GSK-3

GSK-3 functions by phosphorylating a serine or threonine residue on its target substrate. A positively charged pocket adjacent to the active site binds a "priming" phosphate group attached to a serine or threonine four residues C-terminal of the target phosphorylation site. The active site, at residues 181, 200, 97, and 85, binds the terminal phosphate of ATP and transfers it to the target location on the substrate (see figure 1). ==Glycogen synthase==

Glycogen synthase is an enzyme that is responsible in glycogen synthesis. It is activated by glucose 6-phosphate (G6P), and inhibited by glycogen synthase kinases (GSK3). Those two mechanisms play an important role in glycogen metabolism. == Function ==

Phosphorylation of a protein by GSK-3 usually inhibits the activity of its downstream target. GSK-3 is active in a number of central intracellular signaling pathways, including cellular proliferation, migration, glucose regulation, and apoptosis. GSK-3 was originally discovered in the context of its involvement in regulating glycogen synthase. In addition to its role in regulating glycogen synthase, GSK-3 has been implicated in other aspects of glucose homeostasis, including the phosphorylation of insulin receptor IRS1 and of the gluconeogenic enzymes phosphoenolpyruvate carboxykinase and glucose 6 phosphatase. However, these interactions have not been confirmed, as these pathways can be inhibited without the up-regulation of GSK-3. The inactivation of GSK3B by various protein kinases also affects the adaptive immune response by inducing cytokine production and proliferation in naïve and memory CD4+ T cells. GSK-3 is therefore a part of the canonical Beta-catenin/Wnt pathway, which signals the cell to divide and proliferate. GSK-3 phosphorylates cyclins D and E, which are important for the transition from G1 to S phase, and causes their degradation. The transcription factors c-myc and c-fos (also S phase promoters ), which are primarily phosphorylated by the dual-specificity tyrosine phosphorylation-regulated kinase, are also phosphorylated by GSK3, causing them to be degraded. GSK-3 also participates in a number of apoptotic signaling pathways by phosphorylating transcription factors that regulate apoptosis. and inactivating survival-promoting factors through phosphorylation. The role of GSK-3 in regulating apoptosis is controversial, however, as some studies have shown that GSK-3β knockout mice are overly sensitized to apoptosis and die in the embryonic stage, while others have shown that overexpression of GSK-3 can induce apoptosis. Overall, GSK-3 appears to both promote and inhibit apoptosis, and this regulation varies depending on the specific molecular and cellular context. GSK-3 is also involved in nuclear transcriptional activator kappa B (NFκB) signaling pathway, Hedgehog signaling pathway, Notch signaling pathway, and epithelial-mesenchymal transition. The speed and efficacy of GSK-3 phosphorylation is regulated by several factors. Phosphorylation of certain GSK-3 residues can increase or decrease its ability to bind substrate. Phosphorylation at tyrosine-216 in GSK-3β or tyrosine-279 in GSK-3α enhances the enzymatic activity of GSK-3, while phosphorylation of autoinhibitory serine-9 in GSK-3β or serine-21 in GSK-3α significantly decreases active site availability (see figure). Depending on the pathway in which it is being utilized, GSK-3 may be further regulated by cellular localization or the formation of protein complexes. The activity of GSK-3 is far greater in the nucleus and mitochondria than in the cytosol in cortical neurons, while the phosphorylation of Beta-catenin by GSK-3 is mediated by the binding of both proteins to Axin, a scaffold protein, allowing Beta-catenin to access the active site of GSK-3. Insulin indirectly inactivates GSK3 via downstream phosphorylation of the specific serine residues Ser21 and Ser9 in GSK-3 isoforms α and β, respectively, via the PI3K/Akt pathway (protein kinase B). ==Disease relevance==

Due to its involvement in a great number of signaling pathways, GSK-3 has been associated with a host of high-profile diseases. GSK-3 inhibitors are currently being tested for therapeutic effects in Alzheimer's disease, type 2 diabetes mellitus (T2DM), some forms of cancer, and bipolar disorder. There is evidence that lithium, which is used as a treatment for bipolar disorder, acts as a mood stabilizer by selectively inhibiting GSK-3. The mechanism through which GSK-3 inhibition may stabilize mood is not known, though it is suspected that the inhibition of GSK-3's ability to promote inflammation contributes to the therapeutic effect. Elements of the circadian clock may be connected with predisposition to bipolar mood disorder. GSK-3 activity has been associated with both pathological features of Alzheimer's disease, namely the buildup of amyloid-β (Aβ) deposits and the formation of neurofibrillary tangles. GSK-3 is thought to directly promote Aβ production and to be tied to the process of the hyperphosphorylation of tau proteins, which leads to the tangles. In a similar fashion, targeted inhibition of GSK-3 may have therapeutic effects on certain kinds of cancer. Though GSK-3 has been shown to promote apoptosis in some cases, it has also been reported to be a key factor in tumorigenesis in some cancers. Supporting this claim, GSK-3 inhibitors have been shown to induce apoptosis in glioma and pancreatic cancer cells. GSK-3 also seems to be responsible for NFκB aberrant activity in pediatric acute lymphoblastic leukemia and pancreatic cancer cells. In renal cancer cells, GSK-3 inhibitors induce cell cycle arrest, differentiation of the malignant cells, and autophagy. In contrast to the above neoplasms, high expression of inactive pGSK3β-S9 is found in skin, oral, and lung cancers, suggesting tumor suppressive effects of the enzyme in these cancers. In melanoma, the microRNA miR-769 inhibits GSK-3 activity during the tumor development process, also indicating tumor suppressive effects of GSK3. GSK-3 can negatively regulate the insulin signaling pathway by inhibiting IRS1 via phosphorylation of serine-332, GSK-3 inhibitors increased in vivo CD8(+) OT-I CTL function and the clearance of viral infections by murine gamma-herpesvirus 68 and lymphocytic choriomeningitis clone 13 as well as anti-PD-1 in immunotherapy. == Inhibitors ==

Glycogen synthase kinase inhibitors are different chemotypes and have variable mechanisms of action; they may be cations, from natural sources, synthetic ATP and non-ATP competitive inhibitors and substrate-competitive inhibitors. GSK3 is a bi-lobar architecture with N-terminal and C-terminal, the N-terminal is responsible for ATP binding and C-terminal which is called as activation loop mediates the kinase activity, Tyrosine located at the C-terminal it essential for full GSK3 activity. Benefits of GSK-3β inhibitors In diabetes, GSK-3β inhibitors increase insulin sensitivity, glycogen synthesis, and glucose metabolism in skeletal muscles, and reduce obesity by affecting the adipogenesis process. and mood disorders, including bipolar disorder. In vitro studies have shown the beneficial effects of GSK-3 inhibitors in lung cancer, ovarian cancer and neuroblastoma. Specific agents Inhibitors of GSK-3 include: Metal cations • Beryllium • Copper • Lithium (IC50=2mM) • Mercury • Tungsten (Indirect) • Zinc (IC50=15μM) ATP-competitive Marine organism-derived • 6-BIO (IC50=1.5μM) • Dibromocantharelline (IC50=3μM) • Hymenialdesine (IC50=10nM) • Indirubin (IC50=5-50nM) • Meridianin Aminopyrimidines • CHIR99021 (IC50=6.9nM-10nM) • CHIR98014 (IC50=0.58-0.65nM) • CT98014 • CT98023 • CT99021 • TWS119 (IC50=30nM) Arylindolemaleimide • SB-216763 (IC50=34nM) • SB-41528 (IC50=31-78nM) Thiazoles • AR-A014418 (IC50=104nM) • AZD-1080 (IC50=6.9nM-31nM) Paullones IC50=4-80nM: • Alsterpaullone • Cazpaullone • Kenpaullone Aloisines IC50=0.5-1.5μM: Non-ATP competitive Marine organism-derived • Manzamine A (IC50=1.5μM) • Palinurine (IC50=4.5μM) • Tricantine (IC50=7.5μM) Thiazolidinediones • TDZD-8 (IC50=2μM) • NP00111 (IC50=2μM) • NP031115 (IC50=4μM) • Tideglusib (IC50=60nM) Halomethylketones • HMK-32 (IC50=1.5μM) Peptides • L803-mts (IC50=20μM) • L807-mts (IC50=1μM) Unknown Mechanism (small-molecule inhibitors) • COB-187 (IC50=11nM-22nM) • COB-152 (IC50=77nM-132nM) Lithium Lithium which is used in the treatment of bipolar disorder was the first natural GSK-3 inhibitor discovered. It inhibits GSK-3 directly by competition with magnesium ions and indirectly by phosphorylation and auto-regulation of serine. Lithium has been found to have insulin-like effects on glucose metabolism, including stimulation of glycogen synthesis in fat cells, skin, and muscles, increasing glucose uptake, and activation of GS activity. In addition to inhibition of GSK-3, it also inhibits other enzymes involved in the regulation of glucose metabolisms, such as myo-inositol-1-monophosphatase and 1,6 bisphosphatase. Also, it has shown therapeutic benefit in Alzheimer's and other neurodegenerative diseases such as epileptic neurodegeneration. Famotidine Famotidine is a specific, long-acting H2 antagonist that decreases gastric acid secretion. It is used in the treatment of peptic ulcer disease, GERD, and pathological hypersecretory conditions, like Zollinger–Ellison syndrome. (14,15) H2-receptor antagonists affect hormone metabolism, but their effect on glucose metabolism is not well established. (16) A study has revealed a glucose-lowering effect for famotidine. The study of famotidine binding to the enzyme has showed that famotidine can be docked within the binding pocket of GSK-3β making significant interactions with key points within the GSK-3β binding pocket. Strong hydrogen bond interactions with the key amino acids PRO-136 and VAL -135 and potential hydrophobic interaction with LEU-188 were similar to those found in the ligand binding to the enzyme (AR-A014418). Furthermore, famotidine showed high GSK-3β binding affinity and inhibitory activity due to interactions that stabilize the complex, namely hydrogen bonding of guanidine group in famotidine with the sulfahydryl moiety in CYS-199; and electrostatic interactions between the same guanidine group with the carboxyl group in ASP-200, the hydrogen bond between the terminal NH2 group, the OH of the TYR-143, and the hydrophobic interaction of the sulfur atom in the thioether with ILE-62. In vitro studies showed that famotidine inhibits GSK-3β activity and increases liver glycogen reserves in a dose dependent manner. A fourfold increase in the liver glycogen level with the use of the highest dose of famotidine (4.4 mg/kg) was observed. Also, famotidine has been shown to decrease serum glucose levels 30, and 60 minutes after oral glucose load in healthy individuals. As a GSK-3β inhibitor, the IC50 value of famotidine is 1.44μM. Curcumin Curcumin, which Is a constituent of turmeric spice, has flavoring and coloring properties. It has two symmetrical forms: enol (the most abundant forms) and ketone. Curcumin has wide pharmacological activities: anti-inflammatory, anti-microbial, hypoglycemic, anti-oxidant, and wound healing effects. In animal models with Alzheimer disease, it has anti-destructive effect of beta amyloid in the brain, and recently it shows anti-malarial activity. Curcumin also has chemo preventative and anti-cancer effects, and it has been shown to attenuate oxidative stress and renal dysfunction in diabetic animals with chronic use. Curcumin's mechanism of action is anti-inflammatory; it inhibits the nuclear transcriptional activator kappa B (NF-KB) that is activated whenever there is inflammatory response. NF-kB has two regulatory factors, IkB and GSK-3, which suggests curcumin directly binds and inhibits GSK-3B. An in vitro study confirmed GSK-3B inhibition by simulating molecular docking using a silico docking technique. The concentration at which 50% of GK-3B would be inhibited by curcumin is 66.3 nM. Atypical antipsychotics are more commonly used than first generation antipsychotics because they decrease the risk of extrapyramidal symptoms, such as tardive dyskinesia, and have better efficacy. Olanzapine and atypical antipsychotics induce weight gain through increasing body fat. It also affects glucose metabolism, and several studies shows that it may worsen diabetes. A recent study shows that olanzapine inhibits GSK3 activity, suggesting olanzapine permits glycogen synthesis. A study of the effect of olanzapine on mouse blood glucose and glycogen levels showed a significant decrease in blood glucose level and elevation of glycogen level in mice, and the IC50% of olanzapine were 91.0 nm, which is considered a potent inhibitor. The study also illustrates that sub-chronic use of olanzapine results in potent inhibition of GSK3. Some of them have been shown to fit the ATP-binding pocket of GSK-3β to lower blood glucose levels and improve some neuronal diseases. == See also ==