PHOSPHATIDYLINOSITOL 3-KINASE INHIBITORS BLOCK DIFFERENTIATION OF SKELETAL-MUSCLE CELLS

Skeletal muscle differentiation involves myoblast alignment, elongatio n, and fusion into multinucleate myotubes, together with the induction of regulatory and structural muscle-specific genes, were we show that two phosphatidylinositol 3-kinase inhibitors, LY294002 and wortmannin , blocked an essential step in the differentiation of two skeletal mus cle cell models, Both inhibitors abolished the capacity of L6E9 myobla sts to form myotubes, without affecting myoblast proliferation, elonga tion, or alignment, Myogenic events like the induction of myogenin and of glucose carrier GLUT4 were also blocked and myoblasts could not ex it the cell cycle, as measured by the lack of mRNA induction of p21 cy clin-dependent kinase inhibitor, Overexpression of MyoD in 10T1/2 cell s was not sufficient to bypass the myogenic differentiation blockade b y LY294002. Upon serum withdrawal, 10T1/2-MyoD cells formed myotubes a nd showed increased levels of myogenin and p21, In contrast, LY294002- treated cells exhibited none of these myogenic characteristics and mai ntained high levels of Id, a negative regulator of myogenesis. These d ata indicate that whereas phosphatidylinositol 3-kinase is not indispe nsable for cell proliferation or in the initial events of myoblast dif ferentiation, i.e. elongation and alignment, it appears to be essentia l for terminal differentiation of muscle cells.