Irisin was first identified as a peroxisome proliferator‐activated receptor γ co‐activator‐1α (PGC‐1α) dependent myokine with the potential to induce murine brown‐fat‐like development of white adipose tissue. In humans, the regulatory effect of training on muscle FNDC5mRNA expression and subsequently irisin levels in plasma is more controversial. We recruited 26 inactive men (13 normoglycaemic and normal weight, controls; and 13 slightly hyperglycaemic and overweight, pre‐diabetes group) aged 40–65 years for a 12‐week intervention of combined endurance and strength training with four sessions of training per week. Before and after the 12‐week intervention period, participants were exposed to an acute endurance workload of 45 min at 70% of VO_2max, and muscle biopsies were taken prior to and after exercise. Skeletal muscle mRNA for PGC1A and FNDC5 correlated and both PGC1A and FNDC5mRNA levels increased after 12 weeks of training in both control and pre‐diabetes subjects. Circulating irisin was reduced in response to 12 weeks of training, and was increased acutely (~1.2‐fold) just after acute exercise. Plasma concentration of irisin was higher in pre‐diabetes subjects compared with controls. There was little effect of 12 weeks of training on selected browning genes in subcutaneous adipose tissue. UCP1mRNA did not correlate with FNDC5 expression in subcutaneous adipose tissue or skeletal muscle or with irisin levels in plasma. We observed no enhancing effect of long‐term training on circulating irisin levels, and little or no effect of training on browning of subcutaneous white adipose tissue in humans.

The myokine irisin is supposed to be cleaved from a transmembrane precursor, FNDC5 (fibronectin type III domain containing 5) and to mediate beneficial effects of exercise on human metabolism. However, evidence for irisin circulating in blood is largely based on commercial ELISA kits which are based on polyclonal antibodies (pAbs) not previously tested for cross-reacting serum proteins. We have analyzed four commercial pAbs by Western blotting, which revealed prominent cross-reactivity with non-specific proteins in human and animal sera. Using recombinant glycosylated and non-glycosylated irisin as positive controls, we found no immune-reactive bands of the expected size in any biological samples. A FNDC5 signature was identified at ~20 kDa by mass spectrometry in human serum but was not detected by the commercial pAbs tested. Our results call into question all previous data obtained with commercial ELISA kits for irisin and provide evidence against a physiological role for irisin in humans and other species.

Brown adipose tissue has gained interest as a potential target to treat obesity and metabolic diseases. Irisin is a newly identified hormone secreted from skeletal muscle enhancing browning of white fat cells, which improves systemic metabolism by increasing energy expenditure in mice. The discovery of irisin raised expectations of its therapeutic potential to treat metabolic diseases. However, the effect of irisin in humans is unclear. Analyses of genomic DNA, mRNA and expressed sequence tags revealed that FNDC5, the gene encoding the precursor of irisin, is present in rodents and most primates, but shows in humans a mutation in the conserved start codon ATG to ATA. HEK293 cells transfected with a human FNDC5 construct with ATA as start codon resulted in only 1% full-length protein compared to human FNDC5 with ATG. Additionally, in vitro contraction of primary human myotubes by electrical pulse stimulation induced a significant increase in PGC1α mRNA expression. However, FNDC5 mRNA level was not altered. FNDC5 mRNA expression in muscle biopsies from two different human exercise studies was not changed by endurance or strength training. Preadipocytes isolated from human subcutaneous adipose tissue exhibited differentiation to brite human adipocytes when incubated with bone morphogenetic protein (BMP) 7, but neither recombinant FNDC5 nor irisin were effective. In conclusion, our findings suggest that it is rather unlikely that the beneficial effect of irisin observed in mice can be translated to humans.

In addition to generating movement, skeletal muscle may have a function as a secretory organ. The aim of the present study was to identify novel proteins with signaling capabilities secreted from skeletal muscle cells. IL-7 was detected in media conditioned by primary cultures of human myotubes differentiated from satellite cells, and concentrations increased with incubation time. By immunoblotting and real-time RT-PCR IL-7 expression was confirmed at both protein and mRNA levels. Furthermore, with immunofluorescence and specific antisera, multinucleated myotubes were found to coexpress IL-7 and myosin heavy chain. During differentiation of human myotubes from satellite cells, IL-7 expression increased at mRNA and protein levels. In contrast, mRNA expression of the IL-7 receptor was 80% lower in myotubes compared with satellite cells. Incubations with recombinant IL-7 under differentiation conditions caused ∼35% reduction in mRNA for the terminal myogenic markers myosin heavy chain 2 (MYH2) and myogenin (MYOG), suggesting that IL-7 may act on satellite cells to inhibit development of the muscle fiber phenotype. Alternative routes of cell development were investigated, and IL-7 increased migration of satellite cells by 40% after 48 h in a Transwell system, whereas cell proliferation remained unchanged. In vivo, real-time RT-PCR analysis of musculus vastus lateralis ( n = 10) and musculus trapezius ( n = 7) biopsies taken from male individuals undergoing a strength training program demonstrated that after 11 wk mean IL-7 mRNA increased by threefold ( P = 0.01) and fourfold ( P = 0.04), respectively. In conclusion, we have demonstrated that IL-7 is a novel myokine regulated both in vitro and in vivo, and it may play a role in the regulation of muscle cell development.