Elevated skeletal muscle irisin precursor FNDC5 mRNA in obese OLETF rats

Abstract

Objective: There is debate as to whether fibronectin type III domain containing 5 (FNDC5) and its protein product irisin are therapeutic targets for obesity-associated maladies. Thus, we sought to examine FNDC5 mRNA within skeletal muscle of obese/diabetic-prone Otsuka Long-Evans Tokushima Fatty (OLETF) rats versus lean/healthy Long Evans Tokushima Otsuka (LETO) rats. We hypothesized that FNDC5 expression would be greater in obese (OLETF) versus lean (LETO) animals.

Materials/methods: Triceps muscle of 30-32week old OLETF and LETO rats were assayed for FNDC5 and PGC1α mRNA levels. Body composition and circulating biomarkers of the OLETF and LETO rats were also correlated with skeletal muscle FNDC5 mRNA expression patterns in order to examine potential relationships that may exist.

Results: OLETF rats exhibited twice the amount of triceps FNDC5 mRNA compared to LETO rats (p<0.01). Significant positive correlations existed between triceps muscle FNDC5 mRNA expression patterns versus fat mass (r=0.70, p=0.008), as well as plasma leptin (r=0.82, p<0.001). PGC1α mRNA levels were also highly correlated with FNDC5 mRNA (r=0.85, p<0.001). In subsequent culture experiments, low and high physiological doses of leptin had no effect on PGC1α mRNA or FNDC5 mRNA levels in C2C12 myotubes. Paradoxically, circulating irisin concentrations tended to be higher in a second cohort of LETO versus OLETF rats (p=0.085).

Conclusion: These results reveal a positive association between total body adiposity and skeletal muscle FNDC5 gene expression. Of interest, circulating irisin levels tended to be lower in OLETF rats. Further research is needed to examine whether other adipose tissue-derived factors up-regulate FNDC5 transcription and/or inhibit irisin biosynthesis from FNDC5.

Keywords: FNDC5; Irisin; LETO; Long Evans Tokushima Otsuka; OLETF; Obesity; Otsuka Long-Evans Tokushima Fatty; PGC1α; Peroxisome proliferator-activated receptor gamma coactivator 1-alpha; Skeletal muscle; fibronectin type III domain containing 5.

Figure 1

Relationship between muscle FNDC5 mRNA and adiposity. Obese OLETF rats possessed a greater triceps muscle FNDC5 mRNA expression compared to LETO rats (A). In a second cohort of rats, LETO rats tended to present more circulating irisin compared to OLETF rats OLETF (B). In all rats from panel A, positive correlations existed between triceps muscle FNDC5 mRNA expression versus fat mass, triceps muscle PGC1α and plasma leptin (C). Despite the positive association between circulating leptin and triceps muscle FNDC5 mRNA expression, 24-h and 48- treatments with low (8 ng/ml) and high (150 ng/ml) physiological doses of leptin did not alter the mRNA expression of FNDC5 (mRNA precursor to irisin) in C₂C₁₂ myoblasts (D). In panel D, all data are normalized to DM-only-treated cells and are presented as mean ± SE (n = 4 wells per treatment).