Autocrine Phosphorylation of p70S6k in Response to Acute Stretch in Myotubes

Phosphorylation of 70-KDa S6 kinase (p70^S6k) is correlated with in vivo skeletal muscle hypertrophy. Experiments tested whether mechanical stretch is sufficient to increase p70^S6k phosphorylation in skeletal myotubes. Immediately following stretch, there was a small increase in p70^S6k phosphorylation (63.2 ± 8.5%) with maximal phosphorylation at 3 h (129.5 ± 22.2%) and it remained elevated through 24 h (46.0 ± 17.2%). To test whether an autocrine mechanism is involved, unstretched myotubes were incubated with medium from the stretch group for 10 min. Conditioned medium resulted in the phosphorylation of p70^S6k in unstretched myotubes (92.8 ± 28.9%) to levels comparable to the 3-h stretch group. These data indicate that p70^S6k is phosphorylated in stretched myotubes via a mechanism that most likely involves an autocrine signaling pathway.