Skeletal muscle as an immunogenic organ

Available online at www.sciencedirect.com

Skeletal muscle as an immunogenic organ Søren Nielsen and Bente Klarlund Pedersen During the past few years, a possible link between skeletal muscle contractile activity and immune changes has been established. This concept is based on the finding that exercise provokes an increase in a number of cytokines. We have suggested that cytokines and other peptides that are produced; expressed and released by muscle fibers and exert either paracrine or endocrine effects should be classified as ‘myokines’. Human skeletal muscle has the capacity to express several myokines belonging to distinct different cytokine classes and contractile activity plays a role in regulating the expression of cytokines in skeletal muscle. In the present review, we focus on the myokines interleukin (IL)-6, IL-8 and IL-15 and their possible anti-inflammatory, immunoregulatory and metabolic roles. Address The Centre of Inflammation and Metabolism, Department of Infectious Diseases and CMRC, Faculty of Health Sciences, University of Copenhagen, Rigshospitalet 7641, Blegdamsvej 9, DK-2100 Copenhagen, Denmark Corresponding author: Pedersen, Bente ([email protected])

Current Opinion in Pharmacology 2008, 8:346–351 This review comes from a themed issue on Musculoskeletal Edited by Martin Hohenegger Available online 15th April 2008 1471-4892/$ – see front matter # 2008 Elsevier Ltd. All rights reserved. DOI 10.1016/j.coph.2008.02.005

Introduction Research through the past 20 years has demonstrated that exercise induces considerable changes in the immune system. The interactions between exercise and the immune system provide a unique opportunity to evaluate the role of underlying endocrine and cytokine mechanisms [1]. During the past few years, the identification of skeletal muscle as a cytokine-producing organ has led to the discovery that muscle-derived cytokines may account not only for exercise-associated immune changes, but that these muscle-derived cytokines play a role in mediating some of the exercise-associated metabolic changes, as well as the metabolic changes following training adaptation [2].

Muscle-derived interleukin (IL)-6 was the first myokine to be discovered. However, skeletal muscles may produce and express cytokines belonging to distinctly different families. Thus, skeletal muscle has the capacity to express, for example, IL-6, IL-8 and IL-15, and muscle contractions play a regulatory role in the muscular expression of these cytokines. The present review focuses on myokines, their regulation by exercise and their possible anti-inflammatory, immunoregulatory and metabolic roles.

Interleukin-6 IL-6 belongs to the IL-6 family of cytokines, which are characterized by their related structure and their common use of the gp130 [4]. The effects of exercise

A marked increase in circulating levels of IL-6 after prolonged exercise is a remarkably consistent finding. The level of circulating IL-6 increases in an exponential fashion (up to 100-fold) in response to exercise, and declines in the post-exercise period. The increase of IL-6 is independent on concomitant muscle damage and the magnitude by which plasma-IL-6 increases is related to exercise duration, intensity, the muscle mass involved in the mechanical work, and the endurance capacity. The IL-6 mRNA is upregulated in contracting skeletal muscle [3] and the transcriptional rate of the IL-6 gene is markedly enhanced by exercise, and especially so, if muscle glycogen levels are low [5]. It has also been demonstrated that the IL-6 protein is expressed in muscle fibers post exercise [6], and that IL-6 is released from skeletal muscle during exercise [7]. Exercise was further found to increase IL-6 receptor production in human skeletal muscle, suggesting a possible post-exercise sensitizing mechanism to IL-6 [8]. It has been suggested that IL-6 acts as autocrine factor upregulating its mRNA levels, thereby supporting its function as an exerciseactivated factor in skeletal muscle cells [9]. Unlike IL-6 signaling in macrophages, intramuscular IL-6 expression is regulated by a network of signaling events that among other pathways are likely to involve the production of NO [10] and Ca2+ [11] following muscle contraction and p38 MAPK pathways regarding reduction of the glycogen content [12]. The immune-regulatory effects of IL-6

Recently, we suggested that cytokines and other peptides that are produced, expressed, and released by muscle fibers and exert either paracrine or endocrine effects should be classified as ‘myokines’ [3]. Current Opinion in Pharmacology 2008, 8:346–351

The lymphocyte concentration in blood increases during exercise but falls below pre-exercise values following severe long-duration exercise [13]. It appears that the immediate acute exercise effect on lymphocytes is www.sciencedirect.com

Muscle and myokines Nielsen and Pedersen 347

mediated by catecholamines, in particular adrenaline, whereas the post-exercise reduction in lymphocyte number is mediated by both adrenaline and cortisol. The latter is of particular interest, if the exercise is of long duration [14]. We have suggested that the exercise-induced increase in cortisol is mediated by IL-6. Support for this proposal is based on a study from our group in which IL-6 was infused into normal healthy volunteers in low concentrations to mimic the exercise effect on the plasma concentration of IL-6. Interleukin-6 induced increased levels of plasma cortisol and, consequently, an increase in circulating neutrophils and a decrease in the lymphocyte number without effects on plasma epinephrine, body temperature, mean arterial pressure, or heart rate [15]. The link between exercise-induced lymphocyte changes and an effect of IL-6 on cortisol production is supported further by studies demonstrating that carbohydrate loading during exercise attenuates the exercise effect on IL-6, lymphocyte number and function as well as cortisol [16,17], and that anti-oxidant treatment blunts both the release of IL-6 from contracting human muscle as well as the exercise-induced increase in plasma–cortisol levels [18]. The anti-inflammatory effects of IL-6

Interleukin-6 is most often classified as a pro-inflammatory cytokine, although data also suggest that IL-6 and IL-6-regulated acute phase proteins are anti-inflammatory and immuno-suppressive, and may negatively regulate the acute phase response [19]. The exerciseinduced increase in plasma IL-6 is followed by increased circulating levels of well-known anti-inflammatory cytokines such as IL-1ra and IL-10 [20], and infusion of IL-6 to healthy donors replicates the exercise cytokine response and enhances systemic levels of cortisol [15]. Furthermore, both exercise and IL-6 infusion suppress TNF-a production in humans [21]. After adjustment for multiple confounders, including IL-6, high plasma TNF concentrations are associated with insulin resistance [22] and evidence for a direct role of TNF-a in insulin resistance in humans in vivo has been obtained [23]. It is likely that the anti-inflammatory effects of exercise, mediated by muscle-derived IL-6, in part may protect against TNF-induced insulin resistance. Metabolic effects

The metabolic effects of IL-6 are controversial [24]. On one hand, IL-6 is markedly produced and released in the post-exercise period when insulin action is enhanced, but, on the other hand, IL-6 has been associated with obesity and reduced insulin action. The idea of IL-6 being a ‘bad guy’ with regard to metabolic actions is primarily based on (1) correlations in cohort studies; (2) www.sciencedirect.com

animal studies, neglecting that mouse and human IL-6 exhibit only 42% protein sequence identity, and (3) in vitro cell culture studies of supraphysiological concentrations of IL-6. Our research challenge the generally held view of IL-6 based on both in vitro and in human in vivo studies. While IL-6 appears to play a role in endogenous glucose production (EGP) during muscular activity in humans, its action on the liver is totally dependent on other muscle contraction-induced factors [25]. At resting conditions, acute IL-6 administration at physiological concentrations does not impair wholebody glucose disposal, net leg-glucose uptake, or endogenous glucose production in resting healthy young humans [26]. In patients with type 2 diabetes, plasma insulin decreases in response to IL-6 infusion without a corresponding increase of the hepatic glucose production. Recently, we demonstrated that IL-6 may increase glucose infusion rate and glucose oxidation without changes in EGP during a hyperinsulinemic euglycemic clamp in healthy humans [27]. These data are in contrast to observations reported in mice [24], suggesting that the effects of IL-6 on hepatic insulin sensitivity observed in murine models in vivo may not be similar in humans. The finding of an insulin-sensitizing effect of IL-6, in conditions where EGP was completely suppressed, underlines that the main effect of IL-6 on insulin-stimulated glucose metabolism in humans is likely to occur in peripheral tissues (e.g. skeletal muscle and adipose tissue), whereas IL-6 does not influence glucose output from the liver. Infusion of rhIL-6 into healthy humans to obtain physiological concentrations of IL-6 increases lipolysis in the absence of hypertriacylglyceridemia or changes in catecholamines, glucagon, insulin or any adverse effects [28]. These findings identify IL-6 as a novel lipolytic factor. Blocking IL-6 in clinical trials with patients with rheumatoid arthritis leads to enhanced cholesterol and plasma glucose levels, indicating that functional lack of IL-6 may lead to insulin resistance and an atherogenic lipid profile rather than the opposite [29,30]. In accordance, IL-6KO mice develop late onset obesity and impaired glucose tolerance [31]. In vivo, experiments demonstrated that IL-6 may increase basal and insulin-stimulated glucose uptake via an increased GLUT4 translocation [27]. Recent evidence suggests a link between IL-6 and AMP-activated protein kinase (AMPK). AMPK activation stimulates fatty acid oxidation and increases glucose uptake [32] and IL-6 was shown to enhance AMPK activity in both skeletal muscle and adipose tissue [33] and, more recently, the effects of IL-6 on enhanced glucose uptake and fatty acid oxidation in skeletal myotubes were abolished in cells infected with an AMPK dominant negative construct [27]. Interestingly, while IL-6 activates AMPK activity, evidence exists that TNF-alpha blocks AMPK signaling [34]. Current Opinion in Pharmacology 2008, 8:346–351

348 Musculoskeletal

Muscle growth

Skeletal muscles adapt to increasing workload by augmenting their fiber size, through mechanisms that are poorly understood. It has previously been shown that IL-6 expression is induced in C2C12 differentiating myoblasts [35]. In addition, a recent study identified the cytokine interleukin-6 (IL-6) as an essential regulator of satellite cell (muscle stem cell)-mediated hypertrophic muscle growth [36]. In summary, IL-6 is expressed by and released from contracting human skeletal muscle, and has anti-inflammatory, immunoregulatory, metabolic and hypertrophic effects in humans in vivo (Figure 1). The effect of IL-6 on insulin-stimulated glucose disposal and fatty acid oxidation in humans in vivo appears to be mediated via activation of AMPK.

Interleukin-8 IL-8 is a known chemokine that attracts primarily neutrophils. In addition to its chemokine properties, IL-8 acts as an angiogenic factor [37]. The plasma concentration of IL-8 increases in response to exhaustive exercise such as running, which involves eccentric muscle contractions [38,39]. Concentric exercise on the other hand, such as bicycle ergometry [40] or rowing [41] of moderate intensity, does not increase plasma IL-8 concentration. In a study by Nieman and co-workers, a several-fold increase in IL-8 mRNA was found in skeletal

muscle biopsies from subjects having completed a three-hour-treadmill-run concomitant with increased plasma levels of IL-8 [39]. Similarly, IL-8 mRNA increased in response to 1 h of cycle ergometry exercise, but with no change in the plasma concentration of IL-8 [40]. We found that IL-8 protein was clearly expressed in human skeletal muscle as a response to concentric exercise [42]. The finding of a marked increase of IL-8 mRNA in muscle biopsies during and following exercise and IL-8 protein expression within skeletal muscle fibers in the recovery from exercise strongly indicates that exercise per se stimulates muscle cells to produce IL-8. This is in accordance with the finding that muscle cells in vitro have the capacity to express IL-8, both at the mRNA and protein levels [43]. The physiological function of IL-8 within the muscle is still unknown. The main part of the systemic increase in IL-8 as seen during exercise with an eccentric component is most likely because of an inflammatory response. In accordance with this, we and other authors observe no increase in the systemic IL-8 plasma concentration during or after concentric exercise [39,40–42]. However, when measuring the arterio-venous concentration difference across a concentrically exercising limb, we detect a small and transient net release of IL-8, which does not result in an increase in the systemic IL-8 plasma concentration [42]. That a high local IL-8 expression takes place in working muscle with only a small and transient release could indicate that muscle-derived IL-8 acts locally and

Figure 1

A schematic presentation of the biological effects of interleukin (IL)-6, IL-8 and IL-15. Current Opinion in Pharmacology 2008, 8:346–351

www.sciencedirect.com

Muscle and myokines Nielsen and Pedersen 349

exerts its effect in an endocrine or paracrine fashion. A plausible function of the muscle-derived IL-8 would be chemo attraction of neutrophils and macrophages when, in fact, in concentric exercise there is little or no accumulation of neutrophils or macrophages in skeletal muscle. A more likely function of muscle-derived IL-8 is to stimulate angiogenesis. IL-8 associates with the CXC receptor 1 and 2 (CXCR1 and CXCR2). It induces its chemotactic effects via CXCR1, whereas CXCR2, which is expressed by human microvascular endothelial cells, is the receptor responsible for IL-8-induced angiogenesis [44]. Skeletal muscle CXCR2 mRNA increased in response to bicycle exercise in the post-exercise period when compared to pre-exercise samples. The increase in CXCR2 protein was seen in the vascular endothelium, and also mildly within the muscle fibres [45]. In summary, the finding that a high local IL-8 expression takes place in working muscle with only a small and transient release indicates that muscle-derived IL-8 has no systemic immunological effects, but is likely to exert its effect locally, for example stimulating angiogenesis through CXCR2 receptor signaling [45] (Figure 1).

Interleukin-15 In a recent study, we found that the level of IL-15 mRNA was higher in human skeletal muscles dominated by type 2 muscle fibres than in muscles dominated by type 1 muscle fibres, and that IL-15 mRNA content increased 24 hours following a bout of resistance exercise [46]. The increase in IL-15 mRNA levels was not accompanied by an increase in muscular IL-15 protein expression, visualized by Western blot and immunohistochemistry, neither was plasma IL-15 protein increased after a bout of resistance exercise. This was in contrast to Riechman et al. [47] who demonstrated a small, but significant increase in plasma IL-15 protein following resistance exercise. They found an increase in plasma IL-15 of approximately 5% immediately after the end of a resistance exercise bout, but did not investigate later time points. A third study in humans, in which biopsies were obtained immediately after the end of exercise, demonstrated no change in IL15 mRNA level. However, the strength training in this study influenced vastus lateralis to only a minor degree [48], possibly explaining the discrepancy. IL-15 has anabolic effect on muscle cell culture and decreases the muscle degradation rate in a cachexia model, suggesting that IL-15 might be of importance in muscle growth [49]. Furthermore, IL-15 has been suggested to play a role in muscle-adipose tissue interaction [50]. In summary, it is not known if IL-15 is a player in exercise-induced immunoregulation. However, the findings that IL-15 is constitutively expressed by skeletal muscle and regulated by strength training, that IL-15 has anabolic effects and seems to play a role in www.sciencedirect.com

reducing adipose tissue mass, make us suggest that IL-15 may play a role in muscle-fat cross talk (Figure 1).

Conclusion The recent identification of skeletal muscle as an immunogenic organ that produces and releases myokines, expands our knowledge on how the nervous, endocrine and immune systems contribute to the maintenance of homeostasis, also when challenged by physiological demands. Myokines appear to have important local effects within the muscle, including effects on metabolism, angiogenesis and muscle growth. However, during conditions with elevated cytokines, such as sepsis and trauma, high circulating levels of, for example, IL-6 and IL-15 are associated with impaired metabolism and cachexia, rather than the opposite, suggesting that, for example, IL-6-induced hypertrophy requires the secretion of IL-6 from myotubes. Our discovery of contracting muscle as a cytokine producing organ opens for a whole new paradigm: Skeletal muscle can be viewed as both an immunogenic and an endocrine organ, which by contraction stimulates the production and release of cytokines that can influence metabolism and modify cytokine production in tissues and organs.

Acknowledgements The Centre of Inflammation and Metabolism is supported by a grant from the Danish National Research Foundation (# 02-512-55). Our research was further supported by the Danish Medical Research Council and the Commission of the European Communities (contract no. LSHM-CT-2004005272 EXGENESIS). The Copenhagen Muscle Research Centre is supported by grants from the Capital Region of Denmark and the University of Copenhagen.

References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as:  of special interest  of outstanding interest 1.

Pedersen BK, Hoffman- Goetz L: Exercise and the immune system: regulation, integration and adaption. Physiol Rev 2000, 80:1055-1081.

2.

Febbraio MA, Pedersen BK: Muscle-derived interleukin-6: mechanisms for activation and possible biological roles. FASEB J 2002, 16:1335-1347.

3.

Pedersen BK, Akerstrom TC, Nielsen AR, Fischer CP: Role of myokines in exercise and metabolism. J Appl Physiol 2007, 103:1090-1093.

4.

Kamimura D, Ishihara K, Hirano T: IL-6 signal transduction and its physiological roles: the signal orchestration model. Rev Physiol Biochem Pharmacol 2003, 149:1-38.

5.

Keller C, Steensberg A, Pilegaard H, Osada T, Saltin B, Pedersen BK, Neufer PD: Transcriptional activation of the IL-6 gene in human contracting skeletal muscle: influence of muscle glycogen content. FASEB J 2001, 15:2748-2750.

6.

Hiscock N, Chan MH, Bisucci T, Darby IA, Febbraio MA: Skeletal myocytes are a source of interleukin-6 mRNA expression and protein release during contraction: evidence of fiber type specificity. FASEB J 2004, 18:992-994. Current Opinion in Pharmacology 2008, 8:346–351

350 Musculoskeletal

7.

Steensberg A, van HG, Osada T, Sacchetti M, Saltin B, Klarlund PB: Production of interleukin-6 in contracting human skeletal muscles can account for the exercise-induced increase in plasma interleukin-6. J Physiol 2000, 529(Pt 1): 237-242.

8.

Keller P, Penkowa M, Keller C, Steensberg A, Fischer CP, Giralt M, Hidalgo J, Pedersen BK: Interleukin-6 receptor expression in contracting human skeletal muscle: regulating role of IL-6. FASEB J 2005, 19:1181-1183.

9. 

Weigert C, Dufer M, Simon P, Debre E, Runge H, Brodbeck K, Haring HU, Schleicher ED: Upregulation of IL-6 mRNA by IL-6 in skeletal muscle cells: role of IL-6 mRNA stabilization and Ca2+dependent mechanisms. Am J Physiol Cell Physiol 2007, 293:C1139-C1147. The authors provide evidence for the hypothesis that IL-6 upregulates and maintains its own production. Pharmacological activation of AMPK upregulated IL-6 mRNA expression, which was blocked by knockdown of AMPK alpha(1) and alpha(2) using small, interfering RNA (siRNA) oligonucleotides. However, the self-stimulatory effect of IL-6 was shown to be independent of AMPK. The self-stimulatory effect of IL-6 partly involves a Ca(2+)-dependent pathway.

10. Steensberg A, Keller C, Hillig T, Frosig C, Wojtaszewski JF,  Pedersen BK, Pilegaard H, Sander M: Nitric oxide production is a proximal signaling event controlling exercise-induced mRNA expression in human skeletal muscle. FASEB J 2007, 21:2683-2694. Using both NO synthase inhibition (nitro-L-arginine methyl ester, L-NAME) as well as infusion of the NO donor nitroglycerin, the authors provide evidence that NO production contributes to regulation of gene expression in muscle during exercise. 11. Keller C, Hellsten Y, Steensberg A, Pedersen BK: Differential regulation of IL-6 and TNF-alpha via calcineurin in human skeletal muscle cells. Cytokine 2006, 36:141-147. 12. Chan MH, McGee SL, Watt MJ, Hargreaves M, Febbraio MA: 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. FASEB J 2004, 18:1785-1787. 13. McCarthy DA, Dale MM: The leucocytosis of exercise. A review and model. Sports Med 1988, 6:333-363. 14. Lindgarde F, Eriksson KF, Lithell H, Saltin B: Coupling between dietary changes, reduced body weight, muscle fibre size and improved glucose tolerance in middle-aged men with impaired glucose tolerance. Acta Med Scand 1982, 212:99-106. 15. Steensberg A, Fischer CP, Keller C, Moller K, Pedersen BK: IL-6 enhances plasma IL-1ra, IL-10, and cortisol in humans. Am J Physiol Endocrinol Metab 2003, 285:E433-E437. 16. Nieman DC, Fagoaga OR, Butterworth DE, Warren BJ, Utter A, Davis JM, Henson DA, Nehlsen-Canarella SL: Carbohydrate supplementation affects blood granulocyte and monocyte trafficking but not function after 2.5 h of running. J Appl Physiol 1997, 82:1385-1394. 17. Nehlsen Cannarella SL, Fagoaga OR, Nieman DC, Henson DA, Butterworth DE, Schmitt RL, Bailey EM, Warren BJ, Utter A, Davis JM: Carbohydrate and the cytokine response to 2,5 h of running. J Appl Physiol 1997, 82:1662-1667. 18. Fischer CP, Hiscock N, Basu S, Vessby B, Kallner A, Sjo¨berg LB, Febbraio MA, Pedersen BK: Supplementation with vitamins C and E inhibits the release of interleukin-6 from contracting human skeletal muscle. J Physiol 2004, 558:633-645.

Petersen AM et al.: Associations between insulin resistance and TNF-alpha in plasma, skeletal muscle and adipose tissue in humans with and without type 2 diabetes. Diabetologia 2007, 50:2562-2571. 23. Plomgaard P, Bouzakri K, Krogh-Madsen R, Zierath JR,  Pedersen BK: TNF-alpha induces skeletal muscle insulin resistance in healthy human subjects via inhibition of AS160 phosphorylation. Diabetes 2005, 54:2939-2945. This study demonstrates that TNF-alpha infusion in healthy humans induces insulin resistance in skeletal muscle, without effect on endogenous glucose production, as estimated by a combined euglycemic insulin clamp and stable isotope tracer method. A molecular link between lowgrade systemic inflammation and the metabolic syndrome established. 24. Pedersen BK, Febbraio MA: Point: Interleukin-6 does have a beneficial role in insulin sensitivity and glucose homeostasis. J Appl Physiol 2007, 102:814-816. 25. Febbraio MA, Hiscock N, Sacchetti M, Fischer CP, Pedersen BK: Interleukin-6 is a novel factor mediating glucose homeostasis during skeletal muscle contraction. Diabetes 2004, 53:1643-1648. 26. Steensberg A, Fischer CP, Sacchetti M, Keller C, Osada T, Schjerling P, van Hall G, Febbraio MA, Pedersen BK: Acute interleukin-6 administration does not impair muscle glucose uptake or whole body glucose disposal in healthy humans. J Physiol 2003, 548:631-638. 27. Carey AL, Steinberg GR, Macaulay SL, Thomas WG, Holmes AG,  Ramm G, Prelovsek O, Hohnen-Behrens C, Watt MJ, James DE et al.: IL-6 increases insulin stimulated glucose disposal in humans and glucose uptake and fatty acid oxidation in vitro via AMPK. Diabetes 2006, 55:2688-2697. The study demonstrates that acute IL-6 treatment enhances insulinstimulated glucose disposal in humans in vivo, while the effects of IL-6 on glucose and fatty acid metabolism in vitro appear to be mediated by AMPK. 28. van Hall G, Steensberg A, Sacchetti M, Fischer C, Keller C, Schjerling P, Hiscock N, Moller K, Saltin B, Febbraio MA et al.: Interleukin-6 stimulates lipolysis and fat oxidation in humans. J Clin Endocrinol Metab 2003, 88:3005-3010. 29. Nishimoto N, Yoshizaki K, Miyasaka N, Yamamoto K, Kawai S, Takeuchi T, Hashimoto J, Azuma J, Kishimoto T: Treatment of rheumatoid arthritis with humanized anti-interleukin-6 receptor antibody: a multicenter, double-blind, placebocontrolled trial. Arthritis Rheum 2004, 50:1761-1769. 30. Choy EH, Isenberg DA, Garrood T, Farrow S, Ioannou Y, Bird H, Cheung N, Williams B, Hazleman B, Price R et al.: Therapeutic benefit of blocking interleukin-6 activity with an antiinterleukin-6 receptor monoclonal antibody in rheumatoid arthritis: a randomized, double-blind, placebo-controlled, dose-escalation trial. Arthritis Rheum 2002, 46:3143-3150. 31. Wallenius V, Wallenius K, Ahren B, Rudling M, Carlsten H, Dickson SL, Ohlsson C, Jansson JO: Interleukin-6-deficient mice develop mature-onset obesity. Nat Med 2002, 8:75-79. 32. Kahn BB, Alquier T, Carling D, Hardie DG: AMP-activated protein kinase: ancient energy gauge provides clues to modern understanding of metabolism. Cell Metab 2005, 1:15-25. 33. Kelly M, Keller C, Avilucea PR, Keller P, Luo Z, Xiang X, Giralt M, Hidalgo J, Saha AK, Pedersen BK: AMPK activity is deiminished in tissues of the IL-6 knockout mice: the effect of exercise. Biochem Biophys Res Commun 2004, 320:449-454.

21. Starkie R, Ostrowski SR, Jauffred S, Febbraio M, Pedersen BK: Exercise and IL-6 infusion inhibit endotoxin-induced TNFalpha production in humans. FASEB J 2003, 17:884-886.

34. Steinberg GR, Michell BJ, van Denderen BJ, Watt MJ, Carey AL,  Fam BC, Andrikopoulos S, Proietto J, Gorgun CZ, Carling D et al.: Tumor necrosis factor alpha-induced skeletal muscle insulin resistance involves suppression of AMP-kinase signaling. Cell Metab 2006, 4:465-474. The authors demonstrate that TNF-alpha signaling through TNF receptor (TNFR) 1 suppresses AMPK activity via transcriptional upregulation of protein phosphatase 2C (PP2C). In addition, they provide evidence that AMPK is an important TNF-alpha signaling target and is a contributing factor to the suppression of fatty acid oxidation and the development of lipid-induced insulin resistance in obesity.

22. Plomgaard P, Nielsen AR, Fischer CP, Mortensen OH, Broholm C, Penkowa M, Krogh-Madsen R, Erikstrup C, Lindegaard B,

35. Baeza-Raja B, Munoz-Canoves P: p38 MAPK-induced nuclear factor-kappaB activity is required for skeletal

19. Pedersen BK: The anti-inflammatory effect of exercise: its role in diabetes and cardiovascular disease control. Essays Biochem 2006, 42:105-117. 20. Ostrowski K, Schjerling P, Pedersen BK: Physical activity and plasma interleukin-6 in humans – effect of intensity of exercise. Eur J Appl Physiol 2000, 83:512-515.

Current Opinion in Pharmacology 2008, 8:346–351

www.sciencedirect.com

Muscle and myokines Nielsen and Pedersen 351

muscle differentiation: role of interleukin-6. Mol Biol Cell 2004, 15:2013-2026. 36. Serrano AL, Baeza-Raja B, Perdiguero E, Jardi M, MunozCanoves P: Interleukin-6 is an essential regulator of satellite cell-mediated skeletal muscle hypertrophy. Cell Metab 2008, 7:33-44. 37. Baggiolini M: Chemokines in pathology and medicine. J Intern Med 2001, 250:91-104. 38. Ostrowski K, Rohde T, Asp A, Schjerling P, Pedersen BK: Chemokines are elevated in plasma after strenuous exercise in humans. Eur J Appl Physiol 2001, 84:244-245. 39. Nieman DC, Davis JM, Henson DA, Walberg-Rankin J, Shute M, Dumke CL, Utter AC, Vinci DM, Carson JA, Brown A et al.: Carbohydrate ingestion influences skeletal muscle cytokine mRNA and plasma cytokine levels after a 3-h run. J Appl Physiol 2003, 94:1917-1925. 40. Chan MH, Carey AL, Watt MJ, Febbraio MA: Cytokine gene expression in human skeletal muscle during concentric contraction: evidence that IL-8, like IL-6, is influenced by glycogen availability. Am J Physiol Regul Integr Comp Physiol 2004, 287:R322-R327. 41. Henson DA, Nieman DC, Nehlsen-Cannarella SL, Fagoaga OR, Shannon M, Bolton MR, Davis JM, Gaffney CT, Kelln WJ, Austin MD et al.: Influence of carbohydrate on cytokine and phagocytic responses to 2 h of rowing. Med Sci Sports Exerc 2000, 32:1384-1389. 42. Akerstrom TC, Steensberg A, Keller P, Keller C, Penkowa M, Pedersen BK: Exercise induces interleukin-8 expression in human skeletal muscle. J Physiol 2005, 563:507-516. 43. De RM, Bernasconi P, Baggi F, de Waal MR, Mantegazza R: Cytokines and chemokines are both expressed by human

www.sciencedirect.com

myoblasts: possible relevance for the immune pathogenesis of muscle inflammation. Int Immunol 2000, 12:1329-1335. 44. Norrby K: Interleukin-8 and de novo mammalian angiogenesis. Cell Prolif 1996, 29:315-323. 45. Frydelund-Larsen L, Penkowa M, Akerstrom T, Zankari A, Nielsen S, Pedersen BK: Exercise induces interleukin-8 receptor (CXCR2) expression in human skeletal muscle. Exp Physiol 2007, 92:233-240. 46. Nielsen AR, Mounier R, Plomgaard P, Mortensen OH, Penkowa M, Speerschneider T, Pilegaard H, Pedersen BK: Expression of interleukin-15 in human skeletal muscle effect of exercise and muscle fibre type composition. J Physiol 2007, 584:305-312. 47. Riechman SE, Balasekaran G, Roth SM, Ferrell RE: Association of interleukin-15 protein and interleukin-15 receptor genetic variation with resistance exercise training responses. J Appl Physiol 2004, 97:2214-2219. 48. Nieman DC, Davis JM, Brown VA, Henson DA, Dumke CL, Utter AC, Vinci DM, Downs MF, Smith JC, Carson J et al.: Influence of carbohydrate ingestion on immune changes after 2 h of intensive resistance training. J Appl Physiol 2004, 96:1292-1298. 49. Grabstein KH, Eisenman J, Shanebeck K, Rauch C, Srinivasan S, Fung V, Beers C, Richardson J, Schoenborn MA, Ahdieh M: Cloning of a T cell growth factor that interacts with the beta chain of the interleukin-2 receptor. Science 1994, 264:965-968. 50. Argiles JM, Lopez-Soriano J, Almendro V, Busquets S, Lopez Soriano FJ: Cross-talk between skeletal muscle and adipose tissue: a link with obesity? Med Res Rev 2005, 25:49-65. An important and interesting review article, which stimulated our group to study how muscular IL-15 is regulated.

Current Opinion in Pharmacology 2008, 8:346–351