Insulin-like growth factor (IGF-I) induces myotube hypertrophy associated with an increase in anaerobic glycolysis in a clonal skeletal-muscle cell model

Insulin-like growth factor-I (IGF-I) is an important autocrine/paracrine me diator of skeletal-muscle growth and development. To develop a definitive c ultured cell model of skeletal-muscle hypertrophy, C2C12 cells were stably transfected with IGF-I. and clonal lines developed and evaluated. Quantitat ive morphometric analysis showed that IGF-I-transfected myotubes had a larg er area (2381 +/- 60 mu m(2) versus 1429 +/- 39 mu m(2); P < 0.0001) and a greater maximum width (21.4 +/- 0.6 mu m versus 13.9 +/- 0.3 mu m; P < 0.00 01) than control C2C12 myotubes, independent of the number of cell nuclei p er myotube. IGF-I-transfected myotubes had higher levels of protein synthes is but no difference in DNA synthesis when compared with control myotubes, indicating the development of hypertrophy rather than hyperplasia. Both lac tate dehydrogenase and alanine aminotransferase activities were increased ( 3- and 5-fold respectively), and total lactate levels were higher (2.3-fold ) in IGF-I-transfected compared with control myotubes, indicating an increa se in anaerobic glycolysis in the hypertrophied myotubes. However, expressi on of genes involved in skeletal-muscle growth or hypertrophy in vivo, e.g. myocyte nuclear factor and myostatin, was not altered in the IGF-I myotube s. Finally, myotube hypertrophy could also be induced by treatment of C2C12 cells with recombinant ICF-I or by growing C2C12 cells in conditioned medi a from IGF-I-transfected cells. This quantitative model should be uniquely useful for elucidating the molecular mechanisms of skeletal-muscle hypertro phy.