FNDC5 relates to skeletal muscle IGF-I and mitochondrial function and gene expression in obese men with reduced growth hormone

- FNDC5 mRNA and circulating irisin are associated with mRNA expression of IGF-I.
- FNDC5 mRNA correlates with the rate of phosphocreatine recovery after exercise.
- FNDC5 mRNA and irisin do not change after 12 weeks of growth hormone replacement.

ObjectiveTo investigate the relationship of skeletal muscle FNDC5 mRNA expression and circulating irisin to the GH/IGF-I axis and to skeletal muscle mitochondrial function and mitochondria-related gene expression in obese men.DesignFifteen abdominally obese men with reduced growth hormone received 12 weeks of recombinant human GH (rhGH). Before and after treatment, they underwent 31P-magnetic resonance spectroscopy to evaluate phosphocreatine (PCr) recovery as a measure of mitochondrial function and skeletal muscle biopsy to assess expression of mitochondrial-related genes. Serum irisin and IGF-I and skeletal muscle FNDC5 and IGF-I mRNA were measured.ResultsAt baseline, skeletal muscle FNDC5 mRNA was significantly and positively associated with IGF-I mRNA (ρ = 0.81, P = 0.005) and rate of PCr recovery (ρ = 0.79, P = 0.006). Similar relationships of circulating irisin to IGF-I mRNA (ρ = 0.63, P = 0.05) and rate of PCr recovery (ρ = 0.48, P = 0.08) were demonstrated, but were not as robust as those with muscle FNDC5 expression. Both serum irisin and skeletal muscle FNDC5 mRNA were significantly associated with PPARγ (ρ = 0.73, P = 0.02 and ρ = 0.85, P = 0.002), respectively. In addition, FNDC5 mRNA was correlated with skeletal muscle PGC-1α (ρ = 0.68, P = 0.03), NRF1 (ρ = 0.66, P = 0.04) and TFAM (ρ = 0.79, P = 0.007) mRNA. Neither serum irisin nor muscle mRNA expression of FNDC5 changed with rhGH treatment.ConclusionThese novel data in skeletal muscle demonstrate that local expression of FNDC5 is associated with mRNA expression of IGF-I and mitochondrial function and mitochondria-related gene expression in obese subjects with reduced growth hormone and suggest a potential role for FNDC5 acting locally in muscle in a low GH state. Further studies are needed to clarify the relationship between the GH/IGF-I axis and irisin.