Palmitate, but Not Unsaturated Fatty Acids, Induces the Expression of Interleukin-6 in Human Myotubes through Proteasome-dependent Activation of Nuclear Factor-κB

Circulating interleukin-6 (IL-6), insulin, and free fatty acid (FFA) concentrations are associated with impaired insulin action in obese and type 2 diabetic individuals. However, a causal relationship between elevated plasma FFAs and IL-6 has not been shown. Because skeletal muscle represents a major target of impaired insulin action, we studied whether FFAs may affect IL-6 expression in human myotubes. We demonstrate that specifically saturated FFAs, e.g. palmitate (0.25 mm), induce IL-6 mRNA expression and protein secretion by a proteasome-dependent mechanism that leads to a rapid and chronic activation of nuclear factor-κB. Insulin, high glucose concentrations, or unsaturated FFAs did not activate IL-6 expression. In fact, the unsaturated FFA linoleate inhibited palmitate-induced IL-6 production. Because inhibition of palmitate metabolism by the acyl-CoA synthetase inhibitor triacsin C did not abolish IL-6 expression, it appears that the palmitate molecule per se exerts the observed effects. Furthermore, we show that in human myotubes, IL-6 activates the phosphorylation of signal transducer and activator of transcription 3 in concentrations similar to hepatocytes. However, no inhibitory effect of IL-6 on insulin action, determined as phosphatidylinositol 3-kinase association with insulin receptor substrate-1, Akt phosphorylation, and glycogen synthesis, was detected. We conclude that IL-6 expression may be modulated by the composition of circulating FFA, e.g. by diet, and that skeletal muscle cells could be target cells for IL-6. Circulating interleukin-6 (IL-6), insulin, and free fatty acid (FFA) concentrations are associated with impaired insulin action in obese and type 2 diabetic individuals. However, a causal relationship between elevated plasma FFAs and IL-6 has not been shown. Because skeletal muscle represents a major target of impaired insulin action, we studied whether FFAs may affect IL-6 expression in human myotubes. We demonstrate that specifically saturated FFAs, e.g. palmitate (0.25 mm), induce IL-6 mRNA expression and protein secretion by a proteasome-dependent mechanism that leads to a rapid and chronic activation of nuclear factor-κB. Insulin, high glucose concentrations, or unsaturated FFAs did not activate IL-6 expression. In fact, the unsaturated FFA linoleate inhibited palmitate-induced IL-6 production. Because inhibition of palmitate metabolism by the acyl-CoA synthetase inhibitor triacsin C did not abolish IL-6 expression, it appears that the palmitate molecule per se exerts the observed effects. Furthermore, we show that in human myotubes, IL-6 activates the phosphorylation of signal transducer and activator of transcription 3 in concentrations similar to hepatocytes. However, no inhibitory effect of IL-6 on insulin action, determined as phosphatidylinositol 3-kinase association with insulin receptor substrate-1, Akt phosphorylation, and glycogen synthesis, was detected. We conclude that IL-6 expression may be modulated by the composition of circulating FFA, e.g. by diet, and that skeletal muscle cells could be target cells for IL-6. IntroductionInterleukin-6 (IL-6) 1The abbreviations used are: IL-6, interleukin-6; IL-6R, IL-6 receptor; TNF-α, tumor necrosis factor-α; STAT-3, signal transducer and activator of transcription 3; FFA, free fatty acid; IκB-α, inhibitor of κB; NF-κB, nuclear factor κB; IRS-1, insulin receptor substrate-1; BSA, bovine serum albumin. is an important pleiotropic cytokine with both pro- and anti-inflammatory properties (1Akira S. Taga T. Kishimoto T. J. Adv. Immunol. 1993; 54: 1-78Google Scholar). IL-6 is produced by many different cells but in vivo particularly by stimulated monocytes/macrophages, fibroblasts, adipocytes, and vascular endothelial cells (1Akira S. Taga T. Kishimoto T. J. Adv. Immunol. 1993; 54: 1-78Google Scholar, 2Tilg H. Trehu E. Atkin M.B. Dinarello C.A. Mier J.W. Blood. 1994; 83: 113-118Google Scholar, 3Ohshima S. Saeki Y. Mima T. Sasai M. Nishioka K. Nomura S. Kopf M. Katada Y. Tanaka T. Suemura M. Kishimoto T. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 8222-8226Google Scholar). Typical stimuli for IL-6 production are IL-1β, tumor necrosis factor-α (TNF-α), and bacterial endotoxin (1Akira S. Taga T. Kishimoto T. J. Adv. Immunol. 1993; 54: 1-78Google Scholar). Thus, increased circulating levels of IL-6 have been reported in severely ill patients suffering from injury, sepsis, cachexia, or cancer (4Nelson K.A. Walsh D. Sheehan F.A. J. Clin. Oncol. 1994; 12: 213-225Google Scholar). However, recent observations suggest that IL-6 production is also modulated by various conditions in apparently healthy individuals. Pedersen and co-workers (5Febbraio M.A. Pedersen B.K. FASEB J. 2002; 16: 1335-1347Google Scholar, 6Febbraio M.A. Steensberg A. Keller C. Starkie R.L. Nielsen H.B. Krustrup P. Ott P. Secher N.H. Pedersen B.K. J. Physiol. 2003; 549: 607-612Google Scholar, 7Penkowa M. Keller C. Keller P. Jauffred S. Pedersen B.K. FASEB J. 2003; 17: 2166-2168Google Scholar) have demonstrated that exercise can markedly increase plasma IL-6. The marked exercise-induced IL-6 protein expression was shown to occur exclusively within skeletal muscle fibers (7Penkowa M. Keller C. Keller P. Jauffred S. Pedersen B.K. FASEB J. 2003; 17: 2166-2168Google Scholar) and was influenced by muscle glycogen content (6Febbraio M.A. Steensberg A. Keller C. Starkie R.L. Nielsen H.B. Krustrup P. Ott P. Secher N.H. Pedersen B.K. J. Physiol. 2003; 549: 607-612Google Scholar).Several studies have shown that adipose tissue produces and secretes IL-6, and that 10–35% of the body's basal circulating IL-6 is derived from adipose tissue (8Mohamed-Ali V. Goodrick S. Rawesh A. Katz D.R. Miles J.M. Yudkin J.S. Klein S. Coppack S.W. J. Clin. Endocrinol. Metab. 1997; 82: 4196-4200Google Scholar, 9Kern P.A. Ranganathan S. C. Ranganathan J. Physiol. Endocrinol. Metab. Scholar, A. K. S. E. 2003; Scholar). In with and type 2 IL-6 plasma levels are in and are associated with insulin action, insulin J.M. 2003; Scholar, J.M. M. J. 2003; Scholar, Scholar, M.B. D. 1997; Scholar). The IL-6 production in and skeletal muscle tissue of IL-6 in the The is a target of IL-6 produced by skeletal muscle and adipose IL-6 has been shown to increase glucose elevated glucose C. J. Clin. Endocrinol. Metab. 1997; 82: Scholar, T. H. M. K. Y. Y. S. T. 1998; Scholar) and to as a Steensberg A. M. C. Keller C. P. K. M.A. Pedersen B.K. J. Clin. Endocrinol. Metab. 2003; Scholar). IL-6 also appears to an important in insulin in the as has been shown by both IL-6 in vivo in 2003; Scholar) and on a in 2002; Scholar, J. 2003; Scholar). to IL-6 of insulin receptor in the of in insulin 2003; Scholar). In both human and IL-6 insulin receptor signal and insulin action, specifically glycogen 2002; Scholar). However, it is whether IL-6 could also activate IL-6 in skeletal muscle has been that the levels of IL-6 by human are not to IL-6 or that the of to to IL-6 is markedly with e.g. 2003; Scholar, Y. A. J. Physiol. Endocrinol. Metab. of the was to the of insulin, and circulating free fatty on IL-6 expression and protein secretion in skeletal muscle are increased in and skeletal muscle is the tissue for insulin Furthermore, we studied the expression of IL-6 receptor and and the of human to IL-6 by the effect of IL-6 on signal transducer and activator of transcription 3 We demonstrate that specifically saturated FFAs, e.g. induce IL-6 mRNA expression and protein secretion by a proteasome-dependent to inhibitor of and nuclear Insulin, high glucose concentrations, or unsaturated FFAs did not activate IL-6 expression. In fact, the unsaturated FFA linoleate inhibited palmitate-induced IL-6 production. Furthermore, we show that in human myotubes, IL-6 activates the in concentrations similar to no inhibitory effect of IL-6 on insulin action, determined as phosphatidylinositol 3-kinase association with insulin receptor Akt phosphorylation, and glycogen was and from by for synthesis, and the from protein was from fatty fatty bovine serum and human IL-6 from was from STAT-3, and from of phosphatidylinositol 3-kinase and from and from the used for was by IL-6 was from was from was from skeletal muscle cells from cells from on the of muscle as J. C. A. Klein K. M. Scholar). The healthy on the of cells that cells in in a of and with bovine and as J. C. A. Klein K. M. Scholar). the cells for in glucose with bovine serum and cells stimulated in FFAs as or with glucose for cells in with 2 and to H. C. A. M. E. M. 2003; and was with the transcription of was with a for of the transcription in to with the with the by by to that the a of the of the by as by the The of of of The was in a of 2 of 3 and to a of The for for for and for for for for and for for and for for and for for was by for of the the was from the by with the from human used as in The of the the of the of the and the of in the different was with the with of and and of or by to by and with and and with the in with the with or for of was by as C. U. K. A. J. for IL-6 and of of the and IL-6 protein was by the human IL-6 in was by with V. U. A. J. Clin. 1998; as C. U. K. A. J. Scholar). a for with and and with to of nuclear protein in of and on for was as The on a in a and and by was as by J. C. A. Klein K. M. Scholar) with in for with IL-6 in with or insulin for of the and the cells with and in of for of protein The for and glycogen 2 was as a was with of and by for The glycogen was in of and by was as of are derived from and of was of IL-6 mRNA by in studied whether as insulin or high glucose concentrations, affect the expression of IL-6 in human myotubes. glucose concentrations no effect on IL-6 mRNA expression a or in with or insulin, and insulin IL-6 mRNA expression levels by Because we observed that saturated fatty expression in human C. K. C. K. M. 2003; we used to the effect of different FFAs on IL-6 expression. shown in of with palmitate or for in a increase in IL-6 mRNA expression. The effect of unsaturated FFA and linoleate was similar to the effect observed in cells with acid with FFAs no effect on or content not The effect of palmitate was studied in the plasma of FFA is in the and to a fatty or and palmitate is the saturated FFA in plasma we the effect of concentrations of palmitate on IL-6 mRNA expression, we no effect with palmitate but a with palmitate not we used for Because is a of IL-6, we the effect of palmitate and on IL-6 expression in human myotubes. shown in palmitate IL-6 mRNA to a similar as We also observed a effect of BSA, was used as a for FFA In we FFA to the in IL-6 mRNA and IL-6 in we studied the of palmitate-induced IL-6 mRNA and protein expression. of with an increased IL-6 mRNA was that was not the of palmitate The increase in IL-6 mRNA levels and elevated whether the palmitate-induced increase in IL-6 mRNA expression was an IL-6 protein production and IL-6 protein content in was determined by shown in palmitate a increase in IL-6 protein production as as increased with of produced and the and of The that the rapid increase in IL-6 mRNA is IL-6 and that the in IL-6 protein production high and as by the high IL-6 mRNA levels of palmitate-induced IL-6 mRNA and protein expression. stimulated with palmitate for the are as of palmitate-induced expression of IL-6 mRNA was by IL-6 mRNA levels of cells with for as of IL-6 protein by IL-6 protein concentrations in the of cells by and to the content and as of by and to the transcription factor has been shown to be in the activation of IL-6 expression in various and cells D. Scholar, T. K. Y. K. Kishimoto T. S. Proc. Natl. Acad. Sci. U. S. A. 1993; Scholar, C. D. U. J. we the of the palmitate-induced IL-6 expression in human myotubes. The activation of M. Y. Immunol. 16: Scholar, T. S. J. T. D. Scholar) and can be by P. P.A. J. Scholar). whether or is by palmitate and the of IL-6 mRNA expression, inhibition of with palmitate in the of acid did not IL-6 mRNA with acid for was shown to activation of FASEB J. to IL-6 mRNA expression However, the of the inhibitor palmitate-induced IL-6 expression and Thus, activation of by a proteasome-dependent could be in IL-6 by of of fatty acid metabolism and of inhibition on palmitate-induced IL-6 expression. stimulated with palmitate for with or IL-6 mRNA expression was by IL-6 protein in the was by are as of IL-6 mRNA expression and IL-6 protein shown as of of for with inhibitor or acid and stimulated with with acid was for palmitate palmitate for with concentrations of for with triacsin C for with palmitate linoleate for palmitate with triacsin C with or for with palmitate for IL-6 protein in the of for with FFA with or as has been to the activation of e.g. by metabolism to and activation of protein C K. H. J. H. H. M. 2003; Scholar, E. M. M. M. V. K. J. Physiol. Endocrinol. Metab. by of Y. J. or by to J. 2003; Scholar). of activation of palmitate to by acyl-CoA we studied whether is for the effect on IL-6 expression. In the of triacsin an inhibitor of acyl-CoA synthetase J. 2003; no of IL-6 mRNA expression was in the effect of palmitate on IL-6 mRNA expression was we observed that in the of of linoleate and the palmitate-induced of IL-6 was Furthermore, that and both of no effect on palmitate-induced IL-6 expression the that activation of palmitate to is not for the observed palmitate effect inhibitory action of and linoleate was also on palmitate-induced IL-6 protein production In with the on the mRNA acid no effect on IL-6 protein production In the of the acyl-CoA synthetase inhibitor triacsin IL-6 protein was not Thus, palmitate is the molecule to IL-6 mRNA and protein inhibition of with IL-6 mRNA and activation of the transcription factor proteasome-dependent of IκB-α, we the that of with palmitate in activation of we a of in nuclear from cells with no to the in or nuclear from high and was observed in cells was in the as the in cells In the of the inhibitor the activation was and was with The of the was demonstrated by with and of and The activation of was not and of palmitate and the to be and of palmitate to We observed to be the major of the and also the was in nuclear of palmitate activation of with of the a for is marked by the is not shown. with nuclear of stimulated with FFAs or with glucose for is marked by the with nuclear of stimulated with palmitate for or with and show or of and nuclear of stimulated with palmitate for is marked by the nuclear of stimulated with palmitate for or with 2 of for or is marked by the are marked by the is not shown. and with of stimulated with palmitate for the of on a and and with a show the of the The of cells is as is as of of on a and phosphorylation of on was with a is shown with the of the to is the in the of by and NF-κB, to the M. Y. Immunol. 16: Scholar, T. S. J. T. D. Scholar). we the mechanism of palmitate-induced activation by the of and phosphorylation in myotubes. a of protein as as of palmitate and the of protein and of palmitate in with the of of In protein levels In with a increase in phosphorylation of on was of palmitate the phosphorylation to basal levels Thus, a palmitate-induced phosphorylation of is the signal for the proteasome-dependent of to to the and to and to IL-6 mRNA in we could demonstrate that human and of IL-6, we studied the effect of IL-6. IL-6 the expression of and the IL-6 receptor as a in the and signal of IL-6 S. J. 2003; Scholar). we that both in human myotubes, and that the expression was not by palmitate or linoleate and with expression levels in the content of mRNA in was of that in mRNA levels Because an of IL-6 to the is activation of and rapid phosphorylation of S. J. 2003; we studied the effect of concentrations of human IL-6 on phosphorylation in human myotubes. with demonstrated phosphorylation of STAT-3, with a effect of with of IL-6 for The concentrations used in are similar to the of IL-6 protein by the to of IL-6 in Thus, IL-6 are in human myotubes, and the cells are to of IL-6 on myotubes. mRNA expression of and in human myotubes. stimulated with fatty for as and mRNA expression of and was by are as of mRNA expression in cells was as human stimulated with human IL-6 for with the of on a and protein and phosphorylation of on with with the and of the with is shown in the of the show of the to the The of cells is as is as of of stimulated with palmitate for or with in a of for with the different as of on a and protein and phosphorylation of on with with the and of the with is shown in the of the show of the to the The of cells is as is as of we studied whether the palmitate-induced expression of IL-6 could for in the myotubes. of used to induce IL-6 in of the with for phosphorylation of STAT-3, of cells no effect activation is of IL-6 phosphorylation Thus, palmitate-induced IL-6 production activation of IL-6 in of IL-6 on in in circulating IL-6 plasma levels have been associated with insulin in J.M. 2003; Scholar, J.M. M. J. 2003; Scholar, Scholar, M.B. D. 1997; and IL-6 has been demonstrated to in human e.g. by phosphorylation of IRS-1, by association of the of phosphatidylinositol 3-kinase with IRS-1, by impaired phosphorylation of Akt on and by glycogen 2002; Scholar, J. 2003; Scholar). phosphatidylinositol 3-kinase is an and in the insulin in skeletal we in human myotubes, the effect of IL-6 on concentrations of and are to induce insulin receptor in human J. C. A. Klein K. M. Scholar). of with and insulin the association of with IRS-1, as shown by and of and with of IL-6 for been shown to a of in 2002; no inhibitory effect on association in myotubes. phosphorylation of of Akt was not in the IL-6 IL-6 a phosphorylation of Akt glycogen was not by IL-6 with insulin and with IL-6 with with insulin with insulin, we observed a effect of IL-6 on glycogen and Because both and myotubes, to similar insulin concentrations and the IL-6 concentrations for phosphorylation we conclude in human myotubes, IL-6 is not a of insulin of IL-6 on insulin stimulated with insulin as with IL-6 for association of the of phosphatidylinositol 3-kinase with was by are shown in the of the show of to The of cells is as is as of phosphorylation of Akt on of on a and Akt protein and phosphorylation of Akt on with with the and of the is shown in the of the show of the Akt to Akt The of cells is as is as of glycogen of cells is as is as of fatty e.g. palmitate E. M. M. M. V. K. J. Physiol. Endocrinol. Metab. Scholar, M. E. D. A. Scholar, C. Y. D. H. Y. S.W. J. 2002; have been shown to insulin action in skeletal In with of with palmitate for the glycogen stimulated with insulin to with not However, from it that IL-6 is the of fatty insulin in skeletal muscle the we demonstrate that saturated FFAs, in to unsaturated FFAs, high glucose concentrations, or insulin, activate expression and protein production of IL-6 in human myotubes. The within and is the We also a but effect of on IL-6 expression. We suggest that with endotoxin in the for of IL-6 to be by H. T. Y. T. D. A. E. J. Clin. 1997; is in myotubes. The that saturated FFAs induce IL-6 expression is the is that IL-6 expression is by of the pro- and anti-inflammatory In we on a effect was with a plasma that is also in the and the of unsaturated fatty did not IL-6 mRNA of with concentrations of linoleate and palmitate the action of the saturated effect of unsaturated FFAs on has also been for S. Proc. Natl. Acad. Sci. U. S. A. 2003; Scholar, K. H. D. M. 2002; Scholar). In it could be demonstrated that palmitate is and S. Proc. Natl. Acad. Sci. U. S. A. 2003; Scholar). FFAs of palmitate from to Thus, of palmitate metabolism to also be a for the inhibitory effect of linoleate in for is by that the palmitate is for the activation of IL-6 transcription and not or The bacterial triacsin been shown to acyl-CoA synthetase in concentrations J. 2003; did not IL-6 by palmitate in fact, IL-6 mRNA expression with the effect of palmitate that of free palmitate of of activation to in IL-6 expression the metabolism of palmitate in the of the unsaturated fatty acid linoleate the of free palmitate to also the mechanism of palmitate-induced the rapid of within by of phosphorylation on to of the and activation of within Because IL-6 expression could be by activation that was by to a in the of the IL-6 D. Scholar) and by action of and IL-6 T. K. Y. K. Kishimoto T. S. Proc. Natl. Acad. Sci. U. S. A. 1993; of IL-6 expression was of palmitate In we observed a chronic activation of NF-κB, of is also and of with is in with the chronic activation of IL-6 expression and could the that the of is also and The mechanism of of to be phosphorylation of was observed is for the chronic activation of and IL-6 mRNA and protein expression of palmitate the of inhibitor of of was also in healthy 2002; and activation of the has been to fatty insulin in M. J. M. Scholar, Y. J. M. M. P. J. Clin. Scholar). In the activation of protein C e.g. by increased levels was as the mechanism for that palmitate metabolism is not to the of expression not the that is the signal transducer for IL-6 production. The mechanism of free palmitate phosphorylation and protein that is also of the the of to chronic activation A. S. M. M. J. M. T. T. M. M. H. M. M. E. Scholar, M.A. J. 2003; the of whether IL-6 produced by skeletal muscle cells has we whether IL-6 is in human myotubes. We demonstrated that are to IL-6 in a similar as or cells J. M. C. J. 12: as shown by phosphorylation with 3 of IL-6, in a phosphorylation, the expression of was with to the on the IL-6 of human skeletal muscle Thus, may be to the of IL-6 target we the effect of IL-6 on insulin IL-6 has been shown to insulin in and cells 2002; Scholar, J. 2003; Scholar) and to insulin in as demonstrated chronic IL-6 in 2003; Scholar). However, similar conditions as in the studies with cells and we observed no inhibitory effect of IL-6 on phosphatidylinositol 3-kinase and on phosphorylation of of phosphatidylinositol 3-kinase and Akt are in insulin in skeletal muscle that are for insulin of both glucose and glycogen glycogen was not by IL-6 in the myotubes. Thus, we conclude that IL-6 is to be a of insulin in skeletal muscle recent that a of IL-6 to insulin receptor in the skeletal muscle of 2003; is in with in the of elevated muscle IL-6 production to high saturated fatty acid has been that of IL-6 is to TNF-α, as an anti-inflammatory and as an of insulin by (5Febbraio M.A. Pedersen B.K. FASEB J. 2002; 16: 1335-1347Google Scholar). exercise skeletal muscle is with concentrations of fatty from be in the as Thus, we that IL-6 from the muscle the properties of IL-6 on adipose tissue and the effect of IL-6 to glucose that skeletal muscle cells of IL-6 in to saturated FFA a association between palmitate and plasma IL-6 was of was in of the for and type 2 e.g. are associated with an increased of palmitate and a of linoleate 2003; Scholar). Thus, is that IL-6 expression by saturated fatty could also be in The of increased IL-6 production of the skeletal muscle to be IntroductionInterleukin-6 (IL-6) 1The abbreviations used are: IL-6, interleukin-6; IL-6R, IL-6 receptor; TNF-α, tumor necrosis factor-α; STAT-3, signal transducer and activator of transcription 3; FFA, free fatty acid; IκB-α, inhibitor of κB; NF-κB, nuclear factor κB; IRS-1, insulin receptor substrate-1; BSA, bovine serum albumin. is an important pleiotropic cytokine with both pro- and anti-inflammatory properties (1Akira S. Taga T. Kishimoto T. J. Adv. Immunol. 1993; 54: 1-78Google Scholar). IL-6 is produced by many different cells but in vivo particularly by stimulated monocytes/macrophages, fibroblasts, adipocytes, and vascular endothelial cells (1Akira S. Taga T. Kishimoto T. J. Adv. Immunol. 1993; 54: 1-78Google Scholar, 2Tilg H. Trehu E. Atkin M.B. Dinarello C.A. Mier J.W. Blood. 1994; 83: 113-118Google Scholar, 3Ohshima S. Saeki Y. Mima T. Sasai M. Nishioka K. Nomura S. Kopf M. Katada Y. Tanaka T. Suemura M. Kishimoto T. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 8222-8226Google Scholar). Typical stimuli for IL-6 production are IL-1β, tumor necrosis factor-α (TNF-α), and bacterial endotoxin (1Akira S. Taga T. Kishimoto T. J. Adv. Immunol. 1993; 54: 1-78Google Scholar). Thus, increased circulating levels of IL-6 have been reported in severely ill patients suffering from injury, sepsis, cachexia, or cancer (4Nelson K.A. Walsh D. Sheehan F.A. J. Clin. Oncol. 1994; 12: 213-225Google Scholar). However, recent observations suggest that IL-6 production is also modulated by various conditions in apparently healthy individuals. Pedersen and co-workers (5Febbraio M.A. Pedersen B.K. FASEB J. 2002; 16: 1335-1347Google Scholar, 6Febbraio M.A. Steensberg A. Keller C. Starkie R.L. Nielsen H.B. Krustrup P. Ott P. Secher N.H. Pedersen B.K. J. Physiol. 2003; 549: 607-612Google Scholar, 7Penkowa M. Keller C. Keller P. Jauffred S. Pedersen B.K. FASEB J. 2003; 17: 2166-2168Google Scholar) have demonstrated that exercise can markedly increase plasma IL-6. The marked exercise-induced IL-6 protein expression was shown to occur exclusively within skeletal muscle fibers (7Penkowa M. Keller C. Keller P. Jauffred S. Pedersen B.K. FASEB J. 2003; 17: 2166-2168Google Scholar) and was influenced by muscle glycogen content (6Febbraio M.A. Steensberg A. Keller C. Starkie R.L. Nielsen H.B. Krustrup P. Ott P. Secher N.H. Pedersen B.K. J. Physiol. 2003; 549: 607-612Google Scholar).Several studies have shown that adipose tissue produces and secretes IL-6, and that 10–35% of the body's basal circulating IL-6 is derived from adipose tissue (8Mohamed-Ali V. Goodrick S. Rawesh A. Katz D.R. Miles J.M. Yudkin J.S. Klein S. Coppack S.W. J. Clin. Endocrinol. Metab. 1997; 82: 4196-4200Google Scholar, 9Kern P.A. Ranganathan S. C. Ranganathan J. Physiol. Endocrinol. Metab. Scholar, A. K. S. E. 2003; Scholar). In with and type 2 IL-6 plasma levels are in and are associated with insulin action, insulin J.M. 2003; Scholar, J.M. M. J. 2003; Scholar, Scholar, M.B. D. 1997; Scholar). The IL-6 production in and skeletal muscle tissue of IL-6 in the The is a target of IL-6 produced by skeletal muscle and adipose IL-6 has been shown to increase glucose elevated glucose C. J. Clin. Endocrinol. Metab. 1997; 82: Scholar, T. H. M. K. Y. Y. S. T. 1998; Scholar) and to as a Steensberg A. M. C. Keller C. P. K. M.A. Pedersen B.K. J. Clin. Endocrinol. Metab. 2003; Scholar). IL-6 also appears to an important in insulin in the as has been shown by both IL-6 in vivo in 2003; Scholar) and on a in 2002; Scholar, J. 2003; Scholar). to IL-6 of insulin receptor in the of in insulin 2003; Scholar). In both human and IL-6 insulin receptor signal and insulin action, specifically glycogen 2002; Scholar). However, it is whether IL-6 could also activate IL-6 in skeletal muscle has been that the levels of IL-6 by human are not to IL-6 or that the of to to IL-6 is markedly with e.g. 2003; Scholar, Y. A. J. Physiol. Endocrinol. Metab. of the was to the of insulin, and circulating free fatty on IL-6 expression and protein secretion in skeletal muscle are increased in and skeletal muscle is the tissue for insulin Furthermore, we studied the expression of IL-6 receptor and and the of human to IL-6 by the effect of IL-6 on signal transducer and activator of transcription 3 We demonstrate that specifically saturated FFAs, e.g. induce IL-6 mRNA expression and protein secretion by a proteasome-dependent to inhibitor of and nuclear Insulin, high glucose concentrations, or unsaturated FFAs did not activate IL-6 expression. In fact, the unsaturated FFA linoleate inhibited palmitate-induced IL-6 production. Furthermore, we show that in human myotubes, IL-6 activates the in concentrations similar to no inhibitory effect of IL-6 on insulin action, determined as phosphatidylinositol 3-kinase association with insulin receptor Akt phosphorylation, and glycogen was detected.