Myoblast fusion is essential for skeletal muscle development, growth, and regeneration. However, the molecular mechanisms underlying myoblast fusion and differentiation are not fully understood. Previously, we reported that interleukin-4 (IL-4) promotes myoblast fusion; therefore, we hypothesized that IL-4 signaling might regulate the expression of the molecules involved in myoblast fusion. In this study, we showed that in addition to fusion, IL-4 promoted the differentiation of C2C12 myoblast cells by inducing myoblast determination protein 1 (MyoD) and myogenin, both of which regulate the expression of myomerger and myomaker, the membrane proteins essential for myoblast fusion. Unexpectedly, IL-4 treatment increased the expression of myomerger, but not myomaker, in C2C12 cells. Knockdown of IL-4 receptor alpha (IL-4Rα) in C2C12 cells by small interfering RNA impaired myoblast fusion and differentiation. We also demonstrated a reduction in the expression of MyoD, myogenin, and myomerger by knockdown of IL-4Rα in C2C12 cells, while the expression level of myomaker remained unchanged. Finally, cell mixing assays and the restoration of myomerger expression partially rescued the impaired fusion in the IL-4Rα-knockdown C2C12 cells. Collectively, these results suggest that the IL-4/IL-4Rα axis promotes myoblast fusion and differentiation via the induction of myogenic regulatory factors, MyoD and myogenin, and myomerger.

Interleukin-13 receptor α2 (IL-13Rα2) has traditionally been considered a decoy receptor; however, its cellular functions beyond the immune system remain unclear. We aimed to investigate the role of IL-13Rα2 in C2C12 myoblast proliferation and differentiation. IL-13Rα2 expression was knocked down in C2C12 cells using siRNA. Myogenic differentiation was evaluated by myosin heavy chain (MyHC) immunostaining and by quantifying the expression of myogenic regulatory and fusion-related genes. Myoblast proliferation was assessed using BrdU incorporation and cell number analyses, and signaling events induced by IL-13Rα2 knockdown were analyzed via immunoblotting and immunocytochemical analysis. IL-13Rα2 knockdown did not alter myogenic differentiation or the expression of fusion-associated genes. In contrast, IL-13Rα2 knockdown significantly increased BrdU incorporation and cell number, accompanied by increased Akt phosphorylation and decreased ERK phosphorylation. Cyclin D1 and cyclin-dependent kinase 4 (CDK4) levels were also increased. Akt inhibition abolished the enhanced proliferation and normalized Cyclin D1/CDK4 levels, whereas ERK activation did not further modify the knockdown-associated phenotype. These findings demonstrate that IL-13Rα2 negatively regulates myoblast proliferation by modulating the Akt–Cyclin D1–CDK4 signaling pathway, while being dispensable for myogenic differentiation.