MEK1 plays contrary stage-specific roles in skeletal myogenic differentiation

The role of extracellular signal-regulated kinase (ERK) signaling in skeletal myogenesis has been reported to be both stimulatory and inhibitory. We propose that this discrepancy may arise from the stage-specific, different roles of mitogen-activated protein kinase kinase 1 (MEK1). We found that the phosphorylated MEK1 level of differentiating C2C12 cells was low on day 1 (early-stage) and reached a maximum on days 2-3 (mid-stage). Cells treated at early stage with the MEK-specific inhibitors, PD184352 and U0126, reduced both the MHC protein level and MCK promoter activity, demonstrating that high MEK1 activity at the mid-stage is required for myogenic differentiation. In contrast, treatment with the ERK-specific inhibitors, FR180204 and ERK inhibitor I, had no effect. However, because the sustained overexpression of constitutively active MEK1 inhibits myogenic differentiation, we further analyzed the stage-specific role of MEK1 using the Tet-Off expression system. The results demonstrated that myogenic differentiation was inhibited if active MEK1 expression was induced earlier than day 1 in differentiation condition, but stimulated if induced after that, demonstrating that activated MEK1 plays differential roles depending on activation time. In addition, the induction of active MEK1 at 12 h enhanced the Id2 protein level, while the induction at 36 h resulted in reduction. Thus, MEK1 plays stage-specific and contrary roles in myogenesis, and MEK1 activated at the mid-stage promotes muscle differentiation independent of ERK.