Insulin-like Growth Factor-mediated Muscle Differentiation: COLLABORATION BETWEEN PHOSPHATIDYLINOSITOL 3-KINASE-Akt-SIGNALING PATHWAYS AND MYOGENIN

Jolana Tureckova,Elizabeth M Wilson,Jennifer L Cappalonga,Peter Rotwein

doi:10.1074/jbc.m104991200

Jolana Tureckova, Elizabeth M Wilson + Show 2 more

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https://doi.org/10.1074/jbc.m104991200

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Abstract

The differentiation and maturation of skeletal muscle require interactions between signaling pathways activated by hormones and growth factors and an intrinsic regulatory network controlled by myogenic transcription factors. Insulin-like growth factors (IGFs) play key roles in muscle development in the embryo and in regeneration in the adult. To study mechanisms of IGF action in muscle, we developed a myogenic cell line that overexpresses IGF-binding protein-5. C2BP5 cells remain quiescent in low serum differentiation medium until the addition of IGF-I. Here we use this cell line to identify signaling pathways controlling IGF-mediated differentiation. Induction of myogenin by IGF-I and myotube formation were prevented by the phosphatidylinositol (PI) 3-kinase inhibitor, LY294002, even when included 2 days after growth factor addition, whereas expression of active PI 3-kinase could promote differentiation in the absence of IGF-I. Differentiation also was induced by myogenin but was blocked by LY294002. The differentiation-promoting effects of IGF-I were mimicked by a modified membrane-targeted inducible Akt-1 (iAkt), and iAkt was able to stimulate differentiation of C2 myoblasts and primary mouse myoblasts incubated with otherwise inhibitory concentrations of LY294002. These results show that an IGF-regulated PI 3-kinase-Akt pathway controls muscle differentiation by mechanisms acting both upstream and downstream of myogenin.

Highlights

The differentiation and maturation of skeletal muscle require interactions between signaling pathways activated by hormones and growth factors and an intrinsic regulatory network controlled by myogenic transcription factors
In Insulin-like growth factors (IGFs)-I-treated cells myogenin was readily induced within 2 days, and expression was increased for up to 4 days, whereas little myogenin was detected in myoblasts incubated in differentiation medium (DM) alone (Fig. 1C)
We find in C2BP5 myoblasts that IGF-mediated differentiation was completely prevented by the PI 3-kinase inhibitor, LY294002, and that ectopic expression of a constitutively active PI 3-kinase could promote differentiation in the absence of IGF-I to as great an extent as the myogenic transcriptonal regulator, myogenin

Summary

Introduction

The differentiation and maturation of skeletal muscle require interactions between signaling pathways activated by hormones and growth factors and an intrinsic regulatory network controlled by myogenic transcription factors. Induction of myogenin by IGF-I and myotube formation were prevented by the phosphatidylinositol (PI) 3-kinase inhibitor, LY294002, even when included 2 days after growth factor addition, whereas expression of active PI 3-kinase could promote differentiation in the absence of IGF-I.

Results

Conclusion