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 (alpha, beta, gamma, and delta), of which p38 gamma 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. p38 alpha and -gamma mRNA levels were most abundant in differentiating C2 cells with low/negligible contributions from p38 beta and -delta, respectively. Increased phosphorylation of p38 alpha and -gamma occurred during differentiation but via different mechanisms: p38 alpha protein levels remained constant, whereas total p38 gamma levels increased. Following shRNA knockdown of p38 alpha, 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 p38 gamma induced a transient increase in both myogenin and MHC protein levels with no effect on Igf2 mRNA levels or promoter-reporter activity. Knockdown of p38 alpha/beta markedly increased but that of p38 gamma decreased caspase 3 activity, suggesting opposite actions on apoptosis. p38 gamma was initially proposed to have a promyogenic function; however, p38 gamma overexpression could not rescue reduced myoblast differentiation following p38 alpha/beta inhibition. Therefore, p38 alpha 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; p38 gamma has a minor, yet opposing antimyogenic, function. (Endocrinology 151: 4368-4380, 2010)