Myogenic signaling by lithium in cardiomyoblasts is Akt independent but requires activation of the β-catenin-Tcf/Lef pathway

Rat H9c2 cardiomyoblasts can proliferate and maintain an undifferentiated state in the presence of serum. These cardiomyoblasts have been used as a cellular model to study myogenic differentiation after serum withdrawal. Here, we examined the effects of lithium, a known inhibitor of glycogen synthase kinase-3β and activator of Wnt pathway in myogenic differentiation. We show that in the presence of serum, lithium induced the differentiation of H9c2 cells as measured by multinucleated myotube formation and expression of the muscle-specific proteins, myogenin and skeletal α-actin. This differentiation was preceded by nuclear accumulation of β-catenin, which was associated with increased Tcf/Lef-dependent transcription. We also observed that lithium mediated the activation of phosphatidylinositol 3-kinase (PI3-kinase) and its downstream target Akt. Inhibition of PI3-kinase by LY294002 and over-expression of dominant-negative PI3-kinase caused a marked reduction in β-catenin levels. This inhibition was associated with decreased β-catenin-Tcf/Lef-dependent transcription, lack of multinucleated myotube formation, and expression of the muscle-specific proteins. In contrast, expression of dominant-negative Akt failed to inhibit the effects of lithium. We conclude that the capacity of lithium to overcome the inhibitory effects of serum and to induce the differentiation of H9c2 cardiomyoblasts is mediated, in part, by the stabilization and nuclear translocation of β-catenin in a PI3-kinase-dependent but Akt-independent manner. Once activated, β-catenin then interacts with the Lef/Tcf complex to regulate expression of myogenic-inducing genes.