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Journal of Clinical Endocrinology & Metabolism , doi:10.1210/jc.2007-2223
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The Journal of Clinical Endocrinology & Metabolism Vol. 93, No. 7 2851-2858
Copyright © 2008 by The Endocrine Society

Interleukin-6 Markedly Decreases Skeletal Muscle Protein Turnover and Increases Nonmuscle Amino Acid Utilization in Healthy Individuals

Gerrit van Hall, Adam Steensberg, Christian Fischer, Charlotte Keller, Kirsten Møller, Pope Moseley and Bente K. Pedersen

The Centre of Inflammation and Metabolism at Department of Infectious Diseases Rigshospitalet (G.v.H., A.S., C.F., C.K., K.M., P.M., B.K.P.), Faculty of Health Sciences, and Department of Biomedical Sciences (G.v.H.), University of Copenhagen, DK-1017 Copenhagen, Denmark; and Copenhagen Muscle Research Centre (G.v.H., A.S., C.F., C.K., K.M.), Rigshospitalet, DK-2100 Copenhagen, Denmark

Address all correspondence and requests for reprints to: G. van Hall, Copenhagen Muscle Research Centre, Rigshospitalet, section 7652, 9 Blegdamsvej, DK-2100, Copenhagen Ø, Denmark. E-mail: gvhall{at}cmrc.dk.


   Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
Context: IL-6 is a key modulator of immune function and suggested to be involved in skeletal muscle wasting as seen in sepsis.

Objective: Our objective was to determine the role of IL-6 in human in vivo systemic and skeletal muscle amino acid metabolism and protein turnover.

Subjects and Methods: There were 12 healthy men infused for 3 h with saline (saline, n = 6) or recombinant human IL (rhIL)-6 (n = 6). Systemic and muscle protein turnover was determined with a combination of tracer dilution methodology, primed constant infusion of L-[ring-2H5]phenylalanine, and femoral arterial-venous blood differences and m. vastus lateralis biopsies after 2-h basal, 3-h infusion, and 3 h after infusion.

Results: The IL-6 concentration after 30-min infusion was approximately 4 (saline) and 140 pg/ml (rhIL-6). Three-hour rhIL-6 infusion caused an approximate 50% decrease in muscle protein turnover, albeit synthesis was more suppressed than breakdown, causing a small increase in net muscle protein breakdown. Furthermore, rhIL-6 decreased arterial amino acid concentration with 20–40%, despite the increase net release from muscle.

Conclusions: We demonstrated that IL-6 profoundly alters amino acid turnover. A substantial decrease in plasma amino acids was observed with a concomitant 50% decrease in muscle protein turnover, however, modest increase in net muscle degradation. We hypothesize that the profound reduction in muscle protein turnover and modest increase in net degradation are primarily caused by the reduced plasma amino acid availability and not directly mediated by IL-6.


   Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
Recent evidence has expanded our understanding of the pleiotropic cytokine IL-6, resulting in widespread recognition of IL-6 as an important messenger in systemic and skeletal muscle metabolism. This newer attribution reflects the significant muscle IL-6 production during exercise, the ability of IL-6 to increase insulin-stimulated glucose disposal and fatty acid oxidation in vivo (1), as well as the interesting observation that IL-6 knockout animals demonstrate a type 2 diabetic phenotype (2). Although IL-6 is often classified as a proinflammatory cytokine, it also possesses important antiinflammatory properties. IL-6 has been proposed as a key-signaling molecule in the beneficial effects of exercise and is central to the concept of muscle as an endocrine organ. The ability of IL-6 to serve as both myokine and inflammatory cytokine is all the more interesting because of the often-conflicting functions of inflammatory cytokines as insulin resistance factors and as enhancers of protein breakdown (3, 4). In contrast, the inflammatory cytokine TNF-{alpha} decreases both insulin sensitivity and muscle-essential amino acid uptake (5, 6, 7). More recently, studies in humans of endotoxin-induced amino acid turnover revealed a decrease in both plasma and muscle free amino acid concentrations (8). Moreover, alanine and glutamine play a central role in substrate recycling and interorgan nitrogen metabolism. Together, they comprise less than 15% amino acid content of protein, yet account for approximately 60% of amino acids leaving skeletal muscle (9). In the disease states, glutamine consumption exceeds the release from muscle, and a decrease in muscle free glutamine content has been observed and associated with mortality (10). Endotoxin is a complex stressor, and it is not known which, if any, of these effects were mediated by IL-6. More specifically, it is not known how IL-6 influences amino acid turnover and muscle amino acid uptake. Furthermore, because of the complex interaction between cytokines and physiological stressors, it is important to understand this effect in the intact organism. Therefore, we studied the effect of IL-6 on systemic and leg skeletal muscle amino acid and protein metabolism. For this purpose, we infused recombinant human IL (rhIL)-6 for 3 h at a dose that has shown to elicit plasma IL-6 concentration in a physiological range as seen, for example, in healthy individuals during intense prolonged exercise (11), thus much lower than during endotoxemia.


   Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
Subjects

There were 12 healthy, active but not specifically trained males recruited in the study. Each was assigned to the saline infusion (saline) or rhIL-6 infusion group (rhIL-6). The characteristics of the groups were similar for saline and rhIL-6 for age (23 ± 1 and 24 ± 1 yr), weight (78 ± 2 and 80 ± 2 kg), height (183 ± 3 and 184 ± 1 cm), and body mass index (23.4 ± 0.5 and 23.5 ± 0.8 kg/m2, respectively). The study was approved by the Ethical Committee of Copenhagen and Frederiksberg Communities, Denmark, and performed according to the Declaration of Helsinki. Subjects were informed about the possible risks and discomfort involved before giving their written consent to participate.

Protocol

Subjects reported to the laboratory at 0700 h after an overnight fast. They voided, changed into appropriate hospital attire, and remained supine during the entire experiment. The experimental room was kept at 24 C. The subjects were only permitted to consume water ad libitum during the experiment. After 10 min a femoral vein and the femoral arteries of both legs were cannulated under local anesthesia (lidocaine, 20 mg/ml–1). The Seldinger technique was used to insert the catheters (20 gauge; Ohmeda, Wiltshire, UK) one artery for saline or rhIL-6 infusion, the other for blood sampling. Thereafter, a catheter was placed in a forearm vein for infusion of the stable isotope. Immediately after an arterial sample was obtained for background enrichment, a primed constant infusion of [ring-2H5]phenylalanine (0.1 µmol/(min·kg), prime 3 µmol/kg) was started. The [ring-2H5]phenylalanine was purchased from Cambridge Isotope Laboratories (Andover, MA). For each subject the actual infusion rate was calculated from the infusate concentration multiplied by the infusion flow rate. The stable isotope pre-infusion period was 2 h to allow for steady-state tracer measurement, followed by 3-h saline or rhIL-6 infusion and 3 h upon cessation of infusion. The rate of rhIL-6 (Sandoz Pharma AG, Basel, Switzerland) infusion was 30 µg/h, and it was administered in saline. Blood samples for amino acid concentration and phenylalanine enrichment were taken 90, 105, and 120 min after the start of the tracers and every half hour during saline or rhIL-6 infusion, followed by every hour after cessation of the infusions. A blood sample for the measurement of insulin, glucagon, cortisol, and catecholamine concentration was obtained just before the start of the infusion, every hour during infusion, and 2 h after cessation of the infusion. At each blood sample moment, the leg blood flow (LBF) was measured using Doppler ultrasonography (model CFM 800; Vingmed Sound, Horten, Norway).

Sample analysis

The 200-µl plasma was mixed with 50 µl of an internal standard mixture, acidified, and passed through a strong cation-exchange column (Bio-Rad AG 50Wx8; Bio-Rad, Hemel Hempstead, UK). The amino acids were eluated with 2 M NH4OH. The eluent was concentrated by drying under a stream of N2, and 100 µ/liter N-methyl-N-(tert-butyldimethylsilyl)trifluoroacetamide and acetonitrile (1:1) were added to prepare t-butyldimethylsilyl derivatives. After being heated for 1 h at 90 C, the samples were transferred to small vials and sealed, and the enrichments of sample and internal standard were determined by gas chromatography-mass spectrometry (Finnigan Automass II, Paris, France).

Muscle biopsies were immediately frozen in liquid nitrogen and stored at –80 C. Muscle biopsies were cut at –20 C, and approximately 10 mg was freeze dried, dissected free of blood and visible connective tissue under a stereomicroscope. The difference in wet and dry weight is the im free water content. An internal amino acid standard mixture was added to the dry tissue (20 µl/mg dry tissue) and extracted three times with 500 µl sulfosalicylic acid. The supernatants were pooled and further processed as described for plasma samples.

Plasma insulin (Insulin RIA 100; Amersham, Pharmacia, Biotech, Uppsala, Sweden), glucagon (LINCO Research, Inc., St. Charles, MO), and cortisol (Diagnostic Products Corp., Los Angeles, CA) by RIA, and plasma adrenaline and noradrenaline by HPLC. These analyses are described in more detail elsewhere (12, 13). Blood samples for IL-6 measurement were drawn into pre-cooled glass tubes containing EDTA. The tubes were spun immediately at 2200 g for 15 min at 4 C. The plasma was stored at –80 C until analyses were performed. IL-6 was measured with the high-sensitivity ELISA kits from R&D Systems, Inc. (Minneapolis, MN), with a detection limit of 1 pg/ml. The ELISA kit is insensitive to the addition of the recombinant forms of the soluble IL-6 receptor, and the measurements, therefore, correspond to both soluble and receptor-bound cytokine. The intraassay coefficient of variation in our hands was 5.9%.

Calculations

Concentration of free amino acids in plasma and total muscle water were calculated as C = QIS/(V x EIS), where QIS is the amount of internal standard added to the sample, V is the volume of plasma or muscle water, and EIS is the internal standard tracer to tracee ratio in plasma or muscle water. The intracellular water content was calculated as from the difference between wet and dry tissue weight. The whole body phenylalanine rate of appearance (Ra) was calculated by dividing the tracer infusion rate (F) by the arterial enrichment (Ea). Therefore, Ra = F/Ea.

Skeletal muscle phenylalanine kinetics was calculated using the three-compartment model (14, 15). According to this model, the rate of delivery to the leg via the femoral artery (Fin) and the rate of phenylalanine outflow (Fout) equals the product of LBF and phenylalanine concentration in artery and vein, respectively. Fmuscle-artery and Fvein-muscle represent the rates of phenylalanine inward and outward transport from artery to muscle and from muscle back to vein. Fv-a represents the direct phenylalanine flow from artery to vein without entering the intracellular space. F0-muscle and Fmuscle-0 represent the phenylalanine rate of utilization and production because phenylalanine is not metabolized in skeletal muscle, virtually equal the rate of protein synthesis and breakdown. Each kinetic parameter is defined as follows:

Formula

Formula

Formula

Formula

Formula

Formula

Formula
Where E is the enrichment and C the concentration of phenylalanine; subscripts a, v, and m indicate enrichment and concentration in artery, vein, and muscle intracellular free water, respectively.

The net skeletal muscle protein breakdown was estimated from the rate of net phenylalanine release. The skeletal muscle phenylalanine and glutamine content has been reported to be 233 µmol and 460 µmol/g muscle protein, respectively, thus a phenylalanine to glutamine ratio of 1:2 (15, 16). Consequently, the im glutamine rate of net release from muscle protein breakdown was calculated as the net phenylalanine release times two.

Statistics

All data are presented as mean ± SE. To analyze changes over time and between groups, a two-way repeated measures ANOVA was used. If such analysis revealed significant differences, a Newman-Keuls post hoc test was used to locate the specific differences. P < 0.05 was accepted as significant.


   Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
General physiological and hormonal effects of rhIL-6 infusions

The physiological response to infused rhIL-6, effect on stress hormones, and alteration in plasma glucose have been previously described (13). Briefly, subjects receiving the rhIL-6 infusion demonstrated no differences in core temperature, heart rate, or blood pressure. As previously reported, a higher dose of rhIL-6 was associated with some transient discomfort (shivering and hyperesthesias) within 1-h onset of infusion, however, subjects were asymptomatic with the dose used here. The IL-6 concentration during saline and rhIL-6 infusion remained at a steady state after 30-min infusion at approximately 4 and 140 pg/ml, respectively (Fig. 1Go). Saline infusion did not increase the concentration of IL-6. On cessation of the rhIL-6 infusion, IL-6 concentration declined rapidly, and after 1 h the concentrations were only marginally elevated. Plasma insulin decreased over time, but there were no differences when comparing groups and no group by time interaction. The plasma glucagon levels were similar and not affected by time or treatment. There were no differences in plasma cortisol concentrations at rest, whereas concentrations of plasma cortisol declined during saline and rhIL-6 infusion, albeit the decrease was slightly less during rhIL-6 infusion. Three hours after rhIL-6 infusion, cortisol had declined to similar levels as in saline. Cortisol levels for the saline trial were 18 ± 3, 9 ± 2, and 9 ± 2 pg/ml, and for the rhIL-6 trial 20 ± 4, 16 ± 2, and 9 ± 2 pg/ml at basal, after 3-h infusion, and 3 h upon termination of the saline and rhIL-6 infusion, respectively. Plasma epinephrine and norepinephrine were not affected by time or treatment when comparing trials.


Figure 1
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  		FIG. 1. Arterial IL-6 concentration. Data are expressed as means ± SE (n = 6 both groups). *Significantly different from basal, #significantly different from saline (P < 0.05).

 
Systemic and skeletal muscle protein turnover

Baseline phenylalanine concentration of 43 µmol/liter decreased upon the start of rhIL-6 infusion, becoming significant after 1 h at around 34 µmol/liter. Upon cessation of rhIL-6 infusion, phenylalanine concentration returned to basal and saline levels. The Ra was unchanged during rhIL-6 infusion but was higher during the last 1.5-h infusion in the 3 h after infusion compared with the declining Ra over time in saline (Fig. 2Go).


Figure 2
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  		FIG. 2. Phenylalanine arterial concentration (upper panel) and systemic Ra (lower panel). Data are expressed as means ± SE (n = 6 both groups). *Significantly different from basal, #significantly different from saline (P < 0.05).

 
Skeletal muscle protein turnover was estimated with the three-compartment phenylalanine model, before the start of either saline or rhIL-6, at the end of the 3-h infusion period, and 3 h after infusion (Table 1Go and Fig. 3Go). Phenylalanine delivery to the leg was reduced after 3-h rhIL-6 infusion (Fig. 3Go), mainly caused by the decrease in arterial phenylalanine concentration (Table 1Go and Fig. 2Go). In addition, the skeletal muscle inward flux of phenylalanine was reduced with rhIL-6 infusion. Skeletal muscle protein synthesis and breakdown, as reflected by muscle phenylalanine utilization (F0-muscle) and production (Fmuscle-0), were reduced, i.e. about a 50% decrease in mixed muscle protein turnover. However, the decrease in muscle synthesis was more pronounced than the decrease in breakdown, resulting in a significantly more negative phenylalanine balance across the leg during rhIL-6 infusion compared with both baseline and saline. Three hours after rhIL-6 infusion, phenylalanine delivery had returned to basal levels, however, muscle protein turnover was still suppressed with a tendency for an increase in synthesis causing an improvement in the negative muscle.


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  		TABLE 1. Plasma and intracellular muscle phenylalanine concentration and enrichment

 

Figure 3
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  		FIG. 3. The three-compartment model of leg phenylalanine kinetics. Values are means ± SE (n = 5 saline and n = 6 for rhIL-6). *, Significantly different from basal; #, significantly different from saline (P > 0.05). Fin, Rate of arterial delivery to muscle; Fmuscle-0, de novo appearance of phenylalanine from muscle protein breakdown; Fmuscle-artery, rate of entry into intracellular space from femoral artery (inward transport); Fout, rate of exit via femoral vein; F0,muscle, rate of incorporation protein (measure for muscle protein synthesis); Fv-a, rate of shunting from femoral artery to vein; Fvein-muscle, rate of exit from muscle to femoral vein; inf, infusion.

 
Arterial and muscle amino acid concentration and net leg amino acid exchange (Fig. 4Go)

The arterial concentration of glycine, glutamine, and alanine markedly decreased with rhIL-6 infusion, reaching its nadir after about 2-h infusion. The amino acid concentration tended to return to basal levels 3 h after infusion. rhIL-6 infusion caused a substantial increase in the leg net release of amino acids, which returned to basal and saline levels 3 h after infusion. Glutamate, the only amino acid that under postabsorptive condition is taken up by skeletal muscle, showed in the present study only a modest decrease in the last hour of rhIL-6 infusion, especially when compared with the change in concentration seen for glycine, glutamine, and alanine. In addition, the leg net amino acid release returned to basal and saline levels 3 h after infusion. The intramyocellular amino acid concentrations remained unchanged over time or between rhIL-6 and saline.


Figure 4
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  		FIG. 4. Arterial amino acid concentration (left panels) and net amino acid exchange across the leg (right panels). Data are expressed as means ± SE (n = 6 both groups). *, Significantly different from basal; #, significantly different from saline (P < 0.05).

 
Skeletal muscle is the most important tissue for glutamine storage and de novo synthesis. Therefore, the skeletal muscle net balance of glutamine is determined by the glutamine used for muscle protein synthesis, produced by muscle protein degradation, change in the intracellular free glutamine pool, and de novo synthesis. The free glutamine pool was unchanged and did not contribute to the leg glutamine net exchange during basal conditions, after 3-h infusion, and 3 h after infusion, with 70 ± 7, 77 ± 8, and 68 ± 6 for saline, and 60 ± 6, 58 ± 6, and 62 ± 6 mmol/kg dry mass for rhIL-6, respectively. Thus, in the rhIL-6 trial, the basal leg glutamine production from net muscle protein degradation was approximately 1 mmol/min and the de novo synthesis 14 mmol/min. After 3-h rhIL-6 infusion, the net glutamine production from net protein breakdown was 2.5 mmol/min–1 and from de novo synthesis 34 mmol/min. These data indicate that the increase in net glutamine release from the leg is largely caused by an increase in de novo glutamine synthesis.


   Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
In the present study, we demonstrate the profound effect of IL-6 on amino acid metabolism and skeletal muscle protein turnover during 3-h rhIL-6 infusion that elicited IL-6 levels of 140 pg/ml, a concentration that can be found in healthy individuals during intense prolonged exercise (11). With rhIL-6 infusion, muscle protein turnover was half that during baseline conditions, albeit synthesis was more decreased than breakdown, causing an increase in net muscle protein breakdown. Furthermore, skeletal muscle net release of amino acids increased 2-fold, however, this release did not prevent a marked decrease in the arterial concentration (between 20 and 40%), implying that tissues, other than skeletal muscle, have markedly increased their amino acid demand with rhIL-6.

In a prior study, we have shown that IL-6 infusion induces a lipolytic effect as evidenced by increases in plasma free fatty acids and an increase in muscle fatty acid turnover (13). These changes in lipid metabolism were not dependent on alterations in serum epinephrine, norepinephrine, insulin, or glucagon, or on any adverse effects. Thus, it is interesting that the same doses of IL-6 that induced lipolysis also triggered a global decline in serum amino acids, as well as an increase in muscle net amino acid release. In the present study, the decrease in amino acid concentration and the increase in net efflux from the leg were at a level similar or more pronounced to what is seen within healthy individuals injected with endotoxin (8, 17) and in cancer patients injected sc with recombinant human TNF-{alpha} (5). This negative correlation between the decrease in concentration and skeletal muscle efflux of amino acids indicates a larger peripheral, other than skeletal muscle, utilization of amino acids. A remarkable observation was that beginning 1.5 h after rhIL-6 infusion, a new steady state occurred and that this effect was reversible within 3 h upon cessation of rhIL-6 infusion. The glycine and glutamine graphs (Fig. 4Go) demonstrate that the increase in leg net release preceded the decline in arterial concentration. It is tempting to speculate that in the first hour of rhIL-6 infusion, glutamine utilization was higher compared with 2- to 3-h infusion and matched by demand. The pattern of changes observed in the present study does not support the suggestion that low-plasma glutamine concentration initiates the increase in skeletal muscle release, i.e. a pull rather than a push (8) with endotoxin injection in healthy volunteers. Most likely, the splanchnic tissues are responsible for the increase in amino acid and glutamine demand with rhIL-6 infusion because a 50–100% increase in amino acids has been reported during an endotoxin challenge (17). However, other tissues might also have increased demand because rats challenged with endotoxin showed increased protein synthesis in liver (18, 19), spleen, kidney, jejunum, lungs, and skin (19). The notion that the changes with rhIL-6 infusion are very similar, if not more pronounced, compared with endotoxin injection is remarkable. In the present study, volunteers did not have any signs of adverse symptoms or any changes in key hormones, like insulin, glucagons, cortisol, and catecholamines. The slightly higher cortisol level after 3-h rhIL-6 infusion is unlikely to have caused any metabolic effect, and, if so, it would have led to increased amino acids levels and protein turnover (20). This may suggest that the effects on amino acid metabolism and skeletal muscle protein turnover can be directly attributed to IL-6.

Perhaps the most remarkable finding of the present study is the 50% decrease in skeletal muscle protein turnover driven by rhIL-6 infusion. The decrease in muscle protein synthesis was larger than the decrease in breakdown, and as a result, rhIL-6 caused an increase in net muscle protein degradation. The finding of the increase in net muscle protein breakdown supports the notion of an increased loss of skeletal muscle with inflammation. However, as can be seen from this study, this loss may not be mediated by an increase in breakdown. To our knowledge, there is no information available on the role of IL-6 on human muscle protein metabolism. Rat and mouse muscles incubated in the presence of IL-6 have shown no change in muscle protein breakdown (21, 22, 23). Interestingly, when the muscle was obtained from rats pretreated with IL-6, at levels that induced fever, an increased net muscle breakdown was observed (21). Intravenous injection of IL-6 in rat did not cause an effect on skeletal muscle ubiquitin gene expression (24) or cathepsin activities (25). Chronic elevation of IL-6 in IL-6 transgenic mice caused atrophy in gastrocnemius muscles (26, 27) that could be blocked by treatment with antimouse IL-6 receptor antibody. In addition, enzymatic activities and mRNA levels of lysosomal cathepsins B and L and mRNA levels of ubiquitins were increased but completely eliminated with antimouse IL-6 receptor antibody (27). Therefore, studies in animals seem to suggest that IL-6 does not affect skeletal muscle protein breakdown, however, because net protein breakdown was observed to be enhanced, this must be due to a reduction in muscle protein synthesis. The latter conclusion also applies to the present human study; however, muscle protein breakdown was substantially decreased with elevated IL-6. How can we reconcile this and the markedly reduced muscle protein synthesis rate? One can argue based on our results that the decrease in muscle protein turnover is not a direct effect of IL-6 but, rather, the result of the reduced essential amino acid availability. This decrease in availability is caused by a decreased arterial concentration, which in turn may have caused the reduced inward transport and synthesis rate that also drives a decrease in breakdown. This concept follows from the inverse phenomenon that when essential amino acid availability is enhanced via amino acid infusion, both muscle protein synthesis and breakdown increase, albeit in this case the increase in breakdown is less than that of synthesis, causing net muscle protein deposition (28). In addition, hemodialysis induced a 40% reduction in plasma amino acid concentration in swine, which caused a marked decrease in muscle protein synthesis within 1 h (29, 30). Furthermore, a concomitant decrease in skeletal muscle protein breakdown was observed, although it was less than the reduction in synthesis without a change in intracellular amino acids similar to the findings in the present study. Therefore, we hypothesize that IL-6 does not affect human skeletal muscle protein synthesis and breakdown but profoundly increases amino acid requirements by other tissue, either for energy requirements or increased protein synthesis, leading to reduced systemic amino acid concentrations causing skeletal muscle synthesis to decrease followed by breakdown. Future studies on the role of IL-6 on energy and amino acid metabolism should address the effect of amino acid supplementation to prevent hypoaminoacidemia. The observations of the present study and the aforementioned hypothesis also imply that the positive relationship between an increase in muscle protein turnover and IL-6 levels with hypoaminoacidemia in end-stage renal disease patients is not a causal one but most likely is related to other hormonal changes. The changes in IL-1, IL-6, and IL-10 were modest also in view of marked differences in insulin, glucagon, and norepinephrine (31).

Although it is tempting to draw direct parallels between IL-6 infusion, lipopolysaccharide infusion, and sepsis/trauma/postoperative patients, these represent very different situations. However, some similarities seem to exist. In humans, acute elevation of TNF-{alpha} with recombinant human TNF-{alpha} increased forearm net amino acid release with a concomitant decrease in arterial amino acid concentrations (5) quite similar to the IL-6 response in the present study. Moreover, an acute increase in TNF-{alpha} decreased incubated rat skeletal muscle protein synthesis (32, 33), mediated by affecting mammalian target of rapamycin regulation of translation initiation (33) and increased proteolysis in incubated muscle from TNF-{alpha} pretreated rats (21). Chronic elevation of TNF-{alpha} increased both skeletal muscle synthesis and breakdown (34) with a net muscle protein breakdown (17, 35), which is suggested to occur predominantly in type II fiber (35). Moreover, it was reported that acute treatment with TNF-{alpha} increased ubiquitin gene expression in rat skeletal muscle in contrast to IL-6 (24). The aforementioned studies on the effect of TNF-{alpha} on skeletal muscle protein turnover suggest that TNF-{alpha} increases skeletal muscle proteolysis, in contrast to IL-6, and perhaps decreases muscle protein synthesis. In humans, endotoxin injection caused hypoaminoacidemia (8, 17), and increased amino acid release from skeletal muscle (8) and uptake by the splanchnic tissue in humans (17). In mature and neonatal rats, endotoxemia caused a decrease in muscle protein synthesis (18), an increase in muscle protein breakdown (18, 21), and increased protein synthesis in liver (18, 19), spleen, kidney jejunum, lung, and skin (19). Whereas the reduction on protein synthesis mimics the IL-6 response of the present study, the effect on muscle protein breakdown is more in compliance with TNF-{alpha}. However, in endotoxin studies, profound elevations in catabolic hormones are observed like, for example, cortisol, which might be responsible for the increase in muscle breakdown, albeit usually accompanied by also an increase in protein synthesis.

In summary, we have demonstrated a broad and profound alteration in amino acid turnover caused by IL-6. A significant increase in amino acid turnover and the global nature of the decrease in plasma amino acids with a concomitant 50% decrease in muscle protein turnover with a larger decrease in synthesis than breakdown resulted in an increased efflux of amino acids from muscle. These observations coupled strongly suggest that IL-6 drives amino acids to tissues other than skeletal muscle and that the decrease in muscle protein synthesis reflects the lack of substrate availability rather than a direct inhibition of synthesis. When coupled with our previous findings of IL-6 induced lipolysis, these studies provide further evidence for IL-6 as a potent metabolic signal.


   Footnotes
 
This study received support from: The Centre of Inflammation and Metabolism (supported by a grant from the Danish National Research Foundation: DG 02–512-555); The Copenhagen Muscle Research Centre (supported by grants from The University of Copenhagen, The Faculties of Science and of Health Sciences at this university); The Copenhagen Hospital Corporation, The Danish National Research Foundation (Grant 504–14); and the Commission of the European Communities (contract no. LSHM-CT-2004-005272 EXGENESIS).

P.M. was on a sabbatical leave from the Department of Internal Medicine, University of New Mexico, Albuquerque, New Mexico.

Disclosure Statement: The authors have nothing to declare.

First Published Online April 22, 2008

Abbreviations: LBF, Leg blood flow; Ra, rate of appearance; rhIL, recombinant human IL.

Received October 3, 2007.

Accepted April 15, 2008.


   References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

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