Bente Klarlund Pedersen

IL-6 mediates hypoferremia of inflammation by inducing the synthesis of the iron regulatory hormone hepcidin

Hypoferremia is a common response to systemic infections or generalized inflammatory disorders. In mouse models, the development of hypoferremia during inflammation requires hepcidin, an iron regulatory peptide hormone produced in the liver, but the inflammatory signals that regulate hepcidin are largely unknown. Our studies in human liver cell cultures, mice, and human volunteers indicate that IL-6 is the necessary and sufficient cytokine for the induction of hepcidin during inflammation and that the IL-6-hepcidin axis is responsible for the hypoferremia of inflammation.

Gut Microbiota in Human Adults with Type 2 Diabetes Differs from Non-Diabetic Adults

Background Recent evidence suggests that there is a link between metabolic diseases and bacterial populations in the gut. The aim of this study was to assess the differences between the composition of the intestinal microbiota in humans with type 2 diabetes and non-diabetic persons as control. Methods and Findings The study included 36 male adults with a broad range of age and body-mass indices (BMIs), among which 18 subjects were diagnosed with diabetes type 2. The fecal bacterial composition was investigated by real-time quantitative PCR (qPCR) and in a subgroup of subjects (N = 20) by tag-encoded amplicon pyrosequencing of the V4 region of the 16S rRNA gene. The proportions of phylum Firmicutes and class Clostridia were significantly reduced in the diabetic group compared to the control group (P = 0.03). Furthermore, the ratios of Bacteroidetes to Firmicutes as well as the ratios of Bacteroides-Prevotella group to C. coccoides-E. rectale group correlated positively and significantly with plasma glucose concentration (P = 0.04) but not with BMIs. Similarly, class Betaproteobacteria was highly enriched in diabetic compared to non-diabetic persons (P = 0.02) and positively correlated with plasma glucose (P = 0.04). Conclusions The results of this study indicate that type 2 diabetes in humans is associated with compositional changes in intestinal microbiota. The level of glucose tolerance should be considered when linking microbiota with metabolic diseases such as obesity and developing strategies to control metabolic diseases by modifying the gut microbiota.

Muscles, exercise and obesity: skeletal muscle as a secretory organ

During the past decade, skeletal muscle has been identified as a secretory organ. Accordingly, we have suggested that cytokines and other peptides that are produced, expressed and released by muscle fibres and exert either autocrine, paracrine or endocrine effects should be classified as myokines. The finding that the muscle secretome consists of several hundred secreted peptides provides a conceptual basis and a whole new paradigm for understanding how muscles communicate with other organs, such as adipose tissue, liver, pancreas, bones and brain. However, some myokines exert their effects within the muscle itself. Thus, myostatin, LIF, IL-6 and IL-7 are involved in muscle hypertrophy and myogenesis, whereas BDNF and IL-6 are involved in AMPK-mediated fat oxidation. IL-6 also appears to have systemic effects on the liver, adipose tissue and the immune system, and mediates crosstalk between intestinal L cells and pancreatic islets. Other myokines include the osteogenic factors IGF-1 and FGF-2; FSTL-1, which improves the endothelial function of the vascular system; and the PGC-1α-dependent myokine irisin, which drives brown-fat-like development. Studies in the past few years suggest the existence of yet unidentified factors, secreted from muscle cells, which may influence cancer cell growth and pancreas function. Many proteins produced by skeletal muscle are dependent upon contraction; therefore, physical inactivity probably leads to an altered myokine response, which could provide a potential mechanism for the association between sedentary behaviour and many chronic diseases.

Pro- and anti-inflammatory cytokine balance in strenuous exercise in humans

1 The present study investigates to what extent and by which time course prolonged strenuous exercise influences the plasma concentration of pro‐inflammatory and inflammation responsive cytokines as well as cytokine inhibitors and anti‐inflammatory cytokines. 2 Ten male subjects (median age 27.5 years, range 24–37) completed the Copenhagen Marathon 1997 (median running time 3:26 (h:min), range 2:40–4:20). Blood samples were obtained before, immediately after and then every 30 min in a 4 h post‐exercise recovery period. 3 The plasma concentrations of tumour necrosis factor (TNF)α, interleukin (IL)‐1β, IL‐6, IL‐1ra, sTNF‐r1, sTNF‐r2 and IL‐10 were measured by enzyme‐linked immunosorbent assay (ELISA). The highest concentration of IL‐6 was found immediately after the race, whereas IL‐1ra peaked 1 h post exercise (128‐fold and 39‐fold increase, respectively, as compared with the pre‐exercise values). The plasma level of IL‐1β, TNFα, sTNF‐r1 and sTNF‐r2 peaked in the first hour after the exercise (2.1‐, 2.3‐, 2.7‐ and 1.6‐fold, respectively). The plasma level of IL‐10 showed a 27‐fold increase immediately post exercise. 4 In conclusion, strenuous exercise induces an increase in the pro‐inflammatory cytokines TNFα and IL‐1β and a dramatic increase in the inflammation responsive cytokine IL‐6. This is balanced by the release of cytokine inhibitors (IL‐1ra, sTNF‐r1 and sTNF‐r2) and the anti‐inflammatory cytokine IL‐10. The study suggests that cytokine inhibitors and anti‐inflammatory cytokines restrict the magnitude and duration of the inflammatory response to exercise.

Muscle-derived interleukin-6: mechanisms for activation and possible biological roles

It has recently been demonstrated that the marked increase in the systemic concentration of cytokine interleukin‐6 (IL‐6) seen with exercise originates from the contracting limb and that skeletal muscle cells per se are the likely source of the production. This review summarizes the possible mechanisms for activation and biological consequences of muscle‐derived IL‐6. It appears that intramuscular IL‐6 is stimulated by complex signaling cascades initiated by both calcium (Ca2+) ‐dependent and ‐independent stimuli. It also seems likely that skeletal muscle produces IL‐6 to aid in maintaining metabolic homeostasis during periods of altered metabolic demand such as muscular exercise or insulin stimulation. It may do so via local and/or systemic effects. This review also explores the efficacy that IL‐6 may be used as a therapeutic drug in treating metabolic disorders such as obesity, type 2 diabetes, and atherosclerosis.—Febbraio, M. A., Pedersen, B. K. Muscle‐derived interleukin 6: mechanisms for activation and possible biological roles. FASEB J. 16, 1335–1347 (2002)

Production of interleukin-6 in contracting human skeletal muscles can account for the exercise-induced increase in plasma interleukin-6.

1 Plasma interleukin (IL)‐6 concentration is increased with exercise and it has been demonstrated that contracting muscles can produce IL‐ The question addressed in the present study was whether the IL‐6 production by contracting skeletal muscle is of such a magnitude that it can account for the IL‐6 accumulating in the blood. 2 This was studied in six healthy males, who performed one‐legged dynamic knee extensor exercise for 5 h at 25 W, which represented 40% of peak power output (Wmax). Arterial‐femoral venous (a‐fv) differences over the exercising and the resting leg were obtained before and every hour during the exercise. Leg blood flow was measured in parallel by the ultrasound Doppler technique. IL‐6 was measured by enzyme‐linked immunosorbent assay (ELISA). 3 Arterial plasma concentrations for IL‐6 increased 19‐fold compared to rest. The a‐fv difference for IL‐6 over the exercising leg followed the same pattern as did the net IL‐6 release. Over the resting leg, there was no significant a‐fv difference or net IL‐6 release. The work was produced by 2.5 kg of active muscle, which means that during the last 2 h of exercise, the median IL‐6 production was 6.8 ng min−1 (kg active muscle)−1 (range, 3.96‐9.69 ng min−1 kg−1). 4 The net IL‐6 release from the muscle over the last 2 h of exercise was 17‐fold higher than the elevation in arterial IL‐6 concentration and at 5 h of exercise the net release during 1 min was half of the IL‐6 content in the plasma. This indicates a very high turnover of IL‐6 during muscular exercise. We suggest that IL‐6 produced by skeletal contracting muscle contributes to the maintenance of glucose homeostasis during prolonged exercise.

Evidence that interleukin‐6 is produced in human skeletal muscle during prolonged running

1 This study was performed to test the hypothesis that inflammatory cytokines are produced in skeletal muscle in response to prolonged intense exercise. Muscle biopsies and blood samples were collected from runners before, immediately after, and 2 h after a marathon race. 2 The concentration of interleukin (IL)‐6 protein in plasma increased from 1.5 ± 0.7 to 94.4 ± 12.6 pg ml−1 immediately post‐exercise and to 22.1 ± 3.8 pg ml−1 2 h post‐exercise. IL‐1 receptor antagonist (IL‐1ra) protein in plasma increased from 123 ± 23 to 2795 ± 551 pg ml−1, and increased further to 4119 ± 527 pg ml−1 2 h post‐exercise. 3 The comparative polymerase chain reaction technique was used to evaluate mRNA for IL‐6, IL‐1ra, IL‐1β and tumour necrosis factor (TNF)‐α in skeletal muscle and blood mononuclear cells (BMNC) (n= 8). Before exercise, mRNA for IL‐6 could not be detected either in muscle or in BMNC, and was only detectable in muscle biopsies (5 out of 8) after exercise. Increased amounts of mRNA for IL‐1ra were found in two muscle biopsies and five BMNC samples, and increased amounts of IL‐1β mRNA were found in one muscle and four BMNC samples after exercise. TNF‐α mRNA was not detected in any samples. 4 This study suggests that exercise‐induced destruction of muscle fibres in skeletal muscles may trigger local production of IL‐6, which stimulates the production of IL‐1ra from circulating BMNC.

Aging and proinflammatory cytokines

Aging is associated with increased inflammatory activity reflected by increased circulating levels of TNF-&agr;, IL-6, cytokine antagonists and acute phase proteins in vivo. Epidemiologic studies suggest that chronic low-grade inflammation in aging promotes an atherogenic profile and is related to age-associated disorders (eg, Alzheimer disease, atherosclerosis, type 2 diabetes, etc.) and enhanced mortality risk. Accordingly, a dysregulated production of inflammatory cytokines has an important role in the process of aging. Studies of age-related differences in the production of proinflammatory cytokines in response to acute stimulations in vitro have yielded inconsistent results. However, in vivo infectious models show delayed termination of inflammatory activity and a prolonged fever response in elderly humans, suggesting that the acute phase response is altered in aging. However, a causal relation between the acute phase response and the increased mortality because of bacterial infections in older patients remains to be demonstrated.

Interleukin-6 increases insulin-stimulated glucose disposal in humans and glucose uptake and fatty acid oxidation in vitro via AMP-activated protein kinase.

Although interleukin-6 (IL-6) has been associated with insulin resistance, little is known regarding the effects of IL-6 on insulin sensitivity in humans in vivo. Here, we show that IL-6 infusion increases glucose disposal without affecting the complete suppression of endogenous glucose production during a hyperinsulinemic-euglycemic clamp in healthy humans. Because skeletal muscle accounts for most of the insulin-stimulated glucose disposal in vivo, we examined the mechanism(s) by which IL-6 may affect muscle metabolism using L6 myotubes. IL-6 treatment increased fatty acid oxidation, basal and insulin-stimulated glucose uptake, and translocation of GLUT4 to the plasma membrane. Furthermore, IL-6 rapidly and markedly increased AMP-activated protein kinase (AMPK). To determine whether the activation of AMPK mediated cellular metabolic events, we conducted experiments using L6 myotubes infected with dominant-negative AMPK α-subunit. The effects described above were abrogated in AMPK dominant-negative–infected cells. Our results demonstrate that acute IL-6 treatment enhances insulin-stimulated 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.

Exercise and IL-6 infusion inhibit endotoxin-induced TNF-α production in humans

During “nondamaging” exercise, skeletal muscle markedly releases interleukin (IL)‐6, and it has been suggested that one biological role of this phenomenon is to inhibit the production of tumor necrosis factor (TNF)‐α, which is known to cause pathogenesis such as insulin resistance and atherosclerosis. To test this hypothesis, we performed three experiments in which eight healthy males either rested (CON), rode a bicycle for 3 h (EX), or were infused with recombinant human IL‐6 (rhIL‐6) for 3 h while they rested. After 2.5 h, the volunteers received a bolus of Escherichia coli lipopolysaccharide endotoxin (0.06 ng/kg) i.v. to induce low‐grade inflammation. In CON, plasma TNF‐α increased significantly in response to endotoxin. In contrast, during EX, which resulted in elevated IL‐6, and rhIL‐6, the endotoxin‐induced increase in TNF‐α was totally attenuated. In conclusion, physical exercise and rhIL‐6 infusion at physiological concentrations inhibit endotoxin‐induced TNF‐α production in humans. Hence, these data provide the first experimental evidence that physical activity mediates antiinflammatory activity and suggest that the mechanism include IL‐6, which is produced by and released from exercising muscles.