FNDC5 and irisin in humans: I. Predictors of circulating concentrations in serum and plasma and II. mRNA expression and circulating concentrations in response to weight loss and exercise

Abstract

Objective: In mouse, PGC1-α overexpression in muscle stimulates an increase in expression of FNDC5, a membrane protein that is cleaved and secreted as a newly identified hormone, irisin. One prior study has shown that FNDC5 induces browning of subcutaneous fat in mice and mediates beneficial effects of exercise on metabolism, but a more recent study using gene expression arrays failed to detect a robust increase in FNDC5 mRNA in human muscles from exercising subjects. No prior study has reported on the physiological regulation and role of circulating irisin and FNDC5 in humans.

Materials/methods: A. FNDC5 gene expression studies: We first examined tissue distribution of FNDC5 in humans. B. Cross-sectional studies: Predictors of FNDC5 mRNA expression levels were examined in muscle tissues from 18 healthy subjects with a wide range of BMI. Assays were optimized to measure circulating FNDC5 and irisin levels, and their associations with anthropometric and metabolic parameters were analyzed in two cross-sectional studies that examined 117 middle-aged healthy women and 14 obese subjects, respectively. C. Interventional studies: The effect of weight loss on FNDC5 mRNA and/or circulating irisin levels was examined in 14 obese subjects before and after bariatric surgery. The effect of acute and chronic exercise was then assessed in 15 young healthy adults who performed intermittent sprint running sessions over an 8 week period.

Results: Tissue arrays demonstrated that in humans, the FNDC5 gene is predominantly expressed in muscle. Circulating irisin was detected in the serum or plasma of all subjects studied, whereas circulating FNDC5 was detected in only a distinct minority of the subjects. Cross-sectional studies revealed that circulating irisin levels were positively correlated with biceps circumference (used as a surrogate marker of muscle mass herein), BMI, glucose, ghrelin, and IGF-1. In contrast, irisin levels were negatively correlated with age, insulin, cholesterol, and adiponectin levels, indicating a possible compensatory role of irisin in metabolic regulation. Multivariate regression analysis revealed that biceps circumference was the strongest predictor of circulating irisin levels underlying the association between irisin and metabolic factors in humans at baseline. Both muscle FNDC5 mRNA levels and circulating irisin levels were significantly downregulated 6 months after bariatric surgery. Circulating irisin levels were significantly upregulated 30 min after acute exercise and were correlated mainly with ATP levels and secondarily with metabolites related to glycolysis and lipolysis in muscle.

Conclusions: Similar to mice, the FNDC5 gene is expressed in human muscle. Age and muscle mass are the primary predictors of circulating irisin, with young male athletes having several fold higher irisin levels than middle-aged obese women. Circulating irisin levels increase in response to acute exercise whereas muscle FNDC5 mRNA and circulating irisin levels decrease after surgically induced weight loss in parallel to decrease in body mass. Further studies are needed to study the regulation of irisin levels and its physiological effects in humans and to elucidate the mechanisms underlying these effects.

Fig. 1

FNDC5 gene expression in various human tissues. Quantitative real-time PCR was used to analyze the mRNA level of FNDC5 in 47 different human tissues. Data are presented as means of two experiments and as percentage compared to FNDC5 gene expression in muscle.

Fig. 2

Association of FNDC5 gene expression in human muscle. Pearson’s correlation coefficients of muscle FNDC5 mRNA with (A) BMI, (B) age, (C) PGC1-α mRNA, and (D) PPARγ mRNA in healthy subjects (n=14–18). BMI=body mass index, RU=relative unit.

Fig. 3

Association of circulating irisin with anthropometric parameters in 117 healthy women. Pearson’s correlation coefficients of circulating irisin with (A) BMI, (B) Age, (C) Biceps Circumference, and (D) Fat-free Mass are shown. Pearson’s correlation coefficients and p values are shown in each graph. BMI=body mass index, RU=relative unit.

Fig. 4

Association of circulating irisin with hormonal and metabolic parameters in 117 healthy women. Pearson’s correlation coefficients of circulating irisin with (A) LnAdiponectin (logarithmically transformed), (B) Ghrelin, (C) Glucose, and (D) HDL Cholesterol in plasma are shown. Pearson’s correlation coefficients and the p values are shown in each graph.

Fig. 5

Change in (A) BMI, (B) Fat-free Mass, (C) Insulin, (D) Leptin, (E) Circulating Irisin, and (F) muscle FNDC5 mRNA expression in obese subjects (n=8) before and 6 months after bariatric surgery. Data are shown as means±SE. *p<0.05, **p<0.01, ***p<0.001. 6 months post op. indicates 6 months after operation. BMI=body mass index, RU=relative unit.

Fig. 6

Change in (A) Circulating Irisin, muscle content of (B) ATP (C) Glucose, (D) Glucose-6-phosphate, (E) Lactate, (F) Glycerol-3-phosphate in young healthy subjects pre- (black) and post-exercise (white bar) at weeks 1 and 8 of training. Data are shown as means±SE. *p<0.05, **p<0.01, ***p<0.001.