THE MITOGENIC AND MYOGENIC ACTIONS OF INSULIN-LIKE GROWTH-FACTORS UTILIZE DISTINCT SIGNALING PATHWAYS

It is well established that mitogens inhibit differentiation of skelet al muscle cells, but the insulin-like growth factors (IGFs), acting th rough a single receptor, stimulate both proliferation and differentiat ion of myoblasts, Although the IGF-I mitogenic signaling pathway has b een extensively studied in other cell types, little is known about the signaling pathway leading to differentiation in skeletal muscle, Ey u sing specific inhibitors of the IGF signal transduction pathway, we ha ve begun to define the signaling intermediates mediating the two respo nses to IGFs, We found that PD098059, an inhibitor of mitogen-activate d protein (MAP) kinase kinase activation, inhibited IGF-stimulated pro liferation of L6A1 myoblasts and the events associated with it, such a s phosphorylation of the MAP kinases and elevation of c-fos mRNA and c yclin D protein, Surprisingly, PD098059 caused a dramatic enhancement of differentiation, evident both at a morphological (fusion of myoblas ts into myotubes) and biochemical level (elevation of myogenin and p21 cyclin-dependent kinase inhibitor expression, as well as creatine kin ase activity), In sharp contrast, LY294002, an inhibitor of phosphatid ylinositol 3-kinase, and rapamycin, an inhibitor of the activation of p70 S6 kinase (p70(S6k)), completely abolished IGF stimulation of L6A1 differentiation. We found that p70(S6k) activity increased substantia lly during differentiation, and this increase was further enhanced by PD098059, Our results demonstrate that the MAP kinase pathway plays a primary role in the mitogenic response and is inhibitory to the myogen ic response in L6A1 myoblasts, while activation of the phosphatidylino sitol 3-kinase/ p70(S6k) pathway is essential for IGF stimulated diffe rentiation. Thus, it appears that signaling from the IGF-I receptor ut ilizes two distinct pathways leading either to proliferation or differ entiation.