Insulin-Like Growth Factor-Induced Transcriptional Activity of the Skeletal -Actin Gene Is Regulated by Signaling Mechanisms Linked to Voltage-Gated Calcium Channels during Myoblast Differentiation

IGF-I activates signaling pathways that increase the expression of muscle-specific genes in differentiating myoblasts. Induction of skeletal -actin expression occurs during differentiation through unknown mechanisms. The purpose of this investigation was to examine the mechanisms that IGF-I uses to induce skeletal -actin gene expression in C2C12 myoblasts. IGF-I increased skeletal -actin promoter activity by 107% compared with the control condition. Ni [T-type voltagegated Ca channel (VGCC) inhibitor] reduced basal-induced activation of the skeletal -actin promoter by approximately 84%, and nifedipine (L-type VGCC inhibitor) inhibited IGF-Iinduced activation of the skeletal -actin promoter by 29–48%. IGF-I failed to increase skeletal -actin promoter activity in differentiating dysgenic (lack functional L-type VGCC) myoblasts; 30 mM K and 30 mM K IGF-I increased skeletal actin promoter activity by 162% and 76% compared with nonIGF-I or IGF-I-only conditions, respectively. IGF-I increased calcineurin activity, which was inhibited by cyclosporine A. Further, cyclosporine A inhibited K IGF-I-induced activation of the skeletal -actin promoter. Constitutively active calcineurin increased skeletal -actin promoter activity by 154% and rescued the nifedipine-induced inhibition of L-type VGCC but failed to rescue the Ni -inhibition of T-type VGCC. IGF-I-induced nuclear factor of activated T-cells transcriptional activity was not inhibited by nifedipine or Ni . IGF-I failed to increase serum response factor transcriptional activity; however, serum response factor activity was reduced in the presence of Ni . These data suggest that IGF-I-induced activation of the skeletal -actin promoter is regulated by the L-type VGCC and calcineurin but independent of nuclear factor of activated T-cell transcriptional activity as C2C12 myoblasts differentiate into myotubes. (Endocrinology 145: 2054–2063, 2004)