation in skeletal muscle following maximum progressive swimming

Our objective was to determine lipid peroxidation and nuclear factor-κB (NF-κB) activation in skeletal muscle and the plasma cytokine profile following maximum progressive swimming. Adult male Swiss mice (N = 15) adapted to the aquatic environment were randomly divided into three groups: immediately after exercise (EX1), 3 h after exercise (EX2) and control. Animals from the exercising groups swam until exhaustion, with an initial workload of 2% of body mass attached to the tail. Control mice did not perform any exercise but were kept immersed in water for 20 min. Maximum swimming led to reactive oxygen species (ROS) generation in skeletal muscle, as indicated by increased thiobarbituric acid reactive species (TBARS) levels (4062.67 ± 1487.10 vs 19,072.48 ± 8738.16 nmol malondialdehyde (MDA)/mg protein, control vs EX1). Exercise also promoted NF-κB activation in soleus muscle. Cytokine secretion following exercise was marked by increased plasma interleukin-6 (IL-6) levels 3 h post-exercise (P < 0.05). Interleukin-10 (IL-10) levels were reduced following exercise and remained reduced 3 h post-exercise (P < 0.05). Plasma levels of other cytokines investigated, monocyte chemotactic protein-1 (MCP-1), tumor necrosis factor-alpha (TNF-α), interferon-gamma (IFN-γ) and interleukin-12 (IL-12), were not altered by exercise. The present findings showed that maximum swimming, as well as other exercise models, led to lipid peroxidation and NF-κB activation in skeletal muscle and increased plasma IL-6 levels. The plasma cytokine response was also marked by reduced IL-10 levels. These results were attributed to exercise type and intensity.

Reactive oxygen species: impact on skeletal muscle

It is well established that contracting muscles produce both reactive oxygen and nitrogen species. Although the sources of oxidant production during exercise continue to be debated, growing evidence suggests that mitochondria are not the dominant source. Regardless of the sources of oxidants in contracting muscles, intense and prolonged exercise can result in oxidative damage to both proteins and lipids in the contracting myocytes. Further, oxidants regulate numerous cell signaling pathways and modulate the expression of many genes. This oxidant-mediated change in gene expression involves changes at transcriptional, mRNA stability, and signal transduction levels. Furthermore, numerous products associated with oxidant-modulated genes have been identified and include antioxidant enzymes, stress proteins, and mitochondrial electron transport proteins. Interestingly, low and physiological levels of reactive oxygen species are required for normal force production in skeletal muscle, but high levels of reactive oxygen species result in contractile dysfunction and fatigue. Ongoing research continues to explore the redox-sensitive targets in muscle that are responsible for both redox regulation of muscle adaptation and oxidant-mediated muscle fatigue.

MAPKs and NF-κB differentially regulate cytokine expression in the diaphragm in response to resistive breathing: the role of oxidative stress

Inspiratory resistive breathing (IRB) induces cytokine expression in the diaphragm. The mechanism of this cytokine induction remains elusive. The roles of MAPKs and NF-κB and the impact of oxidative stress in IRB-induced cytokine upregulation in the diaphragm were studied. Wistar rats were subjected to IRB (50% of maximal inspiratory pressure) via a two-way nonrebreathing valve for 1, 3, or 6 h. Additional groups of rats subjected to IRB for 6 h were randomly assigned to receive either solvent or N-acetyl-cysteine (NAC) or inhibitors of NF-κB (BAY-11-7082), ERK1/2 (PD98059), and P38 MAPK (SB203580) to study the effect of oxidative stress, NF-κB, and MAPKs in IRB-induced cytokine upregulation in the diaphragm. Quietly breathing animals served as controls. IRB upregulated cytokine (IL-6, TNF-α, IL-10, IL-2, IL-1β) protein levels in the diaphragm and resulted in increased activation of MAPKs (P38, ERK1/2) and NF-κB. Inhibition of NF-κB and ERK1/2 blunted the upregulation of all cytokines except that of IL-6, which was further increased. P38 inhibition attenuated all cytokine (including IL-6) upregulation. Both P38 and ERK1/2 inhibition decreased NF-κB/p65 subunit phosphorylation. NAC pretreatment blunted IRB-induced cytokine upregulation in the diaphragm and resulted in decreased ERK1/2, P38, and NF-κB/p65 phosphorylation. In conclusion, IRB-induced cytokine upregulation in the diaphragm is under the regulatory control of MAPKs and NF-κB. IL-6 is regulated differently from all other cytokines through a P38-dependent and NF-κB independent pathway. Oxidative stress is a stimulus for IRB-induced cytokine upregulation in the diaphragm.

Aging and its effects on inflammation in skeletal muscle at rest and following exercise-induced muscle injury

The world's elderly population is expanding rapidly, and we are now faced with the significant challenge of maintaining or improving physical activity, independence, and quality of life in the elderly. Counteracting the progressive loss of muscle mass that occurs in the elderly, known as sarcopenia, represents a major hurdle in achieving these goals. Indirect evidence for a role of inflammation in sarcopenia is that markers of systemic inflammation correlate with the loss of muscle mass and strength in the elderly. More direct evidence is that compared with skeletal muscle of young people, the number of macrophages is lower, the gene expression of several cytokines is higher, and stress signaling proteins are activated in skeletal muscle of elderly people at rest. Sarcopenia may also result from inadequate repair and chronic maladaptation following muscle injury in the elderly. Macrophage infiltration and the gene expression of certain cytokines are reduced in skeletal muscle of elderly people compared with young people following exercise-induced muscle injury. Further research is required to identify the cause(s) of inflammation in skeletal muscle of elderly people. Additional work is also needed to expand our understanding of the cells, proteins, and transcription factors that regulate inflammation in the skeletal muscle of elderly people at rest and after exercise. This knowledge is critical for devising strategies to restrict sarcopenia, and improve the health of today's elderly population.

Endotoxin-Induced IL-6 Promoter Activation in Skeletal Muscle Requires an NF-κB Site

Previous studies in monocytes and other cell types have provided evidence of a role for the NF-κB pathway in IL-6 induction. The purpose of the present study was to examine the involvement of NF-κB in the induction of the IL-6 promoter in skeletal muscle cells by endotoxin (LPS), TNFα or IL-1α. Transfection of C2C12 mouse myocytes with a luciferase reporter under the control of the IL-6 promoter indicated each immunomodulator enhanced IL-6 promoter activity. Mutation and inhibitor studies indicate this response was dependent on the IL-6 NF-κB binding site, but independent of NF-IL6, AP-1, CREB or C/EBP. Cotransfection with an expression vector which constitutively activates the RelA pathway increased IL-6 promoter activity, and activity could not be further enhanced by cytokines or LPS. However, cotransfecting various dominant negative upstream NF-κB kinase expression vectors which inhibited RelA or RelB pathways either individually or in combination had no effect on LPS-induced activation of the IL-6 promoter, but abolished induction from a NF-κB-based promoter. This lack of effect was not due to a lack of NF-κB pathway activation in C2C12 myocytes because both Western analysis and EMSA supershifting showed an LPS-induced increase in nuclear RelA and RelA phosphorylation. However, another protein was observed bound to the IL-6 NF-κB site that does not bind to a consensus NF-κB site. The present findings provide novel insights regarding inflammation-induced stimulation of IL-6 promoter activity in skeletal muscle which is an important but non-traditional component of the innate immune system.

Calcineurin activates interleukin-6 transcription in mouse skeletal muscle in vivo and in C2C12 myotubes in vitro

Expression of the cytokine interleukin-6 (IL-6) by skeletal muscle is hugely increased in response to a single bout of endurance exercise, and this appears to be mediated by increases in intracellular calcium. We examined the effects of endurance exercise on IL-6 mRNA levels and promoter activity in skeletal muscle in vivo, and the role of the calcium-activated calcineurin signaling pathway on muscle IL-6 expression in vivo and in vitro. IL-6 mRNA levels in the mouse tibialis anterior (TA) were increased 2-10-fold by a single bout of treadmill exercise or by 3 days of voluntary wheel running. Moreover, an IL-6 promoter-driven luciferase transgene was activated in TA by both treadmill and wheel-running exercise and by injection with a calcineurin plasmid. Exercise also increased muscle mRNA expression of the calcineurin regulatory gene MCIP1, as did treatment of C(2)C(12) myotubes with the calcium ionophore A23187. Cotransfection of C(2)C(12) myotubes with a constitutively active calcineurin construct significantly increased while cotransfection with the calcineurin inhibitor CAIN inhibited activity of a mouse IL-6 promoter-reporter construct. Cotransfection with a myocyte enhancer-factor-2 (MEF-2) expression construct increased basal IL-6 promoter activity and augmented the effects of calcineurin cotransfection, while cotransfection with the MEF-2 antagonist MITR repressed calcineurin-activated IL-6 promoter activity in vitro. Surprisingly, cotransfection with a dominant-negative form of another calcineurin-activated transcription factor, nuclear factor activator of T cells (NFAT), greatly potentiated both basal and calcineurin-stimulated IL-6 promoter activity in C(2)C(12) myotubes. Mutation of the MEF-2 DNA binding sites attenuated, while mutation of the NFAT DNA binding sites potentiated basal and calcineurin-activated IL-6 promoter activity. Finally, CREB and C/EBP were necessary for basal IL-6 promoter activity and sufficient to increase IL-6 promoter activity but had minimal roles in calcineurin-activated IL-6 promoter activity. Together, these results suggest that IL-6 transcription in skeletal muscle cells can be activated by a calcineurin-MEF-2 axis which is antagonized by NFAT.

Interleukin-1 stimulates catabolism in C2C12 myotubes

Interleukin-1 (IL-1) is an inflammatory cytokine that has been linked to muscle catabolism, a process regulated by muscle-specific E3 proteins of the ubiquitin-proteasome pathway. To address cellular mechanism, we tested the hypothesis that IL-1 induces myofibrillar protein loss by acting directly on muscle to increase expression of two critical E3 proteins, atrogin1/muscle atrophy F-box (MAFbx) and muscle RING-finger 1 (MuRF1). Experiments were conducted using mature C2C12 myotubes to eliminate systemic cytokine effects and avoid paracrine signaling by nonmuscle cell types. Time-course protocols were used to define the sequence of cellular responses. We found that atrogin1/MAFbx mRNA and MuRF1 mRNA are elevated 60-120 min after myotube exposure to either IL-1alpha or IL-1beta. These responses are preceded by signaling events that promote E3 expression. Both IL-1 isoforms stimulate phosphorylation of p38 mitogen-activated protein kinase and stimulate nuclear factor-kappaB (NF-kappaB) signaling; I-kappaB levels fall and NF-kappaB DNA binding activity increases. Other regulators of E3 expression are unaffected by IL-1 [cytosolic oxidant activity, Forkhead-O (Foxo) activity] or respond paradoxically (AKT). Chronic exposure of C2C12 myotubes over 48 h resulted in reduced myotube width and loss of sarcomeric actin. We conclude that IL-1alpha and IL-1beta act via an oxidant- and AKT/Foxo-independent mechanism to activate p38 MAPK, stimulate NF-kappaB signaling, increase expression of atrogin1/MAFbx and MuRF1, and reduce myofibrillar protein in differentiated myotubes.

Muscle as an endocrine organ: focus on muscle-derived interleukin-6

Skeletal muscle has recently been identified as an endocrine organ. It has, therefore, been suggested that cytokines and other peptides that are produced, expressed, and released by muscle fibers and exert paracrine, autocrine, or endocrine effects should be classified as "myokines." Recent research demonstrates that skeletal muscles can produce and express cytokines belonging to distinctly different families. However, the first identified and most studied myokine is the gp130 receptor cytokine interleukin-6 (IL-6). IL-6 was discovered as a myokine because of the observation that it increases up to 100-fold in the circulation during physical exercise. Identification of IL-6 production by skeletal muscle during physical activity generated renewed interest in the metabolic role of IL-6 because it created a paradox. On one hand, IL-6 is markedly produced and released in the postexercise period when insulin action is enhanced but, on the other hand, IL-6 has been associated with obesity and reduced insulin action. This review focuses on the myokine IL-6, its regulation by exercise, its signaling pathways in skeletal muscle, and its role in metabolism in both health and disease.

Host defence peptide LL-37 induces IL-6 expression in human bronchial epithelial cells by activation of the NF-kappaB signaling pathway

LL-37, the only member of the cathelicidin family of cationic host defence peptides in humans, has been shown to mediate multiple immunomodulatory effects and as such is thought to be an important component of innate immune responses. A growing body of evidence indicates that LL-37 affects lung mucosal responses to pathogens through altered regulation of cell migration, proliferation, wound healing and cell apoptosis. These functions are consistent with LL-37 playing a role in regulating lung epithelial inflammatory responses; however, that role has not been clearly defined. In this report we have demonstrated that host defence peptide LL-37 induced cytokine (IL-6) and chemokine (CXCL-1/GRO-alpha and CXCL-8/IL-8) release from human bronchial epithelial cells. It was demonstrated that LL-37-mediated IL-6 release was time and dose dependent and that LL-37 up-regulated this pleiotropic cytokine at the transcriptional level. Using specific inhibitors it was shown that NF-kappaB signaling led to the LL-37-stimulated production of IL-6. LL-37 stimulation of airway epithelial cells activated NF-kappaB signaling, as demonstrated by the phosphorylation and degradation of Ikappa-Balpha, and consequent nuclear translocation of p65 and p50 NF-kappaB subunits. Furthermore this host defence peptide augmented flagellin-mediated cytokine production, indicating that LL-37 likely modulates Toll-like receptor 5-mediated responses.

Eccentric training impairs NF-kappaB activation and over-expression of inflammation-related genes induced by acute eccentric exercise in the elderly

The present study was aimed to investigate in elderly humans changes in NF-kappaB activation and in the expression of the inflammation-related genes inducible nitric oxide synthase (iNOS), cyclooxygenase-2 (COX-2) and interleukin-6 (IL-6) induced in peripheral blood mononuclear cells (PBMC) by acute eccentric exercise and by submaximal eccentric training. Eleven subjects, aged 66-75 years, carried out 2 bouts of eccentric exercise separated by 8 weeks of training. Following the first bout, NF-kappaB activation, and protein level of p50/p65 subunits, phospho-IkappaBalpha and phospho-IKKalpha increased, while IkappaBalpha protein level was significantly reduced. This was accompanied by a significant increase in iNOS, COX-2 and IL-6 mRNA protein level and protein content. Changes were significantly attenuated following the second exercise bout. In conclusion, acute eccentric exercise increases NF-kappaB activation and the expression of several inflammation-related genes in PBMC from elderly individuals. Regular eccentric training might be an effective method of preventing undesirable inflammatory responses induced by eccentric exercise.

Modulation of skeletal muscle antioxidant defense by exercise: Role of redox signaling

Contraction-induced production of reactive oxygen species has been shown to cause oxidative stress to skeletal muscle. As an adaptive response, muscle antioxidant defense systems are upregulated in response to exercise. Nuclear factor kappaB and mitogen-activated protein kinase are two major oxidative-stress-sensitive signal transduction pathways that have been shown to activate the gene expression of a number of enzymes and proteins that play important roles in maintenance of intracellular oxidant-antioxidant homeostasis. This mini-review will discuss the main mechanisms and gene targets for these signaling pathways during exercise and the biological significance of the adaptation.

NF-kappaB-dependent expression of the antiapoptotic factor c-FLIP is regulated by calpain 3, the protein involved in limb-girdle muscular dystrophy type 2A

Limb-girdle muscular dystrophy type 2A (LGMD2A) is a recessive genetic disorder caused by mutations in the cysteine protease calpain 3 (CAPN3) that leads to selective muscle wasting. We previously showed that CAPN3 deficiency is associated with a profound perturbation of the NF-kappaB/IkappaB alpha survival pathway. In this study, the consequences of altered NF-kappaB/IkappaB alpha pathway were investigated using biological materials from LGMD2A patients. We first show that the antiapoptotic factor cellular-FLICE inhibitory protein (c-FLIP), which is dependent on the NF-kappaB pathway in normal muscle cells, is down-regulated in LGMD2A biopsies. In muscle cells isolated from LGMD2A patients, NF-kappaB is readily activated on cytokine induction as shown by an increase in its DNA binding activity. However, we observed discrepant transcriptional responses depending on the NF-kappaB target genes. IkappaB alpha is expressed following NF-kappaB activation independent of the CAPN3 status, whereas expression of c-FLIP is obtained only when CAPN3 is present. These data lead us to postulate that CAPN3 intervenes in the regulation of the expression of NF-kappaB-dependent survival genes to prevent apoptosis in skeletal muscle. Deregulations in the NF-kappaB pathway could be part of the mechanism responsible for the muscle wasting resulting from CAPN3 deficiency.

Antioxidant signaling in skeletal muscle: A brief review

Generation of reactive oxygen species (ROS) is a ubiquitous biological phenomenon in eukaryotic cell life. During the past two decades, much attention has been paid to the detrimental effects of ROS such as oxidative stress, pathogenesis and aging. However, there is now increasing evidence and recognition that ROS are not merely damaging agents inflicting random destruction to the cell structure and function, but useful signaling molecules to regulate growth, differentiation, proliferation, and apoptosis, at least within the physiological concentration. In skeletal muscle contractile activity has been shown to upregulate antioxidant defense systems and ROS has been postulated to be essential in this adaptation. Available research data suggest that nuclear factor (NF)kappaB and mitogen-activated protein kinase (MAPK) play a critical role in the relay of oxidative stress signals to gene expression apparatus in the myocytes under a variety of physiological and pathological conditions. This mini-review will discuss the main mechanisms and gene targets for these antioxidant signaling pathways during exercise, inflammation and aging.

Local and distant burn injury alter immuno-inflammatory gene expression in skeletal muscle

BACKGROUND

Severe burn trauma mediates immune dysfunction, infection, and multiple organ dysfunction syndrome. We are investigating the immuno-inflammatory response by characterizing gene expression changes in skeletal muscle after local and distant burn injury.

METHODS

Male CD1 mice in three experimental groups, control (unburned), hind limb (local burn), and 30% total body surface area (distant burn), were killed between 6 hours and 10 days postburn; and changes in gastrocnemius muscle global gene expression were assessed using microarrays.

RESULTS

The 35 immuno-inflammatory genes are differentially expressed in both models, with an additional 20 and 30 genes specific to distant and local burn, respectively. These genes encode chemokines, oxidative-stress, complement, and defense/immune functions.

CONCLUSION

Burn mediates a common systemic response, independent of the site or extent of injury, and also specific responses to local versus distant trauma. A transcriptome profile of genes that initiate and sustain systemic inflammation has been identified.

Cytokine profiles during carrageenan-induced inflammatory hyperalgesia in rat muscle and hind paw

It is not known if a cytokine cascade develops during muscle inflammation and whether cytokines contribute to muscle inflammatory pain. We measured plasma and tissue cytokine concentrations, and behavioral responses to noxious mechanical stimuli, after inducing inflammation in the gastrocnemius muscle and the hind paw of rats. Tissue and plasma samples were taken 3, 6, or 24 h after carrageenan or saline injection into one of the 2 sites. Tumor necrosis factor alpha (TNF-alpha), interleukin (IL)-1beta, IL-6, and cytokine-induced neutrophil chemoattractant 1 (CINC-1) concentrations were measured. Hyperalgesia was present 3 h after carrageenan injection into the hind paw and muscle. The TNF-alpha was elevated significantly in the inflamed hind paw tissue (P < .001) but not in inflamed muscle tissue. IL-1beta was elevated 6 h after carrageenan injection in the hind paw tissue but only 24 h in the muscle tissue (P < .001). The IL-6 was elevated 3 h after injection in the hind paw tissue but only after 6 h in the muscle tissue (P < .01). The CINC-1 in plasma, muscle, and hind paw was elevated from 3 h to 24 h after carrageenan injection (P < .01). The release of IL-1beta and IL-6, known to mediate hyperalgesia elsewhere, is delayed in muscle inflammation compared with cutaneous inflammation, whereas TNF-alpha is not elevated during muscle inflammation.

PERSPECTIVE

The quality and mechanisms of muscle pain are different from that of cutaneous pain. So too is the pattern of cytokine release during inflammation. Inhibiting TNF-alpha is unlikely to be effective in managing inflammatory muscle pain, but other cytokines, notably IL-1beta and CINC-1, may prove useful therapeutic targets.

Multiple Toll-like receptor ligands induce an IL-6 transcriptional response in skeletal myocytes

Toll-like receptors (TLRs) comprise a critical sentinel that monitors body compartments for the presence of pathogens. Skeletal muscle expresses TLRs and responds to pathogen-associated molecular patterns (PAMPs), such as lipopolysaccharide (LPS), by mounting an innate immune response. In the present study, we used C2C12myocytes as a model system for skeletal muscle during infection. C2C12cells responded to LPS in a time frame and with a pattern of gene expression that faithfully mimicked the response of skeletal muscle to LPS in vivo. LPS from a variety of Escherichia coli serotypes stimulated IL-6 synthesis. C2C12cells expressed TLR1–7, but not TLR8 or TLR9, mRNA by RT-PCR. A synthetic tripalmitoylated cysteine-, serine-, and lysine-containing peptide (Pam) and LPS from Porphyromonas gingivalis, two TLR2 ligands, also stimulated IL-6 expression. LPS and Pam stimulated luciferase activity driven from NF-κB and IL-6 promoter-containing plasmids, and this response was blunted when the NF-κB binding site was mutated. LPS- and Pam-stimulated IL-6 expression was inhibited by the proteasome inhibitor MG-132 and the IκB kinase-2 (IKK2) inhibitor 2-[(aminocarbonyl)amino]-5-(4-fluorophenyl)-3-thiophenecarboxamide (TPCA-1). Pam-stimulated NF-κB and IL-6 promoter activities were disrupted by a dominant-negative form of TLR2, but not TLR4. Local injection of LPS or Pam into the gastrocnemius muscle stimulated IL-6 mRNA expression in the injected, but not the contralateral, muscle. The LPS- but not Pam-stimulated expression of IL-6 mRNA was blunted in skeletal muscle of mice carrying an inactivating mutation in TLR4. The data suggest that skeletal muscle and muscle cells recognize pathogen-associated molecules with specific TLRs to initiate an IL-6 transcriptional response.

Contraction-induced interleukin-6 transcription in rat slow-type muscle is partly dependent on calcineurin activation

The present work aimed at determining whether interleukin-6 (IL-6) produced by skeletal muscle during exercise is related, at least partly, to calcineurin activity. Rats were treated with two specific calcineurin inhibitors, cyclosporin A (CsA) and FK506, or vehicle (Vhl); they were then subjected to exhaustive treadmill running. Modulatory Calcineurin-Interacting Protein-1 (MCIP-1) mRNA levels, a reliable indicator of calcineurin activity, and IL-6 mRNA levels were measured by real-time RT-PCR in soleus muscles, and IL-6 protein concentration was measured in the plasma. Because low carbohydrates availability enhances IL-6 transcription through p38 Mitogen Activated Protein Kinase (MAPK) pathway, muscle glycogen content and glycaemia were measured and p38 MAPK phosphorylation was determined in skeletal muscle by western blotting. As expected, exercise induced an increase in IL-6 (P < 0.01) and MCIP-1 mRNA (P < 0.01) in soleus muscle of Vhl rats, and enhanced p38 phosphorylation and plasmatic IL-6 protein (P < 0.05). Calcineurin inhibition did not affect running time, glycemia or soleus glycogen content. CsA administration totally inhibited the exercise-induced increase in MCIP-1 mRNA (P < 0.01), blunted the IL-6 gene transcription related to muscle activity, and suppressed the changes in IL-6 protein in plasma. In addition to its inhibition of calcineurin activity, FK506 administration totally suppressed the exercise-induced IL-6 gene transcription, likely by an inhibition of p38 activation. Taken together, these results demonstrate that in addition to p38 MAPK, increased calcineurin activity is one of the signalling events involved in IL-6 gene transcription.

Depolarization-induced slow Ca2+ transients stimulate transcription of IL-6 gene in skeletal muscle cells

Contracting skeletal muscle produces and releases interleukin-6 (IL-6) in high amounts. Nevertheless, the mechanisms underlying IL-6 expression are not understood. Because inositol-1,4,5-trisphosphate (IP(3))-mediated slow Ca(2+) signals evoked by depolarization of skeletal myotubes appears to play a role in the regulation of gene expression, we examined its involvement on IL-6 transcription. With the use of semiquantitative RT-PCR, we have shown that K(+) depolarization of myotubes induces a transient increase in IL-6 mRNA level, which peaks at 3-4 h and is independent of extracellular Ca(2+). Inhibitors of IP(3)-dependent Ca(2+) signals, like 2-aminoethoxydiphenyl borate (2-APB) and U-73122, decreased activation of IL-6 gene expression as did Ca(2+) signals inhibitor BAPTA-AM, whereas ryanodine, a fast Ca(2+) transient inhibitor, had no effect on IL-6 induction. Depolarization of myotubes transiently transfected with a reporter gene construct, containing 651 bp of IL-6 promoter, induced a twofold increase in promoter activity, which was abolished by either 2-APB or U-73122 and remained unaffected after ryanodine treatment. Site-directed mutagenesis of parental construct allowed us to identify activator protein-1 and NF-kappaB sequences as regulatory elements involved in IL-6 upregulation. Our results provide evidence for involvement of IP(3)-mediated Ca(2+) signals on IL-6 transcription in skeletal muscle cells.

Muscle cytokine mRNA changes after 2.5 h of cycling: influence of carbohydrate

PURPOSE

To study the effect of carbohydrate compared to placebo ingestion on plasma cytokines and muscle cytokine mRNA following 2.5 h of intensive cycling in 15 trained cyclists.

METHODS

Fifteen trained cyclists cycled for 2.5 h at 60% Wmax on two occasions while receiving 4 mL.kg.15 min carbohydrate (6%) (CHO) or placebo (PLA) beverages in a randomized, counterbalanced design. Blood and vastus lateralis muscle biopsy samples were collected before and after exercise and 12 h postexercise and compared to samples taken from five cyclists who rested in the lab during the exercise sessions. Blood cell counts were determined, and plasma was analyzed for interleukin (IL)-6, IL-10, IL-1 receptor antagonist (ra), IL-8, cortisol, epinephrine, glucose, and insulin. Muscle was analyzed for glycogen content and relative gene expression of four cytokines, IL-6, IL-8, tumor necrosis factor (TNF) alpha, and IL-1beta, using real-time quantitative reverse transcriptase polymerase chain reaction.

RESULTS

Plasma glucose and insulin were higher, and epinephrine, cortisol, IL-6, IL-10, and IL-1ra, but not IL-8, were significantly lower postexercise in CHO versus PLA. Muscle glycogen content decreased 68% immediately postexercise and the pattern of change did not differ between CHO and PLA. Muscle IL-6, IL-8, TNF-alpha, but not IL-1beta mRNA increased immediately postexercise compared to controls, with no differences between CHO and PLA.

CONCLUSION

CHO compared to PLA beverage ingestion attenuated the increase in plasma cortisol, epinephrine, IL-6, IL-10, and IL-1ra, but not muscle IL-6, IL-8, and TNF-alpha mRNA in athletes cycling 2.5 h at 60% Wmax.

Expression and activity of C/EBPbeta and delta are upregulated by dexamethasone in skeletal muscle

The influence of glucocorticoids on the expression and activity of the transcription factors CCAAT/enhancer binding protein (C/EBP)beta and delta in skeletal muscle was examined by treating rats or cultured L6 myotubes with dexamethasone. Treatment of rats with 10 mg/kg of dexamethasone resulted in increased C/EBPbeta and delta DNA binding activity in the extensor digitorum longus muscle as determined by electrophoretic mobility shift assay (EMSA) and supershift analysis. A similar response was noticed in dexamethasone-treated myotubes. In other experiments, myocytes were transfected with a plasmid containing a promoter construct consisting of multiple C/EBP binding elements upstream of a luciferase reporter gene. Treatment of these cells with dexamethasone resulted in a fourfold increase in luciferase activity, suggesting that glucocorticoids increase C/EBP-dependent gene activation in muscle cells. In addition, dexamethasone upregulated the protein and gene expression of C/EBPbeta and delta in the myotubes in a time- and dose-dependent fashion as determined by Western blotting and real-time PCR, respectively. The results suggest that glucocorticoids increase C/EBPbeta and delta activity and expression through a direct effect in skeletal muscle.

Regulation of IkappaB kinase and NF-kappaB in contracting adult rat skeletal muscle

Nuclear factor-kappaB (NF-kappaB) is a transcription factor with important roles in regulating innate immune and inflammatory responses. NF-kappaB is activated through the phosphorylation of its inhibitor, IkappaB, by the IkappaB kinase (IKK) complex. Physical exercise elicits changes in skeletal muscle gene expression, yet signaling cascades and transcription factors involved remain largely unknown. To determine whether NF-kappaB signaling is regulated by exercise in vivo, rats were run on a motorized treadmill for 5-60 min. Exercise resulted in up to twofold increases in IKKalpha/beta phosphorylation in the soleus and red gastrocnemius muscles throughout the time course studied. In red gastrocnemius muscles, NF-kappaB activity increased 50% 1-3 h after 60 min of treadmill exercise, returning to baseline by 5 h. Contraction of isolated extensor digitorum longus muscles in vitro increased IKKalpha/beta phosphorylation sevenfold and this was accompanied by a parallel increase in IkappaBalpha phosphorylation. Additional kinases that are activated by exercise include p38, extracellular-signal regulated protein kinase (ERK), and AMP-activated protein kinase (AMPK). Inhibitors of p38 (SB-203580) and ERK (U-0126) blunted contraction-mediated IKK phosphorylation by 39 +/- 4% (P = 0.06) and 35 +/- 10% (P = 0.09), respectively, and in combination by 76 +/- 5% (P < 0.05), suggesting that these kinases might influence the activation of IKK and NF-kappaB during exercise. In contrast, 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside, an activator of AMPK, had no effect on either IKK or NF-kappaB activity. In conclusion, acute submaximal exercise transiently stimulates NF-kappaB signaling in skeletal muscle. This activation is a local event because it can occur in the absence of exercise-derived systemic factors.

Expression of mitogen-activated protein kinase phosphatase 1, a negative regulator of the mitogen-activated protein kinases, in rheumatoid arthritis: up-regulation by interleukin-1beta and glucocorticoids

OBJECTIVE

Mitogen-activated protein kinases (MAPKs) are activated by proinflammatory stimuli. MAPK phosphatases (MKPs), in particular MKP-1, have been identified as endogenous negative regulators of MAPK activation. Since MAPKs are known to be important in rheumatoid arthritis (RA) synoviocyte activation, this study assessed the expression, regulation, and function of MKP-1 in RA.

METHODS

MKP-1 expression was measured by Western blotting (WB) and real-time polymerase chain reaction (PCR). RA fibroblast-like synoviocytes (FLS) were treated with interleukin-1beta (IL-1beta), tumor necrosis factor alpha, fetal calf serum, and dexamethasone. Expression of MAPKs in RA FLS was analyzed by WB using phosphospecific antibodies, while IL-6 expression was assessed by real-time PCR.

RESULTS

MKP-1 protein and messenger RNA were detected in cultured RA FLS. IL-1beta rapidly up-regulated MKP-1, coinciding with reciprocal down-regulation of ERK, JNK, and p38 MAPK phosphorylation. Dexamethasone rapidly and sustainably up-regulated MKP-1, and this also coincided with down-regulation of ERK, JNK, and p38 MAPK phosphorylation. In addition, dexamethasone augmented IL-1beta-induced up-regulation of MKP-1, and this was associated with inhibition of ERK, JNK, and p38 MAPK phosphorylation and IL-6 expression. Dexamethasone had no effect on the phosphorylation of upstream kinases such as MEKK-3/6. In the presence of glucocorticoid (GC) receptor antagonist RU 486, the dexamethasone-mediated up-regulation of MKP-1 was impaired. Moreover, inhibition of MKP-1 expression impaired dexamethasone-mediated inhibition of MAPK phosphorylation.

CONCLUSION

This study demonstrates the expression of MKP-1 in RA FLS. Cytokine and GC regulation of MKP-1 may be important in determining the magnitude of the inflammatory response in RA that is mediated via MAPKs. The effects of GCs in RA may be mediated, in part, via GC receptor-dependent up-regulation of MKP-1.

Altering dietary nutrient intake that reduces glycogen content leads to phosphorylation of nuclear p38 MAP kinase in human skeletal muscle: association with IL-6 gene transcription during contraction

To determine the effect of glycogen availability and contraction on intracellular signaling and IL-6 gene transcription, eight males performed 60 min of exercise on two occasions: either with prior ingestion of a normal (Con) or low carbohydrate (LCHO) diet that reduced pre-exercise muscle glycogen content. Muscle biopsies were obtained and analyzed for IL-6 mRNA. In addition, nuclear proteins were isolated from the samples and analyzed for the mitogen- activated protein kinases (MAPK) c-jun amino-terminal kinase (JNK) 1 and 2 and p38 MAPK. Nuclear fractions were also analyzed for the phosphorylated forms of JNK (p-JNK) and p38 MAPK (p-p38 MAPK) and the abundance of the nuclear transcription factors nuclear factor of activated T cells (NFAT) and nuclear factor kappa-beta (NF-kappabeta). No differences were observed in the protein abundance of total JNK 1/2, p38 MAPK, NFAT, or NF-kappabeta before exercise, but the nuclear abundance of p-p38 MAPK was higher (P<0.05) in LCHO. Contraction resulted in an increase (P<0.05) in nuclear p-JNK 1/2, but there were no differences when comparing CON with LCHO. The fold increase in IL-6 mRNA with contraction was potentiated (P<0.05) in LCHO. A correlation between pre-exercise nuclear phosphorylated p38 MAPK and contraction-induced fold increase in IL-6 mRNA was performed, revealing a highly significant correlation (r=0.96; P<0.01). We next incubated L6 myotubes in ionomycin (a compound known to induce IL-6 mRNA) with or without the pyridinylimidazole p38 MAPK inhibitor SB203580. Treatments did not affect total nuclear p38 MAPK, but ionomycin increased (P<0.05) both nuclear p-p38 MAPK and IL-6 mRNA. The addition of SB203580 to ionomycin decreased (P<0.05) nuclear p-p38 MAPK and totally abolished (P<0.05) the ionomycin- induced increase in IL-6 mRNA. These data suggest that reduced carbohydrate intake that results in low intramuscular glycogen leads to phosphorylation of p38 MAPK at the nucleus. Furthermore, phosphorylation of p38 MAPK in the nucleus appears to be an upstream target for IL-6, providing new insights into the regulation of IL-6 gene transcription.

Lipopolysaccharide and proinflammatory cytokines stimulate interleukin-6 expression in C2C12 myoblasts: role of the Jun NH2-terminal kinase

IL-6 is a major inflammatory cytokine that plays a central role in coordinating the acute-phase response to trauma, injury, and infection in vivo. Although IL-6 is synthesized predominantly by macrophages and lymphocytes, skeletal muscle is a newly recognized source of this cytokine. IL-6 from muscle spills into the circulation, and blood-borne IL-6 can be elevated >100-fold due to exercise and injury. The purpose of the present study was to determine whether inflammatory stimuli, such as LPS, TNF-alpha, and IL-1beta, could increase IL-6 expression in skeletal muscle and C2C12 myoblasts. Second, we investigated the role of mitogen-activated protein (MAP) kinases, and the Jun NH2-terminal kinase (JNK) in particular, as a mediator of this response. Intraperitoneal injection of LPS in mice increased the circulating concentration of IL-6 from undetectable levels to 4 ng/ml. LPS also increased IL-6 mRNA 100-fold in mouse fast-twitch skeletal muscle. Addition of LPS, IL-1beta, or TNF-alpha directly to C2C12 myoblasts increased IL-6 protein (6- to 8-fold) and IL-6 mRNA (5- to 10-fold). The response to all three stimuli was completely blocked by the JNK inhibitor SP-600125 but not as effectively by other MAP kinase inhibitors. SP-600125 blocked LPS-stimulated IL-6 synthesis dose dependently at both the RNA and protein level. SP-600125 was as effective as the synthetic glucocorticoid dexamethasone at inhibiting IL-6 expression. SP-600125 inhibited IL-6 synthesis when added to cells up to 60 min after LPS stimulation, but its inhibitory effect waned with time. LPS stimulated IL-6 mRNA in both myoblasts and myotubes, but myoblasts showed a proportionally greater LPS-induced increase in IL-6 protein expression compared with myotubes. SP-600125 and the proteasomal inhibitor MG-132 blocked LPS-induced degradation of IkappaB-alpha/epsilon and LPS-stimulated expression of IkappaB-alpha mRNA. Yet, only SP-600125 and not MG-132 blocked LPS-induced IL-6 mRNA expression. This suggests that IL-6 gene expression is a downstream target of JNK in C2C12 myoblasts.