Control of glycogen synthesis in cultured human muscle cells

The regulation of glycogen synthesis and associated enzymes was studied in human myoblasts and myotubes maintained in culture. Both epidermal growth f actor (EGF) and insulin stimulated glycogen synthesis approximately 2-fold, this stimulation being accompanied by a rapid and stable activation of the controlling enzyme glycogen synthase (GS). EGF also caused inhibition of g lycogen synthase kinase 3 (GSK-3) and activation of the alpha isoform of pr otein kinase B (PKB) with the time-course and magnitude of its effects bein g similar to those induced by insulin. An inhibitor of the mitogen-activate d protein (MAP) kinase pathway did not prevent stimulation of GS by EGF, su ggesting that this pathway is not essential for the effect. A partial decre ase in the fold activation of GS was, however, observed when p70(S6k) activ ation was blocked with rapamycin, suggesting a contribution of this pathway to the control of GS by either hormone. Wortmannin, a selective inhibitor of phosphatidylinositol 3'-kinase (PI-3 kinase) completely blocked the effe cts of both EGF and insulin in these cells. These results demonstrate that EGF, like insulin, activates glycogen synthesis in muscle, acting principal ly via the PKB/GSK-3 pathway but with a contribution from a rapamycin-sensi tive component that lies downstream of PI-3 kinase.