Article

Akt phosphorylation is not sufficient for insulin-like growth factor-stimulated myogenin expression but must be accompanied by down-regulation of mitogen-activated protein kinase/extracellular signal-regulated kinase phosphorylation

Department of Pediatrics, University of California, San Francisco, San Francisco, California, United States
Endocrinology (Impact Factor: 4.64). 12/2004; 145(11):4991-6. DOI: 10.1210/en.2004-0101
Source: PubMed

ABSTRACT IGF-I has a unique biphasic effect on skeletal muscle differentiation. Initially, IGF-I inhibits expression of myogenin, a skeletal muscle-specific regulatory factor essential for myogenesis. Subsequently, IGF-I switches to stimulating expression of myogenin. The mechanisms that mediate this switch in IGF action are incompletely understood. Several laboratories have demonstrated that the phosphatidylinositol-3-kinase/Akt signaling pathway is essential for myogenic differentiation and have suggested that this pathway mediates IGF-I stimulation of myogenin mRNA expression, an early critical step in the differentiation process. These studies, however, did not address concurrent Akt and MAPK/ERK1/2 phosphorylation, the latter of which is also known to regulate myogenic differentiation. In the present study in rat L6E9 muscle cells, we have manipulated ERK1/2 phosphorylation with either an upstream inhibitor or activator and examined concurrent levels of Akt and ERK1/2 phosphorylation and of myogenin mRNA expression in response to treatment with IGF-I. We find that even in the presence of phosphorylated Akt, it is only when ERK1/2 phosphorylation is inhibited that IGF-I can stimulate myogenin mRNA expression. Thus, although Akt phosphorylation may be necessary, it is not sufficient for induction of myogenic differentiation by IGF-I and must be accompanied by a decrease in ERK1/2 phosphorylation.

Download full-text

Full-text

Available from: Nicki Tiffin, Mar 11, 2014
0 Followers
 · 
170 Views
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Skeletal muscle responds to mechanical stimulation by activating p38 MAPK, a key signal for myogenesis. However, the mechanotransduction mechanism that activates p38 is unknown. Here we show that mechanical stimulation of myoblasts activates p38 and myogenesis through stimulating TNF-alpha release by TNF-alpha converting enzyme (TACE). In C2C12 or mouse primary myoblasts cultured in growth medium, static stretch activated p38 along with ERK1/2, JNK and AKT. Disrupting TNF-alpha signaling by TNF-alpha-neutralizing antibody or knocking out TNF-alpha receptors blocked stretch activation of p38, but not ERK1/2, JNK or AKT. Stretch also activated differentiation markers MEF2C, myogenin, p21 and myosin heavy chain in a TNF-alpha- and p38-dependent manner. Stretch stimulated the cleavage activity of TACE. Conversely, TACE inhibitor TAPI or TACE siRNA abolished stretch activation of p38. In addition, conditioned medium from stretched myoblast cultures activated p38 in unstretched myoblasts, which required TACE activity in the donor myoblasts, and TNF-alpha receptors in the recipient myoblasts. These results indicate that posttranscriptional activation of TACE mediates the mechanotransduction that activates p38-dependent myogenesis via the release of TNF-alpha.
    Journal of Cell Science 03/2007; 120(Pt 4):692-701. DOI:10.1242/jcs.03372 · 5.33 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Insulin-like growth factors (IGFs) are regulators that modulate the proliferation and differentiation of muscle tissues. We quantified the messenger RNA (mRNA) expression of IGF-I, IGF-II, and type I and II IGF receptors (IGF-IR and IGF-IIR) in muscle tissues including the breast, leg, and myocardium during an early postnatal development growth stage (post-hatching weeks 1-8) in ducks. The results showed a significant age-related change in mRNA in these muscle tissues. In breast muscle, the developmental expression of IGF-I and IGF-II was highest during week 1 but decreased quickly and was maintained a relatively lower level. Leg muscle had the highest mRNA expression of IGF-I and IGF-II genes at week 3. In myocardial tissues, the expression level of IGF-IR and IGF-IIR genes exhibited a "rise-decline" developmental trend. The expression pattern of IGF-I/IGF-IR and IGF-II/IGF-IIR were different between weeks 4 and 6. The same expression pattern was observed for IGF-I and IGF-IR; however, it was different from that for IGF-II and IGF-IIR. Our results showed a negative correlation between IGF-II mRNA expression and leg muscle weight at week 4 (P < 0.05). A negative correlation was also found between IGF-II mRNA expression and breast muscle weight (P < 0.01), and a positive correlation was found between IGF-IR expression and breast muscle weight. At week 6, a positive correlation was found between IGF-IR expression and breast muscle weight. However, at week 8, a negative correlation was found between IGF-IR expression and breast muscle weight. The results showed that the expression of IGF mRNA in duck tissues exhibits a specific developmental trend and an age-related pattern, suggesting that the regulation mechanism of these 4 genes in proliferation and differentiation of muscle tissues differed.
    Genetics and molecular research: GMR 05/2013; 12(AOP). DOI:10.4238/2013.May.6.3 · 0.85 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: 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 p38gamma 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. p38alpha and -gamma mRNA levels were most abundant in differentiating C2 cells with low/negligible contributions from p38beta and -delta, respectively. Increased phosphorylation of p38alpha and -gamma occurred during differentiation but via different mechanisms: p38alpha protein levels remained constant, whereas total p38gamma levels increased. Following shRNA knockdown of p38alpha, 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 p38gamma induced a transient increase in both myogenin and MHC protein levels with no effect on Igf2 mRNA levels or promoter-reporter activity. Knockdown of p38alpha/beta markedly increased but that of p38gamma decreased caspase 3 activity, suggesting opposite actions on apoptosis. p38gamma was initially proposed to have a promyogenic function; however, p38gamma overexpression could not rescue reduced myoblast differentiation following p38alpha/beta inhibition. Therefore, p38alpha 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; p38gamma has a minor, yet opposing antimyogenic, function.
    Endocrinology 09/2010; 151(9):4368-80. DOI:10.1210/en.2010-0209 · 4.64 Impact Factor