Phosphatidylinositol 3-Kinase Inhibitors Block Differentiation of Skeletal Muscle Cells

Abstract

Skeletal muscle differentiation involves myoblast alignment, elongation, and fusion into multinucleate myotubes, together with the induction of regulatory and structural muscle-specific genes. Here we show that two phosphatidylinositol 3-kinase inhibitors, LY294002 and wortmannin, blocked an essential step in the differentiation of two skeletal muscle cell models. Both inhibitors abolished the capacity of L6E9 myoblasts to form myotubes, without affecting myoblast proliferation, elongation, or alignment. Myogenic events like the induction of myogenin and of glucose carrier GLUT4 were also blocked and myoblasts could not exit the cell cycle, as measured by the lack of mRNA induction of p21 cyclindependent kinase inhibitor. Overexpresssion of MyoD in 10T1/2 cells was not sufficient to bypass the myogenic differentiation blockade by LY294002. Upon serum withdrawal, 10T1/2-MyoD cells formed myotubes and showed increased levels of myogenin and p21. In contrast, LY294002-treated cells exhibited none of these myogenic characteristics and maintained high levels of Id, a negative regulator of myogenesis. These data indicate that whereas phosphatidylinositol 3-kinase is not indispensable for cell proliferation or in the initial events of myoblast differentiation, i.e. elongation and alignment, it appears to be essential for terminal differentiation of muscle cells.

Ras signaling through PI3K confers hormone-independent proliferation that is compatible with differentiation

“…PI3K is also reported to negatively regulate the maturation of undi erentiated fetal pancreatic islet cells (Ptasznik et al, 1997). In many other cells, however, not only is PI3K required for di erentiation (Kimura et al, 1994;Cheatham et al, 1994;Okada et al, 1994;Kaliman et al, 1996), but activation of PI3K or PI3K-dependent signals is su cient to induce markers of the di erentiated phenotype (Katagiri et al, 1996;Martin et al, 1996;Jiang et al, 1998;Kita et al, 1998). Although PI3K is compatible with thyroid di erentiation, activation of PI3K is insu cient to induce thyroid-speci®c gene expression.…”

ErbB kinases and NDF signaling in human prostate cancer cells

“…For example, the di erentiation of PC12 cells in response to nerve growth factor (NGF), and Leydig tumor cells in response to EGF, has been shown to involve PI3-K (Ohmichi et al, 1992;Pignataro and Ascoli, 1990;Solto et al, 1992). Skeletal muscle, hematopoietic, and adipocytic di erentiation have also shown dependency on PI3-K (Kaliman et al, 1996;Kitanaka et al, 1996;Kubota et al, 1996;Tomiyama et al, 1995). By analogy, it is also possible that ErbB3-mediated PI3-K activation could play a role in PCA cell di erentiation.…”

Insulin-like growth factor (IGF-I) induces myotube hypertrophy associated with an increase in anaerobic glycolysis in a clonal skeletal-muscle cell model

“…Recently, the signalling pathways by which IGF-I induces mitogenesis as compared with myogenesis have been elucidated [35,39,40]. Following IGF-I binding to its receptor, at least two pathways are activated : the mitogen-activated protein kinase pathway mediates mitogenesis and the phosphatidylinositol 3-kinase pathway mediates myogenesis, and these responses can be selectively blocked with specific inhibitors of these pathways [35].…”