Lithium-mediated phosphorylation of glycogen synthase kinase-3β involves PI3 kinase-dependent activation of protein kinase C-α

Lithium, a known mood-stabilizer frequently used in treatment of bipolar disorders, is an effective glycogen synthase kinase-3β (GSK-3β) inhibitor. This led to the idea that GSK-3β is an in vivo target directly inhibited by lithium. As lithium is a weak in vitro inhibitor of GSK-3β (IC₅₀ = 2 mM), however, we speculated that it inhibits GSK-3β via an indirect, yet unknown, mechanism. The present studies show that lithium increased the phosphorylation of a key inhibitory site of GSK-3β, serine-9 (Ser-9), in HEK293 cells and in PC12 cells. This phosphorylation was significantly reduced by protein kinase C (PKC) inhibitors GF109203X and Ro31-8425, as well as GÖ6976, an effective inhibitor toward conventional PKC isoforms (cPKC). Consistent with these results, lithium increased PKC-α activity approximately twofold in both cell lines. Because PI3 kinase is a potential upstream regulator of cPKC, its inhibition by wortmannin or LY294002 also abolished the lithium-induced serine phosphorylation of GSK-3β in HEK293 and PC12 cells. Moreover, lithium did not activate PKB, and in addition, its activity was not dependent on the presence of medium inositol nor did it affect the autophosphorylation activity of GSK-3β. Finally, intracerebroventricular injection of lithium increased GSK-3β Ser-9 phosphorylation and enhanced PKC-α activity 1.8-fold in mouse hippocampus, confirming this lithium response in vivo. Our studies propose a new mechanism by which lithium indirectly inhibits GSK-3β via phosphatidylinositol 3 kinase-dependent activation of PKC-α.