Effects of lithium and insulin on glycogen synthesis in L6 myocytes: additive effects on inactivation of glycogen synthase kinase-3.
In insulin-sensitive L6 myocytes, insulin stimulated glycogen synthesis in a dose-dependent manner and lithium further stimulated glycogen synthesis at all insulin concentrations. Lithium alone at 20 mM stimulated glycogen synthesis to the degree similar to the maximal insulin response. Effects of lithium and insulin were fully additive for both glycogen synthesis and glycogen synthase activity. In L6 myocytes, insulin increased phosphorylation of Akt1 and glycogen synthase kinase-3 alpha and beta (GSK-3 alpha and beta), resulting in its activation and inactivation, respectively. Unlike insulin, lithium directly inhibited GSK-3 (both alpha and beta) without affecting phosphorylation of GSK-3. Moreover, lithium in vitro could further inhibit enzyme activity of GSK-3 (both alpha and beta) that was isolated from insulin-stimulated cells (thus already phosphorylated and inactivated by insulin). In summary, insulin increases glycogen synthesis by the Akt1/GSK-3/glycogen synthase pathway, but lithium increases glycogen synthesis by direct inhibition of GSK-3 in L6 myocytes. Inhibitory effects of lithium and insulin on GSK-3 (both alpha and beta) were additive, which may account, at least in part, for their additive effects on glycogen synthase activity and glycogen synthesis in L6 myocytes.