p70 S6 Kinase Activation Is Not Required for Insulin-Like Growth Factor-Induced Differentiation of Rat, Mouse, or Human Skeletal Muscle Cells**This work was supported by research grants from the Direccion General de Investigacion Cientifica y Tecnica (PB95/0971), Fondo de Investigacion Sanitaria (97/2101 and 96/0863), and Generalitat de Catalunya (GRQ 94–1040 and 1995 SGR 537), Spain; a postdoctoral fellowship from Comissionat per a Universitats i Recerca, Generalitat de Catalunya (to P.K.); and a predoctoral fellowship from the Ministerio de Educacion y Cultura (to J.C.).

Abstract

Insulin-like growth factors (IGFs) are potent stimulators of muscle differentiation, and phosphatidylinositol 3-kinase (PI 3-kinase) is an essential second messenger in this process. Little is known about the downstream effectors of the IGF/PI 3-kinase myogenic cascade, and contradictory observations have been reported concerning the involvement of p70 S6 kinase. In an attempt to clarify the role of p70 S6 kinase in myogenesis, here we have studied the effect of rapamycin on rat, mouse, and human skeletal muscle cell differentiation. Both insulin and IGF-II activated p70 S6 kinase in rat L6E9 and mouse Sol8 myoblasts, which was markedly inhibited at 1 ng/ml rapamycin concentrations. Consistent with previous observations in a variety of cell lines, rapamycin exerted a potent inhibitory effect on L6E9 and Sol8 serum-induced myoblast proliferation. In contrast, even at high concentrations (20 ng/ml), rapamycin had no effect on IGF-II-induced proliferation or differentiation. Indeed, neither the morphological differentiation, as assessed by myotube formation, nor the expression of muscle-specific markers such as myogenin, myosin heavy chain, or GLUT4 (glucose transporter-4) glucose carriers was altered by rapamycin. Moreover, here we extended our studies on IGF-II-induced myogenesis to human myoblasts derived from skeletal muscle biopsies. We show that, as observed for rat and mouse muscle cells, human myoblasts can be induced to form multinucleated myotubes in the presence of ex…