The inactive form of glycogen synthase kinase-3 beta is associated with the development of carcinomas in galectin-3 wild-type mice, but not in galectin-3-deficient mice

Galectin-3 has been implicated in the tumor development via its mediation of the Wnt signaling pathway. Likewise, glycogen synthase kinase-3beta (GSK3 beta) also plays a role in the Wnt signaling pathway by controlling the levels of cytoplasmic beta-catenin. Altered GSK3 beta expression has been described in various tumors, but to date, there are no studies evaluating its expression in models of oral carcinogenesis. Additionally, it is unknown whether the absence of galectin-3 regulates the expression of GSK3 beta. To this end, Gal3-deficient (Gal3(-/-)) and wild-type (Gal3(+/+)) male mice were treated with 4NQO for 16 weeks and sacrificed at week 16 and 32. The tongues were removed, processed, and stained with H&E to detect dysplasias and carcinomas. An immunohistochemical assay was performed to determine the level of P-GSK3 beta-Ser9 expression in both groups. Carcinomas were more prevalent in Gal3(+/+) than Gal3(-/-) mice (55.5% vs. 28.5%), but no statistical difference was reached. In the dysplasias, the proportion of cells positive for P-GSK3 beta-Ser9 was slightly higher in Gal3(+/+) than Gal3(-/-) mice (63% vs. 61%). In the carcinomas, a significant difference between Gal3(+/+) and Gal3(-/-) mice was found (74% vs. 59%; p=0.02). P-GSK3 beta-Ser9-positive cells slightly decreased from the progression of dysplasias to carcinomas in Gal3(-/-) mice (61% vs. 59%; p>0.05). However, a significant increase in P-GSK3 beta-Ser9 expression was observed from dysplasias to carcinomas in Gal3(+/+) mice (63% vs. 74%; p=0.01). In conclusion, these findings suggest that fully malignant transformation of the tongue epithelium is associated with increased P-GSK3 beta-Ser9 expression in Gal3(+/+) mice, but not in Gal3(-/-) mice.

Role of glycogen synthase kinase 3 beta as a negative regulator of dorsoventral axis formation in Xenopus embryos.

The dorsoventral axis is established early in Xenopus development and may involve signaling by Wnts, a family of Wnt1-protooncogene-related proteins. The protein kinase shaggy functions in the wingless/Wnt signaling pathway, which operates during Drosophila development. To assess the role of a closely related kinase, glycogen synthase kinase 3 beta (GSK-3 beta), in vertebrate embryogenesis, we cloned a cDNA encoding a Xenopus homolog of GSK-3 beta (XGSK-3 beta). XGSK-3 beta-specific transcripts were detected by Northern analysis in Xenopus eggs and early embryos. Microinjection of the mRNA encoding a catalytically inactive form of rat GSK-3 beta into a ventrovegetal blastomere of eight-cell embryos caused ectopic formation of a secondary body axis containing a complete set of dorsal and anterior structures. Furthermore, in isolated ectodermal explants, the mutant GSK-3 beta mRNA activated the expression of neural tissue markers. Wild-type XGSK-3 beta mRNA suppressed the dorsalizing effects of both the mutated GSK-3 beta and Xenopus dishevelled, a proposed upstream signaling component of the same pathway. These results strongly suggest that XGSK-3 beta functions to inhibit dorsoventral axis formation in the embryo and provide evidence for conservation of the Wnt signaling pathway in Drosophila and vertebrates.

Regulation of glycogen synthase kinase 3 in human platelets: a possible role in platelet function?

In this study we show that both glycogen synthase kinase 3 (GSK3) isoforms, GSK3alpha and GSK3beta, are present in human platelets and are phosphorylated on Ser(21) and Ser(9), respectively, in platelets stimulated with collagen, convulxin and thrombin. Phosphorylation of GSK3alpha/beta was dependent on phosphoinositide 3-kinase (PI3K) activity and independent of platelet aggregation, and correlated with a decrease in GSK3 activity that was preserved by pre-incubating platelets with PI3K inhibitor LY294002. Three structurally distinct GSK3 inhibitors, lithium, SB415286 and TDZD-8, were found to inhibit platelet aggregation. This implicates GSK3 as a potential regulator of platelet function. (C) 2003 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.

Glycogen synthase kinase 3 (GSK3) in the heart: a point of integration in hypertrophic signalling and a therapeutic target? A critical analysis

Glycogen synthase kinase 3 (GSK3, of which there are two isoforms, GSK3alpha and GSK3beta) was originally characterized in the context of regulation of glycogen metabolism, though it is now known to regulate many other cellular processes. Phosphorylation of GSK3alpha(Ser21) and GSK3beta(Ser9) inhibits their activity. In the heart, emphasis has been placed particularly on GSK3beta, rather than GSK3alpha. Importantly, catalytically-active GSK3 generally restrains gene expression and, in the heart, catalytically-active GSK3 has been implicated in anti-hypertrophic signalling. Inhibition of GSK3 results in changes in the activities of transcription and translation factors in the heart and promotes hypertrophic responses, and it is generally assumed that signal transduction from hypertrophic stimuli to GSK3 passes primarily through protein kinase B/Akt (PKB/Akt). However, recent data suggest that the situation is far more complex. We review evidence pertaining to the role of GSK3 in the myocardium and discuss effects of genetic manipulation of GSK3 activity in vivo. We also discuss the signalling pathways potentially regulating GSK3 activity and propose that, depending on the stimulus, phosphorylation of GSK3 is independent of PKB/Akt. Potential GSK3 substrates studied in relation to myocardial hypertrophy include nuclear factors of activated T cells, beta-catenin, GATA4, myocardin, CREB, and eukaryotic initiation factor 2Bvarepsilon. These and other transcription factor substrates putatively important in the heart are considered. We discuss whether cardiac pathologies could be treated by therapeutic intervention at the GSK3 level but conclude that any intervention would be premature without greater understanding of the precise role of GSK3 in cardiac processes.

Glycogen Synthase Kinase 3 is Required for Optimal AKT Activation

The phosphatidylinositol 3-kinase (PI3K) pathway, through its major effector node AKT, is critical for the promotion of cell growth, division, motility and apoptosis evasion. This signaling axis is therefore commonly targeted in the form of mutations and amplifications in a myriad of malignancies. Glycogen synthase kinase 3 (GSK3) was first discovered as the kinase responsible for phosphorylating and inhibiting the activity of glycogen synthase, ultimately antagonizing the storage of glucose as glycogen. Its activity counteracts the effects of insulin in glucose metabolism and AKT has long been recognized as one of the key molecules capable of phosphorylating GSK3 and inhibiting its activity. However, here we demonstrate that GSK3 is required for optimal phosphorylation and activation of AKT in different malignant cell lines, and that this effect is independent of the type of growth factor stimulation and can happen even in basal states. Both GSK3 alpha and GSK3 beta isoforms are necessary for AKT to become fully active, displaying a redundant role in the setting. We also demonstrate that this effect of GSK3 on AKT phosphorylation and full activation is dependent on its kinase activity, since highly specific inhibitors targeting GSK3 catalytic activity also promote a reduction in phosphorylated AKT. Analysis of reverse phase protein array screening of MDA-MB-231 breast cancer cells treated with RNA interference targeting GSK3 unexpectedly revealed an increase in levels of phosphorylated MAPK14 (p38). Treatment with the selective p38 inhibitor SB 202190 rescued AKT activation in that cell line, corroborating the importance of unbiased proteomic analysis in exposing cross-talks between signaling networks and demonstrating a critical role for p38 in the regulation of AKT phosphorylation.

SRF Phosphorylation by Glycogen Synthase Kinase-3 Promotes Axon Growth in Hippocampal Neurons

The growth of axons is an intricately regulated process involving intracellular signaling cascades and gene transcription. We had previously shown that the stimulus-dependent transcription factor, serum response factor (SRF), plays a critical role in regulating axon growth in the mammalian brain. However, the molecular mechanisms underlying SRF-dependent axon growth remains unknown. Here we report that SRF is phosphorylated and activated by GSK-3 to promote axon outgrowth in mouse hippocampal neurons. GSK-3 binds to and directly phosphorylates SRF on a highly conserved serine residue. This serine phosphorylation is necessary for SRF activity and for its interaction with MKL-family cofactors, MKL1 and MKL2, but not with TCF-family cofactor, ELK-1. Axonal growth deficits caused by GSK-3 inhibition could be rescued by expression of a constitutively active SRF. The SRF target gene and actin-binding protein, vinculin, is sufficient to overcome the axonal growth deficits of SRF-deficient and GSK-3-inhibited neurons. Furthermore, short hairpin RNA-mediated knockdown of vinculin also attenuated axonal growth. Thus, our findings reveal a novel phosphorylation and activation of SRF by GSK-3 that is critical for SRF-dependent axon growth in mammalian central neurons.

5-aryl-pyrazolo[3,4-b]pyridazines: Potent inhibitors of glycogen synthase kinase-3 (GSK-3)

Introduction of a nitrogen atom into the 6-position of a series of pyrazolo[3,4-b]pyridines led to a dramatic improvement in the potency of GSK-3 inhibition. Rationalisation of the binding mode suggested participation of a putative structural water molecule, which was subsequently confirmed by X-ray crystallography. (C) 2003 Elsevier Science Ltd. All rights reserved.

3-anilino-4-arylmaleimides: Potent and selective inhibitors of glycogen synthase kinase-3 (GSK-3)

Potent 3-anilino-4-arylmaleimide glycogen synthase kinase-3 (GSK-3) inhibitors have been prepared using automated array methodology. A number of these are highly selective, having little inhibitory potency against more than 20 other protein kinases. (C) 2001 Elsevier Science Ltd. All rights reserved.

Selective small molecule inhibitors of glycogen synthase kinase-3 modulate glycogen metabolism and gene transcription

Background: Glycogen synthase kinase-3 (GSK-8) is a serine/threonine protein kinase, the activity of which is inhibited by a variety of extracellular stimuli including insulin, growth factors, cell specification factors and cell adhesion. Consequently, inhibition of GSK-3 activity has been proposed to play a role in the regulation of numerous signalling pathways that elicit pleiotropic cellular responses. This report describes the identification and characterisation of potent and selective small molecule inhibitors of GSK-3.

Platelet endothelial cell adhesion molecule-1 inhibits platelet response to thrombin and von Willebrand factor by regulating the internalization of glycoprotein Ib via AKT/glycogen synthase kinase-3/dynamin and integrin αIIbβ3

OBJECTIVE: Platelet endothelial cell adhesion molecule-1 (PECAM-1) regulates platelet response to multiple agonists. How this immunoreceptor tyrosine-based inhibitory motif-containing receptor inhibits G protein-coupled receptor-mediated thrombin-induced activation of platelets is unknown. APPROACH AND RESULTS: Here, we show that the activation of PECAM-1 inhibits fibrinogen binding to integrin αIIbβ3 and P-selectin surface expression in response to thrombin (0.1-3 U/mL) but not thrombin receptor-activating peptides SFLLRN (3×10(-7)-1×10(-5) mol/L) and GYPGQV (3×10(-6)-1×10(-4) mol/L). We hypothesized a role for PECAM-1 in reducing the tethering of thrombin to glycoprotein Ibα (GPIbα) on the platelet surface. We show that PECAM-1 signaling regulates the binding of fluorescein isothiocyanate-labeled thrombin to the platelet surface and reduces the levels of cell surface GPIbα by promoting its internalization, while concomitantly reducing the binding of platelets to von Willebrand factor under flow in vitro. PECAM-1-mediated internalization of GPIbα was reduced in the presence of both EGTA and cytochalasin D or latrunculin, but not either individually, and was reduced in mice in which tyrosines 747 and 759 of the cytoplasmic tail of β3 integrin were mutated to phenylalanine. Furthermore, PECAM-1 cross-linking led to a significant reduction in the phosphorylation of glycogen synthase kinase-3β Ser(9), but interestingly an increase in glycogen synthase kinase-3α pSer(21). PECAM-1-mediated internalization of GPIbα was reduced by inhibitors of dynamin (Dynasore) and glycogen synthase kinase-3 (CHIR99021), an effect that was enhanced in the presence of EGTA. CONCLUSIONS: PECAM-1 mediates internalization of GPIbα in platelets through dual AKT/protein kinase B/glycogen synthase kinase-3/dynamin-dependent and αIIbβ3-dependent mechanisms. These findings expand our understanding of how PECAM-1 regulates nonimmunoreceptor signaling pathways and helps to explains how PECAM-1 regulates thrombosis.

Long-term sertraline treatment increases expression and decreases phosphorylation of glycogen synthase kinase-3B in platelets of patients with late-life major depression

Background: Abnormal regulation of glycogen synthase kinase 3-beta (GSK3B) activity has been implicated in the pathophysiology of mood disorders. Many pharmacological agents, including antidepressants, can modulate GSK3B. The aim of the present study was to investigate the effect of short-and long-term sertraline treatment on the expression and phosphorylation of GSK3B in platelets of patients with late-life major depression. Methods: Thirty-nine unmedicated elderly adults with major depressive disorder (MOD) were initially included in this study. The comparison group comprised 18 age-matched, healthy individuals. The expression of total and Ser-9 phosphorylated GSK3B (pGSK3B) was determined by Enzyme Immunometric Assay (EIA) in platelets of patients and controls at baseline, and after 3 and 12 months of sertraline treatments for patients only. During this period, patients were continuously treated with therapeutic doses of sertraline. GSK3B activity was indirectly estimated by calculating the proportion of inactive (phosphorylated) forms (pGSK3B) in relation to the total expression of the enzyme (i.e.. GSK3B ratio). Results: Depressed patients had significantly higher levels of pGSK3B as compared to controls (p < 0.001). Within the MDD group, after 3 months of sertraline treatment no significant changes were observed in GSK3B expression and phosphorylation state, as compared to baseline levels. However, after 12 months of treatment we found a significant increase in the expression of total GSK3B (p = 0.05), in the absence of any significant changes in pGSK3B (p = 0.12), leading to a significant reduction in GSK3B ratio (p = 0.001). Conclusions: Our findings indicate that GSK3B expression was upregulated by the continuous treatment with sertraline, along with an increment in the proportion of active forms of the enzyme. This is compatible with an increase in overall GSK3B activity, which may have been induced by the long-term treatment of late-life depression with sertraline. (C) 2012 Elsevier Ltd. All rights reserved.

Phosphorylation of insulin receptor substrate 1 by glycogen synthase kinase 3 impairs insulin action

The phosphorylation of insulin receptor substrate 1 (IRS-1) on tyrosine residues by the insulin receptor (IR) tyrosine kinase is involved in most of the biological responses of insulin. IRS-1 mediates insulin signaling by recruiting SH2 proteins through its multiple tyrosine phosphorylation sites. The phosphorylation of IRS-1 on serine/threonine residues also occurs in cells; however, the particular protein kinase(s) promoting this type of phosphorylation are unknown. Here we report that glycogen synthase kinase 3 (GSK-3) is capable of phosphorylating IRS-1 and that this modification converts IRS-1 into an inhibitor of IR tyrosine kinase activity in vitro. Expression of wild-type GSK-3 or an “unregulated” mutant of the kinase (S9A) in CHO cells overexpressing IRS-1 and IR, resulted in increased serine phosphorylation levels of IRS-1, suggesting that IRS-1 is a cellular target of GSK-3. Furthermore, insulin-induced tyrosine phosphorylation of IRS-1 and IR was markedly suppressed in cells expressing wild-type or the S9A mutant, indicating that expression of GSK-3 impairs IR tyrosine kinase activity. Taken together, our studies suggest a new role for GSK-3 in attenuating insulin signaling via its phosphorylation of IRS-1 and may provide new insight into mechanisms important in insulin resistance.

Regulation and localization of tyrosine216 phosphorylation of glycogen synthase kinase-3β in cellular and animal models of neuronal degeneration

Inactivation of glycogen synthase kinase-3β (GSK3β) by S9 phosphorylation is implicated in mechanisms of neuronal survival. Phosphorylation of a distinct site, Y216, on GSK3β is necessary for its activity; however, whether this site can be regulated in cells is unknown. Therefore we examined the regulation of Y216 phosphorylation on GSK3β in models of neurodegeneration. Nerve growth factor withdrawal from differentiated PC12 cells and staurosporine treatment of SH-SY5Y cells led to increased phosphorylation at Y216, GSK3β activity, and cell death. Lithium and insulin, agents that lead to inhibition of GSK3β and adenoviral-mediated transduction of dominant negative GSK3β constructs, prevented cell death by the proapoptotic stimuli. Inhibitors induced S9 phosphorylation and inactivation of GSK3β but did not affect Y216 phosphorylation, suggesting that S9 phosphorylation is sufficient to override GSK3β activation by Y216 phosphorylation. Under the conditions examined, increased Y216 phosphorylation on GSK3β was not an autophosphorylation response. In resting cells, Y216 phosphorylation was restricted to GSK3β present at focal adhesion sites. However, after staurosporine, a dramatic alteration in the immunolocalization pattern was observed, and Y216-phosphorylated GSK3β selectively increased within the nucleus. In rats, Y216 phosphorylation was increased in degenerating cortical neurons induced by ischemia. Taken together, these results suggest that Y216 phosphorylation of GSK3β represents an important mechanism by which cellular insults can lead to neuronal death.

Expression and characterization of glycogen synthase kinase-3 mutants and their effect on glycogen synthase activity in intact cells.

In these studies we expressed and characterized wild-type (WT) GSK-3 (glycogen synthase kinase-3) and its mutants, and examined their physiological effect on glycogen synthase activity. The GSK-3 mutants included mutation at serine-9 either to alanine (S9A) or glutamic acid (S9E) and an inactive mutant, K85,86MA. Expression of WT and the various mutants in a cell-free system indicated that S9A and S9E exhibit increased kinase activity as compared with WT. Subsequently, 293 cells were transiently transfected with WT GSK-3 and mutants. Cells expressing the S9A mutant exhibited higher kinase activity (2.6-fold of control cells) as compared with cells expressing WT and S9E (1.8- and 2.0-fold, respectively, of control cells). Combined, these results suggest serine-9 as a key regulatory site of GSK-3 inactivation, and indicate that glutamic acid cannot mimic the function of the phosphorylated residue. The GSK-3-expressing cell system enabled us to examine whether GSK-3 can induce changes in the endogenous glycogen synthase activity. A decrease in glycogen synthase activity (50%) was observed in cells expressing the S9A mutant. Similarly, glycogen synthase activity was suppressed in cells expressing WT and the S9E mutant (20-30%, respectively). These studies indicate that activation of GSK-3 is sufficient to inhibit glycogen synthase in intact cells, and provide evidence supporting a physiological role for GSK-3 in regulating glycogen synthase and glycogen metabolism.