Interleukin-6 Promotes Myogenic Differentiation of Mouse Skeletal Muscle Cells: Role of the STAT3 Pathway

Myogenic differentiation of skeletal muscle cells is characterized by a sequence of events that include activation of signal transducer and activator of transcription 3 (STAT3) and enhanced expression of its target gene Socs3. Autocrine effects of IL-6 may contribute to the activation of the STAT3-Socs3 cascade and thus to myogenic differentiation. The importance of IL-6 and STAT3 for the differentiation process was studied in C2C12 cells and in primary mouse wild-type and IL-6(-/-) skeletal muscle cells. In differentiating C2C12 myoblasts, the upregulation of IL-6 mRNA expression and protein secretion started after increased phosphorylation of STAT3 on tyrosine 705 and increased mRNA expression of Socs3 was observed. Knockdown of STAT3 and IL-6 mRNA in differentiating C2C12 myoblasts impaired the expression of the myogenic markers myogenin and MyHC IIb and subsequently myotube fusion. However, the knockdown of IL-6 did not prevent the induction of STAT3 tyrosine phosphorylation. The IL-6-independent activation of STAT3 was verified in differentiating primary IL-6(-/-) myoblasts. The phosphorylation of STAT3 and the expression levels of STAT3, Socs3, and myogenin during differentiation were comparable in the primary myoblasts independent of the genotype. However, IL-6(-/-) cells failed to induce MyHC IIb expression to the same level as in wild-type cells and showed reduced myotube formation. Supplementation of IL-6 could partially restore the fusion of IL-6(-/-) cells. These data demonstrate that IL-6 depletion during myogenic differentiation does not reduce the activation of the STAT3-Socs3 cascade, while IL-6 and STAT3 are both necessary to promote myotube fusion.