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

The muscle satellite cell is established as the major stem cell contributing to fiber growth and repair. p38 MAPK signaling is essential for myoblast differentiation and in particular for up-regulation of promyogenic Igf2 expression. p38 exists as four isoforms (α, β, γ, and δ), of which p38γ is uniquely abundant in muscle. The aim of this study was to characterize p38 isoform expression and importance (using shRNA knockdown; demonstrated via both reduced protein and kinase activities) during myoblast differentiation. p38α and -γ mRNA levels were most abundant in differentiating C2 cells with low/negligible contributions from p38β and -δ, respectively. Increased phosphorylation of p38α and -γ occurred during differentiation but via different mechanisms: p38α protein levels remained constant, whereas total p38γ levels increased. Following shRNA knockdown of p38α, myoblast differentiation was dramatically inhibited [reduced myosin heavy chain (MHC), myogenin, pAkt protein levels]; significantly, Igf2 mRNA levels and promoter-reporter activities decreased. In contrast, knockdown of p38γ induced a transient increase in both myogenin and MHC protein levels with no effect on Igf2 mRNA levels or promoter-reporter activity. Knockdown of p38α/β markedly increased but that of p38γ decreased caspase 3 activity, suggesting opposite actions on apoptosis. p38γ was initially proposed to have a promyogenic function; however, p38γ overexpression could not rescue reduced myoblast differentiation following p38α/β inhibition. Therefore, p38α is essential for myoblast differentiation, and part of its action is to convert signals that indicate cell density into promyogenic gene expression in the form of the key peptide, IGF-II; p38γ has a minor, yet opposing antimyogenic, function.