Glycogen synthase kinase 3 (GSK-3) is a serine/threonine (S/T) protein kinase. Although GSK-3 originally was identified to have functions in regulation of glycogen synthase, it was subsequently determined to have roles in multiple normal biochemical processes as well as various disease conditions. GSK-3 is sometimes referred to as a moonlighting protein due to the multiple substrates and processes which it controls. Frequently, when GSK-3 phosphorylates proteins, they are targeted for degradation. GSK-3 is often considered a component of the PI3K/PTEN/AKT/GSK-3/mTORC1 pathway as GSK-3 is frequently phosphorylated by AKT which regulates its inactivation. AKT is often active in human cancer and hence, GSK-3 is often inactivated. Moreover, GSK-3 also interacts with WNT/β-catenin signaling and β-catenin and other proteins in this pathway are targets of GSK-3. GSK-3 can modify NF-κB activity which is often expressed at high levels in cancer cells. Multiple pharmaceutical companies developed small molecule inhibitors to suppress GSK-3 activity. In addition, various natural products will modify GSK-3 activity. This review will focus on the effects of small molecule inhibitors and natural products on GSK-3 activity and provide examples where these compounds were effective in suppressing cancer growth.

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Glycogen synthase kinase 3β (GSK3β), originally described as a negative regulator of glycogen synthesis, is a molecular hub linking numerous signaling pathways in a cell. Specific GSK3β inhibitors have anti-depressant effects and reduce depressive-like behavior in animal models of depression. Therefore, GSK3β is suggested to be engaged in the pathogenesis of major depressive disorder, and to be a target and/or modifier of anti-depressants' action. In this review, we discuss abnormalities in the activity of GSK3β and its upstream regulators in different brain regions during depressive episodes. Additionally, putative role(s) of GSK3β in the pathogenesis of depression and the influence of anti-depressants on GSK3β activity are discussed.Cells 2020, 9, 727 2 of 26 any of the groups listed above. Additionally, electroconvulsive therapy, conducted for the first time in 1938, is still widely used in the treatment of MDD, especially in its drug-refractory form [5].Although the monoaminergic hypothesis has led to the invention of many successful therapeutic strategies based on the elevation of levels of NA and 5-HT in the synaptic cleft, it does not explain the anti-depressant effect of lithium and the rapid action of ketamine in the treatment of mood disorders [6]. Therefore, factors other than neurotransmission must be taken into consideration in the context of the MDD pathogenesis. One of them is glycogen synthase kinase 3β (GSK3β) signaling. Glycogen Synthase Kinase 3βGSK3 was isolated in 1980, from rabbit skeletal muscle, and described as a highly specific serine/threonine kinase for glycogen synthase [7]. There are two isozymes of GSK3, α and β, and both are expressed at similar levels in the mouse brain [8]. In the human brain, the β isozyme predominates [9]. Therefore, GSK3β is expected to be crucial for the human central nervous system functioning. The activity of GSK3β is regulated positively and negatively by phosphorylation on Tyr216 and Ser9, respectively [10,11]. Whereas phosphorylation of the residue Tyr216 occurs during the GSK3β translation process and results in a synthesis of the fully activated kinase, Ser9 phosphorylation seems to be the main regulatory modification during the enzyme lifespan [12]. Ser9-phosphorylated GSK3β remains inhibited, and dephosphorylation of the residue results in the disinhibition (activation) of the kinase.GSK3β is part of numerous cellular signaling pathways, and its activity can be regulated, directly or indirectly, by several kinases, phosphatases, and proteases. The wide spectrum of GSK3β substrates, including transcription factors, glycolytic enzymes, pro-and anti-apoptotic factors, mitochondrial channels, membrane receptors, and cytoskeleton-associated proteins, makes GSK3β a central point of the cell homeostasis maintenance [13]. The activity of GSK3β affects energy metabolism, cell survival, proliferation, apoptosis, membrane polarity, internalization of the synaptic receptors, neuroplasticity, neurotransmission, amyloid processing, and many other processes [13].Ex...

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