MEK5 and ERK5 are mediators of the pro-myogenic actions of IGF-2

Abstract

During the differentiation of muscle satellite cells, committed myoblasts respond to specific signalling cues by exiting the cell cycle, migrating, aligning, expressing muscle-specific genes and finally fusing to form multinucleated myotubes. The predominant foetal growth factor, IGF-2, initiates important signals in myogenesis. The aim of this study was to investigate whether ERK5 and its upstream MKK activator, MEK5, were important in the pro-myogenic actions of IGF-2. ERK5 protein levels, specific phosphorylation and kinase activity increased in differentiating C2 myoblasts. ERK5-GFP translocated from the cytoplasm to the nucleus after activation by upstream MEK5, whereas phospho-acceptor site mutated (dominant-negative) ERK5AEF-GFP remained cytoplasmic. Exogenous IGF-2 increased MHC levels, myogenic E box promoter-reporter activity, ERK5 phosphorylation and kinase activity, and rapidly induced nuclear localisation of ERK5. Transfection with antisense Igf2 decreased markers of myogenesis, and reduced ERK5 phosphorylation, kinase and transactivation activity. These negative effects of antisense Igf2 were rescued by constitutively active MEK5, whereas transfection of myoblasts with dominant-negative MEK5 blocked the pro-myogenic action of IGF-2. Our findings suggest that the MEK5-ERK5 pathway is a novel key mediator of IGF-2 action in myoblast differentiation.

Essential Role for p38α MAPK But Not p38γ MAPK in Igf2 Expression and Myoblast Differentiation

“…Next, we investigated the consequences of reduced IGF-II expression in C2 cells by transiently transfecting with an Igf2 antisense construct. In agreement with previous observations (24,25), this delayed differentiation, as assessed by MHC levels (Fig. 1C, P Ͻ 0.01).…”

BTB-Kelch protein Krp1 regulates proliferation and differentiation of myoblasts

“…Withdrawal of serum to initiate differentiation is well established and induces upregulation of the strongly promyogenic growth factor IGF-II (13). As previously observed (7,21), expression of the transcription factor myogenin is observed 24 -48 h after transfer to DM and the later structural protein MHC at 72 h. The abundance of MEF2C gradually increased during differentiation and its apparent mobility decreased due to the increased phosphorylation of MEF2C that occurs as differentiation progresses. Krp1 protein was detectable as early as 24 h after transfer to DM, suggesting that it might have a role in the earlier phases of myogenesis.…”

“…An established scratch wound assay (9, 11), as described in Carter et al (7), was used to investigate whether Krp1 knockdown would change the ability of myoblasts to migrate. Myoblasts were cultured in DM for 48 h and then "scratched"; the distance migrated across the scratch was measured over the next 6 h. No differences were observed between the shMBP, shK1, and shK2 lines (data not shown).…”

Extracellular Signal-Regulated Kinase 5 is Required for Low-Concentration H2O2-Induced Angiogenesis of Human Umbilical Vein Endothelial Cells

“…Activation of ERK5 has previously been shown to regulate myoblast differentiation [37] by controlling the promyogenic actions of insulin-like growth factor 2 (IGF-2) [38]. Together with our results showing that ERK5 also mediates HUVECs tubule formation, these findings indicate that ERK5 regulates the differentiation of multiple cell types.…”