Regulation of follistatin-like protein 1 expression and secretion in primary human skeletal muscle cells.

Follistatin-like protein 1 (Fstl1) is a secreted glycoprotein of the follistatin family. Fstl1 is secreted by C2C12 cells, and Akt1 over-expression in skeletal muscle leads to its induction in muscle and increased circulating levels. So far, secretion of Fstl1 by human myotubes and the effect of exercise on its circulating levels have not been investigated. Here, we examined both the regulation of Fstl1 expression and secretion in primary human skeletal muscle cells and the effect of acute exercise on Fstl1 serum concentrations in humans. We show that human myotubes express and secrete Fstl1 in a differentiation-dependent manner. Furthermore, IFNγ and IL-1β significantly increase Fstl1 secretion. Electrical pulse stimulation (EPS)-induced contractile activity of myotubes did not regulate Fstl1. Interestingly, we observed that 60 min cycling increased serum Fstl1 level by 22%. In conclusion, we demonstrate that Fstl1 is expressed and secreted by human myotubes and plasma Fstl1 levels are increased after exercise.