Identification of multifunctional ATP-citrate lyase kinase as the alpha-isoform of glycogen synthase kinase-3

Oncogenic Control of Metabolism

A cell committed to proliferation must reshape its metabolism to enable robust yet balanced production of building blocks for the assembly of proteins, lipids, nucleic acids, and other macromolecules, from which two functional daughter cells can be produced. The metabolic remodeling associated with proliferation is orchestrated by a number of pro-proliferative signaling nodes, which include phosphatidylinositol-3 kinase (PI3K), the RAS family of small GTPases, and transcription factor c-myc In metazoan cells, these signals are activated in a paracrine manner via growth factor-mediated activation of receptor (or receptor-associated) tyrosine kinases. Such stimuli are limited in duration and therefore allow the metabolism of target cells to return to the resting state once the proliferation demands have been satisfied. Cancer cells acquire activating genetic alterations within common pro-proliferative signaling nodes. These alterations lock cellular nutrient uptake and utilization into a perpetual progrowth state, leading to the aberrant accumulation and spread of cancer cells.

Disrupted mitochondrial response to nutrients is a presymptomatic event in the cortex of the APPSAA knock-in mouse model of Alzheimer's disease

INTRODUCTION

Reduced brain energy metabolism, mammalian target of rapamycin (mTOR) dysregulation, and extracellular amyloid beta (Aβ) oligomer (xcAβO) buildup are some well-known Alzheimer's disease (AD) features; how they promote neurodegeneration is poorly understood. We previously reported that xcAβOs inhibit nutrient-induced mitochondrial activity (NiMA) in cultured neurons. We now report NiMA disruption in vivo.

METHODS

Brain energy metabolism and oxygen consumption were recorded in heterozygous amyloid precursor protein knock-in (APPSAA) mice using two-photon fluorescence lifetime imaging and multiparametric photoacoustic microscopy.

RESULTS

NiMA is inhibited in APPSAA mice before other defects are detected in these Aβ-producing animals that do not overexpress APP or contain foreign DNA inserts into genomic DNA. Glycogen synthase kinase 3 (GSK3β) signals through mTORC1 to regulate NiMA independently of mitochondrial biogenesis. Inhibition of GSK3β with TWS119 stimulates NiMA in cultured human neurons, and mitochondrial activity and oxygen consumption in APPSAA mice.

DISCUSSION

NiMA disruption in vivo occurs before plaques, neuroinflammation, and cognitive decline in APPSAA mice, and may represent an early stage in human AD.

HIGHLIGHTS

Amyloid beta blocks communication between lysosomes and mitochondria in vivo. Nutrient-induced mitochondrial activity (NiMA) is disrupted long before the appearance of Alzheimer's disease (AD) histopathology in heterozygous amyloid precursor protein knock-in (APPSAA/+) mice. NiMA is disrupted long before learning and memory deficits in APPSAA/+ mice. Pharmacological interventions can rescue AD-related NiMA disruption in vivo.

Acetyl-CoA metabolism in cancer

Few metabolites can claim a more central and versatile role in cell metabolism than acetyl coenzyme A (acetyl-CoA). Acetyl-CoA is produced during nutrient catabolism to fuel the tricarboxylic acid cycle and is the essential building block for fatty acid and isoprenoid biosynthesis. It also functions as a signalling metabolite as the substrate for lysine acetylation reactions, enabling the modulation of protein functions in response to acetyl-CoA availability. Recent years have seen exciting advances in our understanding of acetyl-CoA metabolism in normal physiology and in cancer, buoyed by new mouse models, in vivo stable-isotope tracing approaches and improved methods for measuring acetyl-CoA, including in specific subcellular compartments. Efforts to target acetyl-CoA metabolic enzymes are also advancing, with one therapeutic agent targeting acetyl-CoA synthesis receiving approval from the US Food and Drug Administration. In this Review, we give an overview of the regulation and cancer relevance of major metabolic pathways in which acetyl-CoA participates. We further discuss recent advances in understanding acetyl-CoA metabolism in normal tissues and tumours and the potential for targeting these pathways therapeutically. We conclude with a commentary on emerging nodes of acetyl-CoA metabolism that may impact cancer biology.

The Role of Hypoxia-Inducible Factor Post-Translational Modifications in Regulating Its Localisation, Stability, and Activity

The hypoxia signalling pathway enables adaptation of cells to decreased oxygen availability. When oxygen becomes limiting, the central transcription factors of the pathway, hypoxia-inducible factors (HIFs), are stabilised and activated to induce the expression of hypoxia-regulated genes, thereby maintaining cellular homeostasis. Whilst hydroxylation has been thoroughly described as the major and canonical modification of the HIF-α subunits, regulating both HIF stability and activity, a range of other post-translational modifications decorating the entire protein play also a crucial role in altering HIF localisation, stability, and activity. These modifications, their conservation throughout evolution, and their effects on HIF-dependent signalling are discussed in this review.

A novel GSK3 inhibitor that promotes self-renewal in mouse embryonic stem cells

Small molecules that regulate cell stemness have the potential to make a major contribution to regenerative medicine. In the course of screening for small molecules that affect stemness in mouse embryonic stem cells (mESCs), we discovered that NPD13432, an aurone derivative, promoted self-renewal of mESCs. Normally, mESCs start to differentiate upon withdrawal of 2i/LIF. However, cells treated with the compound continued to express endogenous Nanog, a pluripotency marker protein essential for sustaining the undifferentiated state, even in the absence of 2i/LIF. Biochemical characterization revealed that NPD13432 inhibited GSK3α and GSK3β with IC₅₀ values of 92 nM and 310 nM, respectively, suggesting that the compound promotes self-renewal in mESCs by inhibiting GSK3. The chemical structure of the compound is unique among known molecules with this activity, providing an opportunity to develop new inhibitors of GSK3, as well as chemical tools for investigating cell stemness.

Control of neuronal excitability by GSK-3beta: Epilepsy and beyond

Glycogen synthase kinase 3beta (GSK-3β) is an enzyme with a variety of cellular functions in addition to the regulation of glycogen metabolism. In the central nervous system, different intracellular signaling pathways converge on GSK-3β through a cascade of phosphorylation events that ultimately control a broad range of neuronal functions in the development and adulthood. In mice, genetically removing or increasing GSK-3β cause distinct functional and structural neuronal phenotypes and consequently affect cognition. Precise control of GSK-3β activity is important for such processes as neuronal migration, development of neuronal morphology, synaptic plasticity, excitability, and gene expression. Altered GSK-3β activity contributes to aberrant plasticity within neuronal circuits leading to neurological, psychiatric disorders, and neurodegenerative diseases. Therapeutically targeting GSK-3β can restore the aberrant plasticity of neuronal networks at least in animal models of these diseases. Although the complete repertoire of GSK-3β neuronal substrates has not been defined, emerging evidence shows that different ion channels and their accessory proteins controlling excitability, neurotransmitter release, and synaptic transmission are regulated by GSK-3β, thereby supporting mechanisms of synaptic plasticity in cognition. Dysregulation of ion channel function by defective GSK-3β activity sustains abnormal excitability in the development of epilepsy and other GSK-3β-linked human diseases.

Effects of PKB/Akt inhibitors on insulin-stimulated lipogenesis and phosphorylation state of lipogenic enzymes in white adipose tissue

We investigated acute effects of two allosteric protein kinase B (PKB) inhibitors, MK-2206 and Akti-1/2, on insulin-stimulated lipogenesis in rat epididymal adipocytes incubated with fructose as carbohydrate substrate. In parallel, the phosphorylation state of lipogenic enzymes in adipocytes and incubated epididymal fat pads was monitored by immunoblotting. Preincubation of rat epididymal adipocytes with PKB inhibitors dose-dependently inhibited the following: insulin-stimulated lipogenesis, increased PKB Ser473 phosphorylation, increased PKB activity and decreased acetyl-CoA carboxylase (ACC) Ser79 phosphorylation. In contrast, the effect of insulin to decrease the phosphorylation of pyruvate dehydrogenase (PDH) at Ser293 and Ser300 was not abolished by PKB inhibition. Insulin treatment also induced ATP-citrate lyase (ACL) Ser454 phosphorylation, but this effect was less sensitive to PKB inhibitors than ACC dephosphorylation by insulin. In incubated rat epididymal fat pads, Akti-1/2 treatment reversed insulin-induced ACC dephosphorylation, while ACL phosphorylation by insulin was maintained. ACL and ACC purified from white adipose tissue were poor substrates for PKBα in vitro. However, effects of wortmannin and torin, along with Akti-1/2 and MK-2206, on recognized PKB target phosphorylation by insulin were similar to their effects on insulin-induced ACL phosphorylation, suggesting that PKB could be the physiological kinase for ACL phosphorylation by insulin. In incubated epididymal fat pads from wild-type versus ACC1/2 S79A/S212A knockin mice, effects of insulin to increase lipogenesis from radioactive fructose or from radioactive acetate were reduced but not abolished. Together, the results support a key role for PKB in mediating insulin-stimulated lipogenesis by decreasing ACC phosphorylation, but not by decreasing PDH phosphorylation.

The vital role of ATP citrate lyase in chronic diseases

Chronic or non-communicable diseases are the leading cause of death worldwide; they usually result in long-term illnesses and demand long-term care. Despite advances in molecular therapeutics, specific biomarkers and targets for the treatment of these diseases are required. The dysregulation of de novo lipogenesis has been found to play an essential role in cell metabolism and is associated with the development and progression of many chronic diseases; this confirms the link between obesity and various chronic diseases. The main enzyme in this pathway-ATP-citrate lyase (ACLY), a lipogenic enzyme-catalyzes the critical reaction linking cellular glucose catabolism and lipogenesis. Increasing lines of evidence suggest that the modulation of ACLY expression correlates with the development and progressions of various chronic diseases such as neurodegenerative diseases, cardiovascular diseases, diabetes, obesity, inflammation, and cancer. Recent studies suggest that the inhibition of ACLY activity modulates the glycolysis and lipogenesis processes and stimulates normal physiological functions. This comprehensive review aimed to critically evaluate the role of ACLY in the development and progression of different diseases and the effects of its downregulation in the prevention and treatment of these diseases.

Gastrodin attenuates proliferation and inflammatory responses in activated microglia through Wnt/β-catenin signaling pathway

Microglia contribute to the regulation of neuroinflammation and play an important role in the pathogenesis of brain disorders. Thus, regulation of neuroinflammation triggered by activation of microglia has become a promising therapeutic strategy. Here, we investigated the beneficial effects of Gastrodin in activated microglia and analyzed the underlying molecular mechanisms. Microglia activation was regulated by Gastrodin not only in terms of microglia population size but also production of inflammatory mediators. Gastrodin inhibited the expression of inducible nitric oxide synthase (iNOS), tumor necrosis factor-α (TNF-α), cyclin-D1 and Ki67 in lipopolysaccharide (LPS)-stimulated BV-2 or primary microglia. Gastrodin also suppressed the expression of iNOS and Ki67 in activated microglia in three-day-old LPS-injected postnatal rats. In addition, the present results have shown that Gastrodin inhibited LPS-induced phosphorylation of glycogen synthase kinase-3β (GSK-3β) at Ser 9 and β-catenin activity. We further extended our investigation to determine whether Wnt/β-catenin signaling pathway was involved in the anti-inflammatory and anti-proliferation function of Gastrodin. β-Catenin antagonist (XAV939) was used to block LPS-mediated upregulation of iNOS, TNF-α, cyclin-D1, nitric oxide (NO) and the number of cells in the G2/M+S phase of cell cycle. Moreover, treatment with LiCl, a special Wnt/β-catenin pathway agonist significantly blocked Gastrodin-mediated down-regulation of iNOS, TNF-α, cyclin-D1, NO and the number of cells in the G2/M+S phase of cell cycle in LPS-stimulated BV-2 microglia. Taken together, the present results suggested that Gastrodin mediated anti-inflammatory and anti-proliferation effects in activated microglia by modulating the Wnt/β-catenin signaling pathway.

Mechanisms of Insulin Action and Insulin Resistance

The 1921 discovery of insulin was a Big Bang from which a vast and expanding universe of research into insulin action and resistance has issued. In the intervening century, some discoveries have matured, coalescing into solid and fertile ground for clinical application; others remain incompletely investigated and scientifically controversial. Here, we attempt to synthesize this work to guide further mechanistic investigation and to inform the development of novel therapies for type 2 diabetes (T2D). The rational development of such therapies necessitates detailed knowledge of one of the key pathophysiological processes involved in T2D: insulin resistance. Understanding insulin resistance, in turn, requires knowledge of normal insulin action. In this review, both the physiology of insulin action and the pathophysiology of insulin resistance are described, focusing on three key insulin target tissues: skeletal muscle, liver, and white adipose tissue. We aim to develop an integrated physiological perspective, placing the intricate signaling effectors that carry out the cell-autonomous response to insulin in the context of the tissue-specific functions that generate the coordinated organismal response. First, in section II, the effectors and effects of direct, cell-autonomous insulin action in muscle, liver, and white adipose tissue are reviewed, beginning at the insulin receptor and working downstream. Section III considers the critical and underappreciated role of tissue crosstalk in whole body insulin action, especially the essential interaction between adipose lipolysis and hepatic gluconeogenesis. The pathophysiology of insulin resistance is then described in section IV. Special attention is given to which signaling pathways and functions become insulin resistant in the setting of chronic overnutrition, and an alternative explanation for the phenomenon of ‟selective hepatic insulin resistanceˮ is presented. Sections V, VI, and VII critically examine the evidence for and against several putative mediators of insulin resistance. Section V reviews work linking the bioactive lipids diacylglycerol, ceramide, and acylcarnitine to insulin resistance; section VI considers the impact of nutrient stresses in the endoplasmic reticulum and mitochondria on insulin resistance; and section VII discusses non-cell autonomous factors proposed to induce insulin resistance, including inflammatory mediators, branched-chain amino acids, adipokines, and hepatokines. Finally, in section VIII, we propose an integrated model of insulin resistance that links these mediators to final common pathways of metabolite-driven gluconeogenesis and ectopic lipid accumulation.

Targeting ATP-Citrate Lyase in Hyperlipidemia and Metabolic Disorders

Chronic overnutrition and a sedentary lifestyle promote imbalances in metabolism, often manifesting as risk factors for life-threating diseases such as atherosclerotic cardiovascular disease (ASCVD) and nonalcoholic fatty liver disease (NAFLD). Nucleocytosolic acetyl-coenzyme A (CoA) has emerged as a central signaling node used to coordinate metabolic adaptations in response to a changing nutritional status. ATP-citrate lyase (ACL) is the enzyme primarily responsible for the production of extramitochondrial acetyl-CoA and is thus strategically positioned at the intersection of nutrient catabolism and lipid biosynthesis. Here, we discuss recent findings from preclinical studies, as well as Mendelian and clinical randomized trials, demonstrating the importance of ACL activity in metabolism, and supporting its inhibition as a potential therapeutic approach to treating ASCVD, NAFLD, and other metabolic disorders.

Imbalanced insulin action in chronic over nutrition: Clinical harm, molecular mechanisms, and a way forward

The growing worldwide prevalence of overnutrition and underexertion threatens the gains that we have made against atherosclerotic cardiovascular disease and other maladies. Chronic overnutrition causes the atherometabolic syndrome, which is a cluster of seemingly unrelated health problems characterized by increased abdominal girth and body-mass index, high fasting and postprandial concentrations of cholesterol- and triglyceride-rich apoB-lipoproteins (C-TRLs), low plasma HDL levels, impaired regulation of plasma glucose concentrations, hypertension, and a significant risk of developing overt type 2 diabetes mellitus (T2DM). In addition, individuals with this syndrome exhibit fatty liver, hypercoagulability, sympathetic overactivity, a gradually rising set-point for body adiposity, a substantially increased risk of atherosclerotic cardiovascular morbidity and mortality, and--crucially--hyperinsulinemia. Many lines of evidence indicate that each component of the atherometabolic syndrome arises, or is worsened by, pathway-selective insulin resistance and responsiveness (SEIRR). Individuals with SEIRR require compensatory hyperinsulinemia to control plasma glucose levels. The result is overdrive of those pathways that remain insulin-responsive, particularly ERK activation and hepatic de-novo lipogenesis (DNL), while carbohydrate regulation deteriorates. The effects are easily summarized: if hyperinsulinemia does something bad in a tissue or organ, that effect remains responsive in the atherometabolic syndrome and T2DM; and if hyperinsulinemia might do something good, that effect becomes resistant. It is a deadly imbalance in insulin action. From the standpoint of human health, it is the worst possible combination of effects. In this review, we discuss the origins of the atherometabolic syndrome in our historically unprecedented environment that only recently has become full of poorly satiating calories and incessant enticements to sit. Data are examined that indicate the magnitude of daily caloric imbalance that causes obesity. We also cover key aspects of healthy, balanced insulin action in liver, endothelium, brain, and elsewhere. Recent insights into the molecular basis and pathophysiologic harm from SEIRR in these organs are discussed. Importantly, a newly discovered oxide transport chain functions as the master regulator of the balance amongst different limbs of the insulin signaling cascade. This oxide transport chain--abbreviated 'NSAPP' after its five major proteins--fails to function properly during chronic overnutrition, resulting in this harmful pattern of SEIRR. We also review the origins of widespread, chronic overnutrition. Despite its apparent complexity, one factor stands out. A sophisticated junk food industry, aided by subsidies from willing governments, has devoted years of careful effort to promote overeating through the creation of a new class of food and drink that is low- or no-cost to the consumer, convenient, savory, calorically dense, yet weakly satiating. It is past time for the rest of us to overcome these foes of good health and solve this man-made epidemic.

Decreased store operated Ca2+ entry in dendritic cells isolated from mice expressing PKB/SGK-resistant GSK3

Dendritic cells (DCs), key players of immunity, are regulated by glycogen synthase kinase GSK3. GSK3 activity is suppressed by PKB/Akt and SGK isoforms, which are in turn stimulated by the PI3K pathway. Exposure to bacterial lipopolysaccharides increases cytosolic Ca²⁺-concentration ([Ca²⁺]_i), an effect augmented in DCs isolated from mutant mice expressing PKB/SGK-resistant GSK3α,β (gsk3^KI). Factors affecting [Ca²⁺]_i include Ca²⁺-release from intracellular stores (CRIS), store-operated Ca²⁺-entry (SOCE) through STIM1/STIM2-regulated Orai1, K⁺-dependent Na⁺/Ca²⁺-exchangers (NCKX), K⁺-independent Na⁺/Ca²⁺-exchangers (NCX) and calbindin-D28k. The present study explored whether PKB/SGK-dependent GSK3α, β-activity impacts on CRIS, SOCE, NCKX, NCX or calbindin. DCs were isolated from gsk3^KI mice and respective wild-type mice (gsk3^WT), [Ca²⁺]_i estimated from Fura2 fluorescence, Orai1, STIM1, STIM2 as well as calbindin-D28k protein abundance determined by Western blotting and mRNA levels quantified by real time PCR. As a result, thapsigargin-induced CRIS and SOCE were significantly blunted by GSK3-inhibitors SB216763 (1–10 µM, 30 min) or GSK-XIII (10 µM, 30 min) but were significantly lower in gsk3^WT than in gsk3^KIDCs. Orai1, STIM1 and STIM2 protein abundance was significantly lower and calbindin-D28k abundance significantly higher in gsk3^KI than in gsk3^WTDCs. Activity of NCKX and NCX was significantly higher in gsk3^KI than in gsk3^WTDCs and was significantly increased by SB216763 (1 µM, 30 min) or GSK-XIII (10 µM, 30 min). Treatment of gsk3^WT DCs with SB216763 (1 µM, 4–24 h) or GSK-XIII (10 µM, 4–24 h) did not significantly modify the protein abundance of Orai1, STIM1 and STIM2. The present observations point to a dual role of GSK3 in the regulation of Ca²⁺ in DCs. Acute inhibition of GSK3 blunted the increase of [Ca²⁺]_i following CRIS and SOCE and stimulated NCKX/NCX activity. However, expression of PKB/SGK-resistant GSK3α, β downregulated the increase of [Ca²⁺]_i following CRIS and SOCE, an effect at least partially due to downregulation of Orai1, STIM1 and STIM2 expression as well as upregulation of Na⁺/Ca²⁺-exchanger activity and calbindin D28k expression.

Glycogen synthase kinase-3 (GSK3) controls deoxyglucose-induced mitochondrial biogenesis in human neuroblastoma SH-SY5Y cells

Mitochondrial biogenesis, a mitochondrial growth and division process, is crucial for adaptation to metabolic stress. The present study demonstrated that treatment with a specific inhibitor of GSK3, SB216763, attenuated induction of mitochondrial biogenesis by a glycolysis inhibitor, 2-deoxyglucose (2-DG), without affecting this biogenesis at basal condition. Additionally, overexpression of WT-GSK3β promoted whereas GSK3β-KD attenuated 2-DG-induced mitochondrial protein expression. The mitochondrial biogenesis attenuation by GSK3 inhibitor was not due to inhibition of protein degradation. Furthermore, GSK3 inhibition further reduced transcription of mitochondrial (COXII), but not nuclear (VDAC) gene by 2-DG suggesting its participation in 2-DG-induced mitochondrial transcription. Together, our results show that GSK3 regulates mitochondrial biogenesis induced by glycolysis inhibition.

Cytosolic functions of MORC2 in lipogenesis and adipogenesis

Microrchidia (MORC) family CW-type zinc finger 2 (MORC2) has been shown to be involved in several nuclear processes, including transcription modulation and DNA damage repair. However, its cytosolic function remains largely unknown. Here, we report an interaction between MORC2 and adenosine triphosphate (ATP)-citrate lyase (ACLY), an enzyme that catalyzes the formation of acetyl-coA and plays a central role in lipogenesis, cholesterogenesis, and histone acetylation. Furthermore, we demonstrate that MORC2 promotes ACLY activation in the cytosol of lipogenic breast cancer cells and plays an essential role in lipogenesis, adipogenesis and differentiation of 3T3-L1 preadipocytic cells. Consistently, the expression of MORC2 is induced during the process of 3T3-L1 adipogenic differentiation and mouse mammary gland development at a stage of increased lipogenesis. This observation was accompanied by a high ACLY activity. Together, these results demonstrate a cytosolic function of MORC2 in lipogenesis, adipogenic differentiation, and lipid homeostasis by regulating the activity of ACLY.

PKB/SGK-dependent GSK3-phosphorylation in the regulation of LPS-induced Ca2+ increase in mouse dendritic cells

The function of dendritic cells (DCs) is modified by glycogen synthase kinase GSK3 and GSK3 inhibitors have been shown to protect against inflammatory disease. Regulators of GSK3 include the phosphoinositide 3 kinase (PI3K) pathway leading to activation of protein kinase B (PKB/Akt) and serum and glucocorticoid inducible kinase (SGK) isoforms, which in turn phosphorylate and thus inhibit GSK3. The present study explored, whether PKB/SGK-dependent inhibition of GSK3 contributes to the regulation of cytosolic Ca(2+) concentration following stimulation with bacterial lipopolysaccharides (LPS). To this end DCs from mutant mice, in which PKB/SGK-dependent GSK3α,β regulation was disrupted by replacement of the serine residues in the respective SGK/PKB-phosphorylation consensus sequence by alanine (gsk3(KI)), were compared to DCs from respective wild type mice (gsk3(WT)). According to Western blotting, GSK3 phosphorylation was indeed absent in gsk3(KI) DCs. According to flow cytometry, expression of antigen-presenting molecule major histocompatibility complex II (MHCII) and costimulatory molecule CD86, was similar in unstimulated and LPS (1μg/ml, 24h)-stimulated gsk3(WT) and gsk3(KI) DCs. Moreover, production of cytokines IL-6, IL-10, IL-12 and TNFα was not significantly different in gsk3(KI) and gsk3(WT) DCs. In gsk3(WT) DCs, stimulation with LPS (1μg/ml) within 10min led to transient phosphorylation of GSK3. According to Fura2 fluorescence, LPS (1μg/ml) increased cytosolic Ca(2+) concentration, an effect significantly more pronounced in gsk3(KI) DCs than in gsk3(WT) DCs. Conversely, GSK3 inhibitor SB216763 (3-[2,4-Dichlorophenyl]-4-[1-methyl-1H-indol-3-yl]-1H-pyrrole-2,5-dione, 10μM, 30min) significantly blunted the increase of cytosolic Ca(2+) concentration following LPS exposure. In conclusion, PKB/SGK-dependent GSK3α,β activity participates in the regulation of Ca(2+) signaling in dendritic cells.

The Role of Glycogen Synthase Kinase 3 Beta in Neuroinflammation and Pain

Neuroinflammation is a crucial mechanism related to many neurological diseases. Extensive studies in recent years have indicated that dysregulation of Glycogen Synthase Kinase 3 Beta (GSK3β) contributes to the development and progression of these disorders through regulating the neuroinflammation processes. Inhibitors of GSK3β have been shown to be beneficial in many neuroinflammatory disease models including Alzheimer's disease, multiple sclerosis and AIDS dem entia complex. Glial activation and elevated pro-inflammation cytokines (signs of neuroinflammation) in the spinal cord have been widely recognized as a pivotal mechanism underlying the development and maintenance of many types of pathological pain. The role of GSK3β in the pathogenesis of pain has recently emerged. In this review, we will first review the GSK3β structure, regulation, and mechanisms by which GSK3βregulates inflammation. We will then describe neuroinflammationin general and in specific types of neurological diseases and the potential beneficial effects induced by inhibiting GSK3β. Finally, we will provide new evidence linking aberrant levels of GSK3β in the development of pathological pain.

The role of pathway-selective insulin resistance and responsiveness in diabetic dyslipoproteinemia

PURPOSE OF REVIEW

Type 2 diabetes mellitus (T2DM) and related syndromes exhibit a deadly triad of dyslipoproteinemia, which leads to atherosclerosis, hyperglycemia, which causes microvascular disease, and hypertension. These features share a common, but unexplained, origin--namely, pathway-selective insulin resistance and responsiveness (SEIRR). Here, we review recent work on hepatic SEIRR indicating that deranged insulin signaling may have a remarkably simple molecular basis.

RECENT FINDINGS

Comprehensive examination of a set of 18 insulin targets revealed that T2DM liver in vivo exhibits a specific defect in the ability of the NAD(P)H oxidase 4 (NOX4) to inactivate protein tyrosine phosphatase gene family members after stimulation with insulin, and that impairment of this single molecule, NOX4, in cultured hepatocytes recapitulates all features of hepatic SEIRR in vivo. These features include insulin-stimulated generation of an unusual monophosphorylated form of AKT at Thr308 (pT308-AKT) with only weak phosphorylation at Ser473, impaired insulin-stimulated pathways for lowering plasma levels of lipids and glucose, but continued lipogenic pathways and robust extracellular signal-regulated kinase activation. This new study, in combination with important prior work, provides clues to several long-standing mysteries, such as how AKT might regulate lipid-lowering and glucose-lowering pathways that become insulin-resistant but also lipogenic pathways that remain insulin-responsive, as well as a potential role for NOX4 in insulin-stimulated generation of oxysterol ligands for LXR, a key lipogenic factor.

SUMMARY

These findings suggest a unified molecular explanation for fatty liver, atherogenic dyslipoproteinemia, hyperglycemia, and hence accelerated atherosclerosis and microvascular disease in T2DM, obesity, and related syndromes of positive caloric imbalance.

AKT/SGK-sensitive phosphorylation of GSK3 in the regulation of L-selectin and perforin expression as well as activation induced cell death of T-lymphocytes

Survival and function of T-lymphocytes critically depends on phosphoinositide (PI) 3 kinase. PI3 kinase signaling includes the PKB/Akt and SGK dependent phosphorylation and thus inhibition of glycogen synthase kinase GSK3α,β. Lithium, a known unspecific GSK3 inhibitor protects against experimental autoimmune encephalomyelitis. The present study explored, whether Akt/SGK-dependent regulation of GSK3 activity is a determinant of T cell survival and function. Experiments were performed in mutant mice in which Akt/SGK-dependent GSK3α,β inhibition was disrupted by replacement of the serine residue in the respective SGK/Akt-phosphorylation consensus sequence by alanine (gsk3(KI)). T cells from gsk3(KI) mice were compared to T cells from corresponding wild type mice (gsk3(WT)). As a result, in gsk3(KI) CD4(+) cells surface CD62L (L-selectin) was significantly less abundant than in gsk3(WT) CD4(+) cells. Upon activation in vitro T cells from gsk3(KI) mice reacted with enhanced perforin production and reduced activation induced cell death. Cytokine production was rather reduced in gsk3(KI) T cells, suggesting that GSK3 induces effector function in CD8(+) T cells. In conclusion, PKB/Akt and SGK sensitive phosphorylation of GSK3α,β is a potent regulator of perforin expression and activation induced cell death in T lymphocytes.

NOX4 pathway as a source of selective insulin resistance and responsiveness

OBJECTIVE

Type 2 diabetes mellitus and related syndromes exhibit a deadly triad of dyslipoproteinemia, which leads to atherosclerosis; hyperglycemia, which causes microvascular disease; and hypertension. These features share a common, but unexplained, origin-namely, pathway-selective insulin resistance and responsiveness. Here, we undertook a comprehensive characterization of pathway-selective insulin resistance and responsiveness in liver and hepatocytes by examining 18 downstream targets of the insulin receptor, surveying the AKT, ERK, and NAD(P)H oxidase 4 pathways.

METHODS AND RESULTS

Injection of insulin into hyperphagic, obese type 2 diabetic db/db mice failed to inactivate hepatic protein tyrosine phosphatase gene family members, a crucial action of NAD(P)H oxidase 4 previously thought to be required for all signaling through AKT and ERK. Insulin-stimulated type 2 diabetic livers unexpectedly produced an unusual form of AKT that was phosphorylated at Thr308 (pT308), with only weak insulin-stimulated phosphorylation at Ser473. Remarkably, knockdown or inhibition of NAD(P)H oxidase 4 in cultured hepatocytes recapitulated the entire complicated pattern of pathway-selective insulin resistance and responsiveness seen in vivo-namely, monophosphorylated pT308-AKT, impaired insulin-stimulated pathways for lowering plasma lipids and glucose, but continued lipogenic pathways and robust ERK activation.

CONCLUSIONS

Functional disturbance of a single molecule, NAD(P)H oxidase 4, is sufficient to induce the key harmful features of deranged insulin signaling in type 2 diabetes mellitus, obesity, and other conditions associated with hyperinsulinemia and pathway-selective insulin resistance and responsiveness.

GSK-3: Functional Insights from Cell Biology and Animal Models

Glycogen synthase kinase-3 (GSK-3) is a widely expressed and highly conserved serine/threonine protein kinase encoded in mammals by two genes that generate two related proteins: GSK-3α and GSK-3β. GSK-3 is active in cells under resting conditions and is primarily regulated through inhibition or diversion of its activity. While GSK-3 is one of the few protein kinases that can be inactivated by phosphorylation, the mechanisms of GSK-3 regulation are more varied and not fully understood. Precise control appears to be achieved by a combination of phosphorylation, localization, and sequestration by a number of GSK-3-binding proteins. GSK-3 lies downstream of several major signaling pathways including the phosphatidylinositol 3′ kinase pathway, the Wnt pathway, Hedgehog signaling and Notch. Specific pools of GSK-3, which differ in intracellular localization, binding partner affinity, and relative amount are differentially sensitized to several distinct signaling pathways and these sequestration mechanisms contribute to pathway insulation and signal specificity. Dysregulation of signaling pathways involving GSK-3 is associated with the pathogenesis of numerous neurological and psychiatric disorders and there are data suggesting GSK-3 isoform-selective roles in several of these. Here, we review the current knowledge of GSK-3 regulation and targets and discuss the various animal models that have been employed to dissect the functions of GSK-3 in brain development and function through the use of conventional or conditional knockout mice as well as transgenic mice. These studies have revealed fundamental roles for these protein kinases in memory, behavior, and neuronal fate determination and provide insights into possible therapeutic interventions.

What Are the bona fide GSK3 Substrates?

Nearly 100 proteins are proposed to be substrates for GSK3, suggesting that this enzyme is a fundamental regulator of almost every process in the cell, in every tissue in the body. However, it is not certain how many of these proposed substrates are regulated by GSK3 in vivo. Clearly, the identification of the physiological functions of GSK3 will be greatly aided by the identification of its bona fide substrates, and the development of GSK3 as a therapeutic target will be highly influenced by this range of actions, hence the need to accurately establish true GSK3 substrates in cells. In this paper the evidence that proposed GSK3 substrates are likely to be physiological targets is assessed, highlighting the key cellular processes that could be modulated by GSK3 activity and inhibition.

ATP-citrate lyase reduction mediates palmitate-induced apoptosis in pancreatic beta cells

Elevated extracellular lipids, such as the free fatty acid palmitate, can induce pancreatic beta cell endoplasmic reticulum (ER) stress and apoptosis, thereby contributing to the initiation and progression of type 2 diabetes. ATP-citrate lyase (ACLY), a key enzyme in cellular lipid production, was identified as a palmitate target in a proteomic screen. We investigated the effects of palmitate on ACLY activity and phosphorylation and its role in beta cell ER stress and apoptosis. We demonstrated that treatment of MIN6 cells, mouse islets and human islets with palmitate reduced ACLY protein levels. These in vitro results were validated by our finding that islets from high fat-fed mice had a significant decrease in ACLY, similar to that previously observed in type 2 diabetic human islets. Palmitate decreased intracellular acetyl-CoA levels to a similar degree as the ACLY inhibitor, SB-204990, suggesting a reduction in ACLY activity. ACLY inhibitors alone were sufficient to induce CCAAT/enhancer-binding protein homologues protein (CHOP)-dependent ER stress and caspase-3-dependent apoptosis. Similarly, even modest shRNA-mediated knockdown of ACLY caused a significant increase in beta cell apoptosis and ER stress. The effects of chemical ACLY inhibition and palmitate were non-additive and therefore potentially mediated by a common mechanism. Indeed, overexpression of ACLY prevented palmitate-induced beta cell death. These observations provide new evidence that ACLY expression and activity can be suppressed by exogenous lipids and demonstrate a critical role for ACLY in pancreatic beta cell survival. These findings add to the emerging body of evidence linking beta cell metabolism with programmed cell death.

Steroid hormone release as well as renal water and electrolyte excretion of mice expressing PKB/SGK-resistant GSK3

Insulin and insulin-like growth factor (IGF1) participate in the regulation of renal electrolyte excretion. Insulin- and IGF1-dependent signaling includes phosphatidylinositide-3 (PI3)-kinase, phosphoinositide-dependent kinase PDK1 as well as protein kinase B (PKB) and serum and glucocorticoid inducible kinase (SGK) isoforms, which in turn phosphorylate and thus inhibit glycogen synthase kinase GSK3alpha,beta. Replacement of the serines in the PKB/SGK consensus sequences by alanine (gsk3 ( KI )) confers resistance of GSK3 to PKB/SGK. To explore the role of PKB/SGK-dependent inhibition of GSK3 in the regulation of water/electrolyte metabolism, mice carrying the PKB/SGK resistant mutant (gsk3 ( KI )) were compared to their wild-type littermates (gsk3 ( WT ) ). Body weight was similar in gsk3 ( KI ) and gsk3 ( WT ) mice. Plasma aldosterone at 10 A.M: . and corticosterone concentrations at 5 P.M: . were significantly lower, but 24-h urinary aldosterone was significantly higher, and corticosterone excretion tended to be higher in gsk3 ( KI ) than in gsk3 ( WT ) mice. Food and water intake, fecal excretion, glomerular filtration rate, urinary flow rate, urine osmolarity, as well as urinary Na+, K+, urea excretion were significantly larger, and plasma Na+, urea, but not K+ concentration, were significantly lower in gsk3 ( KI ) than in gsk3 ( WT ) mice. Body temperature was significantly higher in gsk3 ( KI ) than in gsk3 ( WT ) mice. When allowed to choose between tap water and saline, gsk3 ( WT ) mice drank more saline, whereas gsk3 ( KI ) mice drank similar large volumes of tap water and saline. During high-salt diet, urinary vasopressin excretion increased to significantly higher levels in gsk3 ( KI ) than in gsk3 ( WT ) mice. After water deprivation, body weight decreased faster in gsk3 ( KI ) than in gsk3 ( WT ) mice. Blood pressure, however, was significantly higher in gsk3 ( KI ) than in gsk3 ( WT ) mice. The observations disclose a role of PKB/SGK-dependent GSK3 activity in the regulation of steroid hormone release, renal water and electrolyte excretion and blood pressure control.

Regulation of Human Cytidine Triphosphate Synthetase 1 by Glycogen Synthase Kinase 3

Cytidine triphosphate synthetase (CTPS) catalyzes the rate-limiting step in the de novo synthesis of CTP, and both the yeast and human enzymes have been reported to be regulated by protein kinase A or protein kinase C phosphorylation. Here, we provide evidence that stimulation or inhibition of protein kinase A and protein kinase C does not alter the phosphorylation of endogenous human CTPS1 in human embryonic kidney 293 cells under the conditions tested. Unexpectedly, we found that low serum conditions increased phosphorylation of endogenous CTPS1 and this phosphorylation was inhibited by the glycogen synthase kinase 3 (GSK3) inhibitor indirubin-3'-monoxime and GSK3beta short interfering RNAs, demonstrating the involvement of GSK3 in phosphorylation of endogenous human CTPS1. Separating tryptic peptides from [(32)P]orthophosphate-labeled cells and analyzing the phosphopeptides by mass spectrometry identified Ser-574 and Ser-575 as phosphorylated residues. Mutation of Ser-571 demonstrated that Ser-571 was the major site phosphorylated by GSK3 in intact human embryonic kidney 293 cells by GSK3 in vitro. Furthermore, mutation of Ser-575 prevented the phosphorylation of Ser-571, suggesting that phosphorylation of Ser-575 was necessary for priming the GSK3 phosphorylation of Ser-571. Low serum was found to decrease CTPS1 activity, and incubation with the GSK3 inhibitor indirubin-3'-monoxime protected against this decrease in activity. Incubation with an alkaline phosphatase increased CTPS1 activity in a time-dependent manner, demonstrating that phosphorylation inhibits CTPS1 activity. This is the first study to investigate the phosphorylation and regulation of human CTPS1 in human cells and suggests that GSK3 is a novel regulator of CTPS activity.

The role of GSK3 in glucose homeostasis and the development of insulin resistance

GSK3 has been implicated in the development of insulin resistance, primarily based on its role in regulation of glycogen synthesis. However, GSK3 is involved in many other important signaling cascades which may regulate glucose homeostasis and the development of insulin resistance. In addition, GSK3 is composed of two isoforms, GSK3alpha and beta, which do not completely share their physiological roles, and this raises a possibility that GSK3alpha and beta may function differently in glucose homeostasis. In this review, we will give an overview to examine potential mechanisms for the roles of GSK3 in the development of insulin resistance.

Inhibition of GSK-3beta as a target for cardioprotection: the importance of timing, location, duration and degree of inhibition

Cardiovascular disease is the major cause of morbidity and mortality in western countries such as the US. Myocardial infarction leads to loss of myocytes and with extremely limited ability to replenish cardiomyocytes, the heart exhibits depressed contractility. This ultimately results in hypertrophy of the remaining viable myocytes, which is the primary predictor for heart failure. Thus, drug therapies which can reduce myocyte cell death and reduce postischaemic dysfunction would be expected to greatly reduce cardiac hypertrophy and subsequent heart failure and death. Inhibition of glycogen synthase kinase (GSK)-3beta has been proposed as a strategy to improve postischaemic cardiomyocyte survival, as inhibition of GSK-3beta has been shown to reduce myocardial cell death following ischaemia and reperfusion. Therapies for inhibiting GSK are feasible as there are a number of newly developed specific inhibitors of GSK available, although most of these drugs have not been tested in long-term animal studies.

The twentieth century struggle to decipher insulin signalling

Following the discovery of insulin, it took the rest of the twentieth century to understand how this hormone regulates intracellular metabolism. What are the main discoveries that led to our current understanding of this process? And how is this new knowledge being exploited in an attempt to develop improved drugs to treat the epidemic of type-2 diabetes?

Glycogen synthase kinase-3 in insulin and Wnt signalling: a double-edged sword?

Glycogen synthase kinase-3 is an unusual protein serine/threonine kinase that, unlike most of its 500-odd relatives in the genome, is active under resting conditions and is inactivated upon cell stimulation. The two mammalian isoforms, GSK-3alpha and beta, play largely overlapping roles and have been implicated in a variety of human pathologies, including Type II diabetes, Alzheimer's disease, bipolar disorder and cancer. Recently, the modes of regulation of this enzyme have been elucidated through a combination of structural and cell biological studies. A series of relatively selective small molecules have facilitated chemical manipulation of the enzyme in intact cells and tissues, and new roles for the protein kinase in embryonic stem cell differentiation and motility have emerged. Despite these advances, the therapeutic value of this enzyme as a drug target remains clouded by uncertainty over the potential of antagonists to promote tumorigenesis. This article describes the state of understanding of this intriguing enzyme, and weighs current evidence regarding whether there is a therapeutic window for amelioration of diseases in which it is implicated, in the absence of inducing new pathologies.

Physiological roles of glycogen synthase kinase-3: potential as a therapeutic target for diabetes and other disorders

Glycogen synthase kinase-3 (GSK-3) has perplexed signal transduction researchers since its detection in skeletal muscle 25 years ago. The enzyme confounds most of the rules normally associated with protein kinases in that it exhibits significant activity, even in resting, unstimulated cells. However, the protein is highly regulated and potently inactivated in response to signals such as insulin and polypeptide growth factors. The enzyme also displays a distinct and unusual preference for substrates that have been previously phosphorylated by other protein kinases which provides obvious opportunities for cross-talk. Its substrates are diverse and are predominantly regulatory molecules. The molecular cloning of the kinase revealed it to be encoded by two related but distinct genes. Moreover, the mammalian proteins showed remarkable similarity to a fruitfly protein isolated on the basis of its role in cell fate determination. From these humble beginnings, study of the enzyme has accrued further surprises such as its inhibition by lithium, its regulation by serine and tyrosine phosphorylation and its implication in several human disorders including Alzheimers disease, bipolar disorder, cancer and diabetes. Most recently, small molecule inhibitors of GSK-3 have been developed and assessed for therapeutic potential in several of models of pathophysiology. The question is whether modulation of such an "involved" enzyme could lead to selective restoration of defects without multiple unwanted side effects. This review summarizes current knowledge of GSK-3 with respect to its known functions, together with an assessment of its real-life potential as a drug target for chronic conditions such as type 2 diabetes.

GSK-3: tricks of the trade for a multi-tasking kinase

Glycogen synthase kinase 3 (GSK-3) is a multifunctional serine/threonine kinase found in all eukaryotes. The enzyme is a key regulator of numerous signalling pathways, including cellular responses to Wnt, receptor tyrosine kinases and G-protein-coupled receptors and is involved in a wide range of cellular processes, ranging from glycogen metabolism to cell cycle regulation and proliferation. GSK-3 is unusual in that it is normally active in cells and is primarily regulated through inhibition of its activity. Another peculiarity compared with other protein kinases is its preference for primed substrates, that is, substrates previously phosphorylated by another kinase. Several recent advances have improved our understanding of GSK-3 regulation in multiple pathways. These include the solution of the crystal structure of GSK-3, which has provided insight into GSK-3's penchant for primed substrates and the regulation of GSK-3 by serine phosphorylation, and findings related to the involvement of GSK-3 in the Wnt/beta-catenin and Hedgehog pathways. Finally, since increased GSK-3 activity may be linked to pathology in diseases such as Alzheimer's disease and non-insulin-dependent diabetes mellitus, several new GSK-3 inhibitors, such as the aloisines, the paullones and the maleimides, have been developed. Although they are just starting to be characterized in cell culture experiments, these new inhibitors hold promise as therapeutic agents.

The identification of ATP-citrate lyase as a protein kinase B (Akt) substrate in primary adipocytes

Protein kinase B (Akt) plays a central role in cellular regulation, although many of the physiologically relevant substrates for the kinase remain to be identified. In this study, we have isolated a protein from primary epididymal adipocytes with an apparent molecular weight of 125,000. This protein exhibited immunoreactivity, in an insulin-dependent manner, with a phosphospecific antibody raised against the protein kinase B substrate consensus sequence RXRXX(pS/pT) as well as a phosphospecific antibody that recognizes serine 21/9 of GSK-3alpha/beta. MALDI-TOF mass spectrometry revealed the protein to be ATP-citrate lyase, suggesting that the two phosphospecific antibodies recognize phosphoserine 454, a previously reported insulin- and isoproterenol-stimulated ATP-citrate lyase phosphorylation site. Indeed, both insulin and isoproterenol stimulated the phosphorylation of this protein on the site recognized by the phosphospecific antibodies in a wortmannin-sensitive and -insensitive manner, respectively. In addition, transient expression of a constitutively active protein kinase B in primary adipocytes mimicked the effect of insulin on ATP-citrate lyase phosphorylation. Furthermore, ATP-citrate lyase was phosphorylated in vitro by recombinant protein kinase B on the same site. Taken together, these results demonstrate that serine 454 of ATP-citrate lyase is a novel and major in vivo substrate for protein kinase B.

Kinetic and biochemical analyses on the reaction mechanism of a bacterial ATP-citrate lyase

The prokaryotic ATP-citrate lyase is considered to be a key enzyme of the carbon dioxide-fixing reductive tricarboxylic acid (RTCA) cycle. Kinetic examination of the ATP-citrate lyase from the green sulfur bacterium Chlorobium limicola (Cl-ACL), an alpha(4)beta(4) heteromeric enzyme, revealed that the enzyme displayed typical Michaelis-Menten kinetics toward ATP with an apparent K(m) value of 0.21 +/- 0.04 mm. However, strong negative cooperativity was observed with respect to citrate binding, with a Hill coefficient (n(H)) of 0.45. Although the dissociation constant of the first citrate molecule was 0.057 +/- 0.008 mm, binding of the first citrate molecule to the enzyme drastically decreased the affinity of the enzyme for the second molecule by a factor of 23. ADP was a competitive inhibitor of ATP with a K(i) value of 0.037 +/- 0.006 mm. Together with previous findings that the enzyme catalyzed the reaction only in the direction of citrate cleavage, these kinetic features indicated that Cl-ACL can regulate both the direction and carbon flux of the RTCA cycle in C. limicola. Furthermore, in order to gain insight on the reaction mechanism, we performed biochemical analyses of Cl-ACL. His273 of the alpha subunit was indicated to be the phosphorylated residue in the catalytic center, as both catalytic activity and phosphorylation of the enzyme by ATP were abolished in an H273A mutant enzyme. We found that phosphorylation of the subunit was reversible. Nucleotide preference for activity was in good accordance with the preference for phosphorylation of the enzyme. Although residues interacting with nucleotides in the succinyl-CoA synthetase from Escherichia coli were conserved in AclB, AclA alone could be phoshorylated with the same nucleotide specificity observed in the holoenzyme. However, AclB was necessary for enzyme activity and contributed to enhance phosphorylation and stabilization of AclA.

Glycogen synthase kinase-3 beta mutagenesis identifies a common binding domain for GBP and Axin

Glycogen synthase kinase-3 beta (GSK-3) is a key downstream target of Wnt signaling and is regulated by its interactions with activating and inhibitory proteins. We and others have shown that GSK-3 activity toward non-primed substrates is regulated in part through a competition between its activating (Axin) and inhibitory (GBP/FRAT) binding partners. Here we use a reverse two-hybrid screen to identify mutations in GSK-3 that alter binding to GBP and Axin. We find that these mutations overlap and propose that GBP and Axin compete for binding to the same region of GSK-3. We use these mutations to examine the ability of GSK-3 to block eye development in Xenopus embryos and suggest that GSK-3 regulates eye development through a non-Wnt pathway.

GSK-3 and the neurodevelopmental hypothesis of schizophrenia

The Neurodevelopmental Hypothesis of schizophrenia suggests that interaction between genetic and environmental events occurring during critical early periods in neuronal growth may negatively influence the way by which nerve cells are laid down, differentiated and selectively culled by apoptosis. Recent advances offer insights into the regulation of brain development. The Wnt family of genes plays a central role in normal brain development. Activation of the Wnt cascade leads to inactivation of glycogen synthase kinase-3beta (GSK-3beta), accumulation and activation of beta-catenin and expression of genes involved in neuronal development. Alteration in the Wnt transduction cascade, which may represent an aberrant neurodevelopment in schizophrenia, is discussed. Programmed cell death is also an essential component of normal brain development. Abnormal neuronal distribution found in schizophrenic patients' brains may imply aberrant programmed cell death. GSK-3 participates in the signal transduction cascade of apoptosis. The possible role of aberrant GSK-3 in the etiology of schizophrenia is discussed.

Differing substrate specificities of members of the DYRK family of arginine-directed protein kinases

The mammalian DYRK (dual specificity tyrosine phosphorylated and regulated kinase) family of protein kinases comprises a number of related, but poorly understood enzymes. DYRK1A is nuclear while DYRKs 2 and 3 are cytoplasmic. We recently showed that DYRK2 phosphorylates the translation initiation factor eIF2B at Ser539 in its epsilon-subunit and thereby "primes" its phosphorylation by glycogen synthase kinase-3. Here we have used peptides based on the sequence around Ser539 to help define the specificity of DYRK2/3 in comparison with DYRK1A. These kinases require an arginine N-terminal to the target residue for efficient substrate phosphorylation. This cannot be replaced even by lysine. A peptide with arginine at -2 is phosphorylated much less well by all three kinases than one with arginine at -3. Replacement of the +1 proline by alanine almost completely eliminates substrate phosphorylation, but valine here does allow phosphorylation especially by DYRK2. This study reveals both similarities and differences in the specificities of these arginine-dependent protein kinases.

The multifaceted roles of glycogen synthase kinase 3beta in cellular signaling

Glycogen synthase kinase-3beta (GSK3beta) is a fascinating enzyme with an astoundingly diverse number of actions in intracellular signaling systems. GSK3beta activity is regulated by serine (inhibitory) and tyrosine (stimulatory) phosphorylation, by protein complex formation, and by its intracellular localization. GSK3beta phosphorylates and thereby regulates the functions of many metabolic, signaling, and structural proteins. Notable among the signaling proteins regulated by GSK3beta are the many transcription factors, including activator protein-1, cyclic AMP response element binding protein, heat shock factor-1, nuclear factor of activated T cells, Myc, beta-catenin, CCAAT/enhancer binding protein, and NFkappaB. Lithium, the primary therapeutic agent for bipolar mood disorder, is a selective inhibitor of GSK3beta. This raises the possibility that dysregulation of GSK3beta and its inhibition by lithium may contribute to the disorder and its treatment, respectively. GSK3beta has been linked to all of the primary abnormalities associated with Alzheimer's disease. These include interactions between GSK3beta and components of the plaque-producing amyloid system, the participation of GSK3beta in phosphorylating the microtubule-binding protein tau that may contribute to the formation of neurofibrillary tangles, and interactions of GSK3beta with presenilin and other Alzheimer's disease-associated proteins. GSK3beta also regulates cell survival, as it facilitates a variety of apoptotic mechanisms, and lithium provides protection from many insults. Thus, GSK3beta has a central role regulating neuronal plasticity, gene expression, and cell survival, and may be a key component of certain psychiatric and neurodegenerative diseases.

GSK3 takes centre stage more than 20 years after its discovery

Identified originally as a regulator of glycogen metabolism, glycogen synthase kinase-3 (GSK3) is now a well-established component of the Wnt signalling pathway, which is essential for setting up the entire body pattern during embryonic development. It may also play important roles in protein synthesis, cell proliferation, cell differentiation, microtubule dynamics and cell motility by phosphorylating initiation factors, components of the cell-division cycle, transcription factors and proteins involved in microtubule function and cell adhesion. Generation of the mouse knockout of GSK3beta, as well as studies in neurons, also suggest an important role in apoptosis. The substrate specificity of GSK3 is unusual in that efficient phosphorylation of many of its substrates requires the presence of another phosphorylated residue optimally located four amino acids C-terminal to the site of GSK3 phosphorylation. Recent experiments, including the elucidation of its three-dimensional structure, have enhanced our understanding of the molecular basis for the unique substrate specificity of GSK3. Insulin and growth factors inhibit GSK3 by triggering its phosphorylation, turning the N-terminus into a pseudosubstrate inhibitor that competes for binding with the 'priming phosphate' of substrates. In contrast, Wnt proteins inhibit GSK3 in a completely different way, by disrupting a multiprotein complex comprising GSK3 and its substrates in the Wnt signalling pathway, which do not appear to require a 'priming phosphate'. These latest findings have generated an enormous amount of interest in the development of drugs that inhibit GSK3 and which may have therapeutic potential for the treatment of diabetes, stroke and Alzheimer's disease.

The renaissance of GSK3

Glycogen synthase kinase 3 (GSK3) was initially described as a key enzyme involved in glycogen metabolism, but is now known to regulate a diverse array of cell functions. The study of the substrate specificity and regulation of GSK3 activity has been important in the quest for therapeutic intervention.

Regulation of glycogen synthase kinase-3 in human skeletal muscle: effects of food intake and bicycle exercise

Studies of skeletal muscle from rodents performed both in vivo and in vitro suggest a regulatory role of glycogen synthase kinase (GSK) 3 in glycogen synthase (GS) activation in response to insulin. Recently, hyperinsulinemic clamp studies in humans support such a role under nearly physiological conditions. In addition, in rats the activation of GS in skeletal muscle during treadmill running is time-related to the deactivation of GSK3. We investigated whether GSK3 was deactivated in human muscle during low- (approximately 50% VO2max for 1.5 h) and high-intensity (approximately 75% VO2max for 1 h) bicycle exercise as well as food intake. We observed a small but significant increase in GSK3alpha (10-20%) activity in biopsies obtained from vastus lateralis after both low- and high-intensity exercise, whereas GSK3beta activity was unaffected. Subsequent food intake increased Aktphosphorylation (approximately 2-fold) and deactivated GSK3alpha (approximately 40%), whereas GSK3beta activity was unchanged. GS activity increased in response to both exercise and food intake. We conclude that GSK3alpha but not GSK3beta may have a role in the regulation of GS activity in response to meal-associated hyperinsulinemia in humans. However, in contrast to findings in muscle from rats, exercise does not deactivate GSK3 in humans, suggesting a GSK3-independent mechanism in the regulation of GS activity in muscle during physical activity.

Activation of mRNA translation in rat cardiac myocytes by insulin involves multiple rapamycin-sensitive steps

Insulin acutely activates protein synthesis in ventricular cardiomyocytes from adult rats. In this study, we have established the methodology for studying the regulation of the signaling pathways and translation factors that may be involved in this response and have examined the effects of acute insulin treatment on them. Insulin rapidly activated the 70-kDa ribosomal S6 kinase (p70 S6k), and this effect was inhibited both by rapamycin and by inhibitors of phosphatidylinositol 3-kinase. The activation of p70 S6k is mediated by a signaling pathway involving the mammalian target of rapamycin (mTOR), which also modulates other translation factors. These include the eukaryotic initiation factor (eIF) 4E binding proteins (4E-BPs) and eukaryotic elongation factor 2 (eEF2). Insulin caused phosphorylation of 4E-BP1 and induced its dissociation from eIF4E, and these effects were also blocked by rapamycin. Concomitant with this, insulin increased the binding of eIF4E to eIF4G. Insulin also activated protein kinase B (PKB), which may lie upstream of p70 S6k and 4E-BP1, with the activation of the different isoforms being in the order alpha>beta>gamma. Insulin also caused inhibition of glycogen synthase kinase 3, which lies downstream of PKB, and of eEF2 kinase. The phosphorylation of eEF2 itself was also decreased by insulin, and this effect and the inactivation of eEF2 kinase were attenuated by rapamycin. The activation of overall protein synthesis by insulin in cardiomyocytes was substantially inhibited by rapamycin (but not by inhibitors of other specific signaling pathways, e.g., mitogen-activated protein kinase), showing that signaling events linked to mTOR play a major role in the control of translation by insulin in this cell type.

Selective interaction between leptin and insulin signaling pathways in a hepatic cell line

Leptin is a 16-kDa hormone secreted by adipocytes and plays an important role in control of feeding behavior and energy expenditure. In obesity, circulating levels of leptin and insulin are high because of the presence of increased body fat mass and insulin resistance. Recent reports have suggested that leptin can act through some of the components of the insulin signaling cascade, such as insulin receptor substrates (IRS-1 and IRS-2), phosphatidylinositol 3-kinase (PI 3-kinase), and mitogen-activated protein kinase, and can modify insulin-induced changes in gene expression in vitro and in vivo. Well differentiated hepatoma cells (Fao) possess both the long and short forms of the leptin receptor and respond to leptin with a stimulation of c-fos gene expression. In Fao cells, leptin alone had no effects on the insulin signaling pathway, but leptin pretreatment transiently enhanced insulin-induced tyrosine phosphorylation and PI 3-kinase binding to IRS-1, while producing an inhibition of tyrosine phosphorylation and PI 3-kinase binding to IRS-2. Leptin alone also induced serine phosphorylation of Akt and glycogen synthase kinase 3 but to a lesser extent than insulin, and the combination of these hormones was not additive. These results suggest complex interactions between the leptin and insulin signaling pathways that can potentially lead to differential modification of the metabolic and mitotic effects of insulin exerted through IRS-1 and IRS-2 and the downstream kinases that they activate.

Phosphorylation of recombinant human ATP:citrate lyase by cAMP-dependent protein kinase abolishes homotropic allosteric regulation of the enzyme by citrate and increases the enzyme activity. Allosteric activation of ATP:citrate lyase by phosphorylated sugars

Recombinantly expressed human ATP:citrate lyase was purified from E. coli, and its kinetic behavior was characterized before and after phosphorylation. Cyclic AMP-dependent protein kinase catalyzed the incorporation of only 1 mol of phosphate per mole of enzyme homotetramer, and glycogen synthase kinase-3 incorporated an additional 2 mol of phosphate into the phosphorylated protein. Isoelectric focusing revealed that all of the phosphates were incorporated into only one of the four enzyme subunits. Phosphorylation resulted in a 6-fold increase in V(max) and the conversion of citrate dependence from sigmoidal, displaying negative cooperativity, to hyperbolic. The phosphorylated recombinant enzyme is more similar to the enzyme isolated from mammalian tissues than unphosphorylated enzyme with respect to the K(m) for citrate, CoA, and ATP, and the specific activity. Fructose 6-phosphate was found to be a potent activator (60-fold) of the unphosphorylated recombinant enzyme, with half-maximal activation at 0.16 mM, which results in a decrease in the apparent K(m) for citrate and ATP, as well as an increase in the V(max) of the reaction. Thus, human ATP:citrate lyase activity is regulated in vitro allosterically by phosphorylated sugars as well as covalently by phosphorylation.

Role of protein kinase B and the MAP kinase cascade in mediating the EGF-dependent inhibition of glycogen synthase kinase 3 in Swiss 3T3 cells

Epidermal growth factor (EGF), insulin-like growth factor 1 (IGF1) and phorbol myristate acetate (PMA) induce the inhibition of glycogen synthase kinase 3 (GSK3) by stimulating the phosphorylation of an N-terminal serine. Here, we show that protein kinase B (PKB) plays a key role in mediating EGF-induced inhibition of GSK3alpha and that the classical MAP kinase (MAPK) cascade has two functions in this process. Firstly, it makes a transient contribution to EGF-induced inhibition of GSK3alpha. Secondly, it shortens the duration of PKB activation and GSK3alpha inhibition. In contrast, PKB alone mediates the IGF1-induced inhibition of GSK3alpha, while the MAPK cascade mediates the inhibition of GSK3alpha by PMA.

Insulin and exercise decrease glycogen synthase kinase-3 activity by different mechanisms in rat skeletal muscle

Glycogen synthase activity is increased in response to insulin and exercise in skeletal muscle. Part of the mechanism by which insulin stimulates glycogen synthesis may involve phosphorylation and activation of Akt, serine phosphorylation and deactivation of glycogen synthase kinase-3 (GSK-3), leading to dephosphorylation and activation of glycogen synthase. To study Akt and GSK-3 regulation in muscle, time course experiments on the effects of insulin injection and treadmill running exercise were performed in hindlimb skeletal muscle from male rats. Both insulin and exercise increased glycogen synthase activity (%I-form) by 2-3-fold over basal. Insulin stimulation significantly increased Akt phosphorylation and activity, whereas exercise had no effect. The time course of the insulin-stimulated increase in Akt was closely matched by GSK-3alpha Ser(21) phosphorylation and a 40-60% decrease in GSK-3alpha and GSK-3beta activity. Exercise also deactivated GSK-3alpha and beta activity by 40-60%. However, in contrast to the effects of insulin, there was no change in Ser(21) phosphorylation in response to exercise. Tyrosine dephosphorylation of GSK-3, another putative mechanism for GSK-3 deactivation, did not occur with insulin or exercise. These data suggest the following: 1) GSK-3 is constitutively active and tyrosine phosphorylated under basal conditions in skeletal muscle, 2) both exercise and insulin are effective regulators of GSK-3 activity in vivo, 3) the insulin-induced deactivation of GSK-3 occurs in response to increased Akt activity and GSK-3 serine phosphorylation, and 4) there is an Akt-independent mechanism for deactivation of GSK-3 in skeletal muscle.