UP-REGULATION OF INSULIN-LIKE-GROWTH-FACTOR BINDING PROTEIN-5 IS INDEPENDENT OF MUSCLE-CELL DIFFERENTIATION, SENSITIVE TO RAPAMYCIN, BUT INSENSITIVE TO WORTMANNIN AND LY294002

Skeletal myoblast differentiation is stimulated by insulin-like growth factors (IGFs). The autocrine action of IGFs is mediated through the type-1 IGF receptor (IGFR-1) and modulated by IGF binding proteins (IG FBPs) secreted by the cells. The mouse C2 myoblast cell line stably tr ansfected with a vector producing IGF-II antisense RNA was used to sho w that specific IGFBP expression changes with the state of the cells: high levels of IGFBP-2 messenger RNA (mRNA) were found only in prolife rating myoblasts, whereas IGFBP-3 mRNA was induced in quiescent cells. Secretion of IGFBP5 was strongly stimulated during differentiation. I nsulin and IGF dose-response experiments showed that up-regulation of IGFBP-5 resulted from IGFR-1 activation. Drugs interfering with IGFR-1 signaling and inhibiting myoblast differentiation had different effec ts on IGFBP-5 up-regulation. Two phosphatidylinositol 3-kinase (PI 3-k inase) inhibitors, wortmaninn and LY294002 2-(4-morpholinyl)-8-phenyl- 4H-1-benzopyran-4-one], failed to alter IGFBP-5 up-regulation, which p ersisted in the absence of differentiation. Rapamycin which indirectly prevents activation of the p70 ribosomal protein-S6 kinase (p70(S6k)) , suppressed IGFBP-5 induction. Because the PI3-kinase inhibitors bloc k p70(S6k), neither kinase would be required for IGFR-1-dependent IGFB P-5 induction. In C2 anti-IGF-II myoblasts, IGFBP-5 induction is there fore rapamycin-sensitive and independent of differentiation.