RESUMO
Sonic Hedgehog (Shh) has been shown to promote adult myoblast proliferation and differentiation and affect Akt phosphorylation via its effector Smoothened (Smo). Here, the relationship between Shh and insulin-like growth factor I (IGF-I) was examined with regard to myogenic differentiation via signaling pathways which regulate this process. Each factor enhanced Akt and MAPK/ERK (p42/44) phosphorylation and myogenic factor expression levels in a dose-responsive manner, while combinations of Shh and IGF-I showed additive effects. Blockage of the IGF-I effects by neutralizing antibody partially reduced Shh's effects on signaling pathways, suggesting that IGF-I enhances, but is not essential for Shh effects. Addition of cyclopamine, a Smo inhibitor, reduced Shh- and IGF-I-induced Akt phosphorylation in a similar manner, implying that Shh affects gain of the IGF-I signaling pathway. This implication was also examined via a genetic approach. In cultures derived from Smo(mut) (MCre;Smo(flox/flox)) mice lacking Smo expression specifically in hindlimb muscles, IGF-I-induced Akt and p42/44 phosphorylation was significantly reduced compared to IGF-I's effect on Smo(cont) cells. Moreover, remarkable inhibition of the stimulatory effect of IGF-I on myogenic differentiation was observed in Smo(mut) cultures, implying that intact Smo is required for IGF-I effects in myoblasts. Immunoprecipitation assays revealed that tyrosine-phosphorylated proteins, including the regulatory unit of PI3K (p85), are recruited to Smo in response to Shh. Moreover, IGF-IR was found to associate with Smo in response to Shh and to IGF-I, suggesting that Shh and IGF-I are already integrated at the receptor level, a mechanism by which their signaling pathways interact in augmenting their effects on adult myoblasts.