Growth hormone (GH) regulates insulin-like growth factor one (IGF-1) production predominantly through the intracellular signallers Stat5a and Stat5b. Loss of Stat5b results in reduced production of liver IGF-1 and loss of sexually dimorphic growth in males. However, no study has observed the phenotype beyond twelve weeks, or looked at expression of IGF-1, IGF-1 receptors (IGF-1R), GH receptors (GHR), myostatin, androgen receptors (AR) and SOCS2 mRNA in skeletal muscles. The hindlimb muscles and abdominal fat of male and female Stat5b knockout mice and wild type mice were collected at 6, 12 and 24 weeks of age (n=16 per time and sex). Muscle mass was normalised to tibia length and abdominal fat mass was normalised to body mass. C2C12 myoblast cell lines were treated with viral Stat5b siRNA, Stat5a siRNA or a scrambled vector as a control, then differentiated and treated with 100ng/mL of GH for 24 hours. RNA and protein were harvested for quantitative PCR and Western blotting.

Nose-to-tail length, tibia length and normalised hindlimb muscle mass were decreased to a greater extent in male (29.8%) than in female (11.5%) Stat5b knockout mice versus wild type mice at all ages (P <0.001). Both male and female Stat5b knockout mice had a greater abundance of abdominal fat than wild type mice at 24 weeks (P <0.05). Individual Stat5a and Stat5b knockdown both blocked the GH induced increase in IGF-1, IGF-1R and GHR mRNA. Additionally Stat5a knockdown blocked the increase in AR and SOCS2 mRNA. Stat5b knockdown reduced the abundance of mature myostatin protein. We conclude that (1), sexual dimorphism persists in Stat5b-null mice, (2) both Stat5a and Stat5b have essential signalling roles in skeletal muscle wherein Stat5a regulates SOCS2 and AR, Stat5b regulates myostatin and both Stat5a and Stat5b independently, or a heterodimer between the two, regulate IGF-1, IGF-1R and GHR.