Abstract
Glycogen synthase kinase-3beta (GSK-3beta) is a key target and effector of downstream insulin signalling. Using comparative protein kinase assays and molecular docking studies we characterize the emodin-derivative 4-[N-2-(aminoethyl)-amino]-emodin (L4) as a sensitive and potent inhibitor of GSK-3beta with peculiar features. Compound L4 shows a low cytotoxic potential compared to other GSK-3beta inhibitors determined by the 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide assay and cellular ATP levels. Physiologically, L4 acts as an insulin-sensitizing agent that is able to enhance hepatocellular glycogen and fatty acid biosynthesis. These functions are particularly stimulated in the presence of elevated concentrations of glucose and in synergy with the hormone action at moderate but not high insulin levels. In contrast to other low molecular weight GSK-3beta inhibitors (SB216763 and LiCl) or Wnt-3alpha-conditioned medium, however, L4 does not induce reporter and target genes of activated beta-catenin such as TOPflash, Axin2 and glutamine synthetase. Moreover, when present together with SB216763 or LiCl, L4 counteracts expression of TOPflash or induction of glutamine synthetase by these inhibitors. Because L4 slightly activates beta-catenin on its own, these results suggest that a downstream molecular step essential for activation of gene transcription by beta-catenin is also inhibited by L4. It is concluded that L4 represents a potent insulin-sensitizing agent favouring physiological effects of insulin mediated by GSK-3beta inhibition but avoiding hazardous effects such as activation of beta-catenin-dependent gene expression which may lead to aberrant induction of cell proliferation and cancer.