Akt2, a Novel Functional Link between p38 Mitogen-Activated Protein Kinase and Phosphatidylinositol 3-Kinase Pathways in Myogenesis

Abstract

Activation of either the phosphatidylinositol 3-kinase (PI 3-kinase)/Akt or the p38 mitogen-activated protein kinase (MAPK) signaling pathways accelerates myogenesis but only when the reciprocal pathway is functional. We therefore examined the hypothesis that cross-activation between these signaling cascades occurs to orchestrate myogenesis. We reveal a novel and reciprocal cross-talk and activation between the PI 3-kinase/Akt and p38 MAPK pathways that is essential for efficient myoblast differentiation. During myoblast differentiation, Akt kinase activity correlated with S473 but not T308 phosphorylation and occurred 24 h after p38 activation. Inhibition or activation of p38 with SB203580, dominant-negative p38, or MKK6EE regulated Akt kinase activity. Analysis of Akt isoforms revealed a specific increase in Akt2 protein levels that coincided with AktS473 phosphorylation during myogenesis and an enrichment of S473-phosphorylated Akt2. Akt2 promoter activity and protein levels were regulated by p38 activation, thus providing a mechanism for communication. Subsequent Akt activation by S473 phosphorylation was PI 3-kinase dependent and specific for Akt2 rather than Akt1. Complementary to p38-mediated transactivation of Akt, activation or inhibition of PI 3-kinase regulated p38 activity upstream of MKK6, demonstrating reciprocal communication and positive feedback characteristic of myogenic regulation. Our findings have identified novel communication between p38 MAPK and PI 3-kinase/Akt via Akt2.A hallmark of cellular differentiation in many lineages is the mutual exclusivity of proliferation and differentiation. Skeletal myogenesis is the precisely orchestrated process by which committed but proliferating myoblasts irreversibly exit from the cell cycle, acquire an apoptosis-resistant phenotype, and finally form multinucleated myotubes (44). Myogenesis therefore provides an excellent model for understanding the fundamental mechanisms that regulate cell fate specification and the apparent antagonism between cell multiplication and differentiation. Two groups of transcription factors, the myogenic determination factors (such as MyoD and myogenin) and the myocyte enhancer factor 2 (MEF2) proteins, are central to the coordination of myogenesis; these interact to modify chromatin structure and initiate muscle-specific gene expression (64).The p38 mitogen-activated protein kinase (MAPK) family was identified as part of the mechanism by which bacterial endotoxin induces cytokine expression (25, 38); they were therefore defined as stress-activated protein kinases. The results of subsequent studies of other cell systems suggest a significant role for p38 in differentiation (reviewed in reference 42); thus, its function is not confined to stress response. p38 has also been implicated in the regulation of cell cycle exit (as evidenced by direct phosphorylation of cyclin D1) (13) and of the retinoblastoma protein independent of cdk activity (58). p38 MAPKs exist as four isoforms: p38␣, p38␤, p38␥, and p38␦. They are mainly ac...

The phosphoinositide 3-kinase/Akt pathway is essential for the retinoic acid-induced differentiation of F9 cells

“…This is consistent with the emerging concept that PI3K/Akt signaling has diversified capacities and is apparently capable not only to promote cell growth but also to contribute in cell differentiation. Indeed, recent studies reported that activation of the PI3K/Akt pathway is required for the differentiation of myoblasts (Cabane et al, 2004;Gonzalez et al, 2004;Sarker and Lee, 2004), keratinocytes (Calautti et al, 2005) as well as for the RA-induced differentiation of neuroblastoma cells (Lopez-Carballo et al, 2002), HL60 cells (Ishida et al, 2004), endometrial adenocarcinoma cells (Carter, 2003) and APL cells (Lal et al, 2005). The precise mechanisms through which the PI3K/Akt pathway contributes to cell differentiation have not been determined yet.…”

Interleukin-6 Gene Ablation in a Transgenic Mouse Model of Malignant Skin Melanoma

“…Such cross-talk between the MAPK and PI3K signaling pathways has already been described. 46 Interestingly, the PI3K inhibitor LY294002 markedly increased the amount of STAT3 activation achieved by stimulating cells with IL-6 (Figure 6c), suggesting that activation of …”

Protein Phosphatase 2A Isoforms Utilizing Aβ Scaffolds Regulate Differentiation through Control of Akt Protein

“…Akt Activity Controls Differentiation of C2C12 and 3T3-L1 Cells-Akt activity has been implicated in differentiation of preadipocytes and myoblasts (25)(26)(27)(28)(29)(30)(31). As expected, Akt was activated during differentiation as seen by increased phosphorylation at Ser-473 (Fig.…”

“…Both Akt1 and Akt2 play a role in myoblast differentiation (28,30,31), and knockdown of either blocks myoblast differentiation (30,31). Conversely, Akt2 expression is increased during the differentiation process (28).…”

“…Akt activity has been implicated as a necessary component in preadipocyte (25,26) and myoblast (27)(28)(29) differentiation. Both Akt1 and Akt2 play a role in myoblast differentiation (28,30,31), and knockdown of either blocks myoblast differentiation (30,31).…”