Ca(2+) calmodulin kinase and calcineurin mediate IGF-1-induced skeletal muscle dihydropyridine receptor alpha(1S) transcription.

The skeletal muscle L-type Ca(2+) channel or dihydropyridine(DHP)-sensitive receptor is a key molecule involved in membrane voltage-sensing, sarcoplasmic reticulum Ca(2+) release, and muscle contraction. Previous work from our laboratory has shown that the insulin-like growth factor-1 (IGF-1) increases skeletal muscle L-type Ca(2+) channel or dihydropyridine-sensitive receptor DHPRalpha(1S) transcriptional activity by acting on the cyclic AMP response element binding protein (CREB) element of the promoter region; however, the cellular signaling mediating this process is not known. In this study, we investigated the signaling pathway whereby IGF-1 enhances the expression of DHPRalpha(1S) in C2C12 myotubes, using a molecular, pharmacological and electrophysiological approach. We found that inhibition of the Ca(2+)/Calmodulin (CaM)-dependent protein kinase or calcineurin, influenced IGF-1-induced increase in DHPRalpha(1S) expression, as detected by recording the luminescence of the DHPRalpha(1S) promoter-luciferase fusion construct and by immunoblot analysis of the DHPR alpha1 subunit. IGF-1 significantly increased CaM kinase and calcineurin activity and the cellular levels of phosphorylated CREB in a time-dependent manner. The role of CaM kinase and calcineurin in DHPRalpha(1S) expression was confirmed by functional recording of the effects of the inhibition of the kinase and phosphatase on IGF-1-mediated enhancement of charge movement. These results support the conclusion that IGF-1 controls CREB phosphorylation by activating a phosphorylation and dephosphorylation cascade, which ultimately modulates the DHPRalpha(1S) gene transcription.