INSULIN ACTION IN CULTURED HUMAN MYOBLASTS - CONTRIBUTION OF DIFFERENT SIGNALING PATHWAYS TO REGULATION OF GLYCOGEN-SYNTHESIS

A key metabolic action of insulin is the stimulation of nonoxidative g lucose utilization in skeletal muscle, by increasing both glucose upta ke and glycogen synthesis. The molecular mechanism underlying this pro cess has been investigated using a variety of experimental systems. We report here the use of cultured human myoblasts to study insulin cont rol of glycogen synthesis in humans. In these cells insulin stimulates glycogen synthesis approx. 2.2-fold, associated with a similar activa tion of glycogen synthase (GS) which occurs within 5-10 min of the add ition of insulin. Insulin also causes inactivation of glycogen synthas e kinase-3 (GSK-3) and activation of protein kinase B, both processes being sufficiently rapid to account for the effects of insulin on GS. Activation by insulin of the protein kinases p70(s6K), pg90(s6K) and e xtracellular signal-regulated kinase 2 (ERK2) is observed, but is sign ificantly slower than the activation of GS. Selective inhibitors of th e p70(s6K) pathway (rapamycin), the ERK2/p90(s6K) pathway (PD98059) an d phosphatidylinositol 3-kinase (wortmannin) have been used to probe t he contribution of these components to insulin signalling in human mus cle. Wortmannin blocks activation of both glycogen synthesis and GS an d inactivation of GSK-3. PD98059 is without effect an these events, wh ile rapamycin is without effect on inactivation of GSK-3 but partially blocks activation of glycogen synthesis and GS. Taken together, these findings suggest that protein kinase B is responsible for the inactiv ation of GSK-3, but that an additional rapamycin-sensitive mechanism m ay contribute to the activation of GS and stimulation of glycogen synt hesis.