S-myotrophin promotes the hypertrophy of myotube as insulin-like growth factor-I does

We have reported previously that a novel muscle cell growth factor, having a structure of a peptide with sugar chains, was successfully purified from porcine skeletal muscle. It was named s-myotrophin. To determine the role o f s-myotrophin in skeletal muscle growth, the effect of s-myotrophin on pri mary cultured chick skeletal muscle cells (composed almost totally of multi nucleated myotubes) was investigated by comparing s-myotrophin with Insulin -like growth factor-I (IGF-I). Both s-myotrophin and IGF-I significantly in creased creatine kinase activity of the cultures; both substances gave simi lar responses. Intracellar protein content was also increased by the additi on of these factors. The content of myosin and actin in s-myotrophin treate d culture in the differentiation medium was significantly higher than that of the control (unstimulated). The content of those proteins in IGF-I treat ed culture was also higher than that of control, but the differences were n ot statistically significant. Immunoblot analysis confirmed that the amount s of myosin and actin in the myocytes were greatly increased by s-myotrophi n stimulation and also by IGF-I stimulation. Morphological observations usi ng an anti-desmin antibody staining procedure demonstrated that the size of both s-myotrophin and IGF-I treated myotubes was appreciably larger than t hat of control myotubes. These results suggest that s-myotrophin is a poten t mediator of skeletal muscle cell hypertrophy thorough the accumulations o f muscle structural proteins. (C) 2001 Elsevier Science Ltd. All rights res erved.