FNDC5/irisin is not only a myokine but also an adipokine

Abstract

Exercise provides clear beneficial effects for the prevention of numerous diseases. However, many of the molecular events responsible for the curative and protective role of exercise remain elusive. The recent discovery of FNDC5/irisin protein that is liberated by muscle tissue in response to exercise might be an important finding with regard to this unsolved mechanism. The most striking aspect of this myokine is its alleged capacity to drive brown-fat development of white fat and thermogenesis. However, the nature and secretion form of this new protein is controversial. The present study reveals that rat skeletal muscle secretes a 25 kDa form of FNDC5, while the 12 kDa/irisin theoretical peptide was not detected. More importantly, this study is the first to reveal that white adipose tissue (WAT) also secretes FNDC5; hence, it may also behave as an adipokine. Our data using rat adipose tissue explants secretomes proves that visceral adipose tissue (VAT), and especially subcutaneous adipose tissue (SAT), express and secrete FNDC5. We also show that short-term periods of endurance exercise training induced FNDC5 secretion by SAT and VAT. Moreover, we observed that WAT significantly reduced FNDC5 secretion in fasting animals. Interestingly, WAT of obese animals over-secreted this hormone, which might suggest a type of resistance. Because 72% of circulating FNDC5/irisin was previously attributed to muscle secretion, our findings suggest a muscle-adipose tissue crosstalk through a regulatory feedback mechanism.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1. Muscle and adipose tissue sample viability and FNDC5 secretion characterization.

The presence of IL-6 and adiponectin in muscle (oxidative-soleus and glycolytic-gastrocnemius) and adipose tissue (subcutaneous and visceral) secretomes assessed by western blot are shown in corresponding histograms for band quantification (A and B). PGC1α expression was tested in the same samples by real-time PCR (C and D). The presence of whole FNDC5 (E) and the soluble form of irisin (F) in muscle and adipose tissues are shown in representative images. Histograms of the band quantification from at least 3 independent experiments are shown, and significance is indicated with regard to the control soleus (*p#p<0.05) (G). CONT: control ad libitum animals; 1 and 3 W: 1 and 3 weeks of exercise training.

Figure 2. Adipose tissue secretion of FNDC5/irisin.

FNDC5/irisin secretion was assessed in adipose tissue secretomes from SAT, VAT and BAT under ad libitum conditions (A); in SAT and VAT secretomes from animals after one and three weeks of exercise training (B); and in SAT, VAT and BAT from animals with 36-h food restriction or re-feed after 15 minutes (C). FNDC5/irisin SAT and VAT secretion from the DIO and Zucker animal models and anorexic animals are also shown (D, E). Irisin circulating levels for all previous situations are shown taking the most intense band at 25 kDa (F). Representative images of western blots and histograms of band quantifications from at least 3 independent experiments are shown. The results are represented as percentages with regard to each control as well as significance. In the situation of 3 weeks of exercise, results are compared with 1 week of exercise (*p

Figure 3. Adipose tissue expression of FNDC5.

FNDC5 expression was assessed in whole adipose tissue using QRT-PCR and western blot in the same secretion study samples shown in Figure 2. Representative images of western blots and histograms of band quantification towards GAPDH from at least 3 independent experiments are shown. Significant protein results are represented as percentages with regard to each control as well as statistically significance (*p

Figure 4. UCP1 detection in adipose tissue.

Representative western blot images of UCP1 for SAT, VAT and VAT under ad libitum condition and after 3 weeks of exercise are shown (A); UCP1 expression in SAT and VAT among anorexic and control counterparts is shown (B). Histograms of band quantification from at least 3 independent experiments are shown in those cases in which bands exist. Significant protein results are shown as percentages compared with controls in A and with regard to EXER in B (*p

Figure 5. Adipose tissue expression of PGC1α.

PGC1α expression was assessed in whole adipose tissue using western blot in the same secretion study samples shown in Figure 2. Representative images of western blots and histograms of band quantification towards GAPDH from at least 3 independent experiments are shown. Significant protein results are represented as percentages with regard to each control (*p

Figure 6. Mature adipocytes in rats and human adipose tissue secrete FNDC5.

The occurrence of FNDC5 in rat and human adipocytes as well as SVCs secretomes (A); in differentiated and non-differentiated 3T3-L1 cell secretomes (B); and in human subcutaneous and visceral adipose tissues and secretomes (C).