S6K1 regulates GSK3 under conditions of mTOR-dependent feedback inhibition of Akt

Different interactomes for p70-S6K1 and p54-S6K2 revealed by proteomic analysis

S6Ks are major effectors of the mTOR (mammalian target of rapamycin) pathway, signaling for increased protein synthesis and cell growth in response to insulin, AMP/ATP levels, and amino acids. Deregulation of this pathway has been related to disorders and diseases associated with metabolism, such as obesity, diabetes, and cancer. S6K family is composed of two main members, S6K1 and S6K2, which comprise different isoforms resulted from alternative splicing or alternative start codon use. Although important molecular functions have been associated with p70-S6K1, the most extensively studied isoform, the S6K2 counterpart lacks information. In the present study, we performed immunoprecipitation assays followed by mass spectrometry (MS) analysis of FLAG-tagged p70-S6K1 and p54-S6K2 interactomes, after expression in HEK293 cells. Protein lists were submitted to CRAPome (Contaminant Repository for Affinity Purification) and SAINT (Significance Analysis of INTeractome) analysis, which allowed the identification of high-scoring interactions. By a comparative approach, p70-S6K1 interacting proteins were predominantly related to "cytoskeleton" and "stress response," whereas p54-S6K2 interactome was more associated to "transcription," "splicing," and "ribosome biogenesis." Moreover, we have found evidences for new targets or regulators of the S6K protein family, such as proteins NCL, NPM1, eIF2α, XRCC6, PARP1, and ILF2/ILF3 complex. This study provides new information about the interacting networks of S6Ks, which may contribute for future approaches to a better understanding of the mTOR/S6K pathway.

Activation of mTORC1 in B Lymphocytes Promotes Osteoclast Formation via Regulation of β-Catenin and RANKL/OPG

The cytokine receptor activator of nuclear factor-κB ligand (RANKL) induces osteoclast formation from monocyte/macrophage lineage cells. However, the mechanisms by which RANKL expression is controlled in cells that support osteoclast differentiation are still unclear. We show that deletion of TSC1 (tuberous sclerosis complex 1) in murine B cells causes constitutive activation of mechanistic target of rapamycin complex 1 (mTORC1) and stimulates RANKL but represses osteoprotegerin (OPG) expression and subsequently promotes osteoclast formation and causes osteoporosis in mice. Furthermore, the regulation of RANKL/OPG and stimulation of osteoclastogenesis by mTORC1 was confirmed in a variety of RANKL-expressing cells and in vivo. Mechanistically, mTORC1 controls RANKL/OPG expression through negative feedback inactivation of Akt, destabilization of β-catenin mRNA, and downregulation of β-catenin. Our findings demonstrate that mTORC1 activation-stimulated RANKL expression in B cells is sufficient to induce bone loss and osteoporosis. The study also established a link between mTORC1 and the RANKL/OPG axis via negative regulation of β-catenin. © 2016 American Society for Bone and Mineral Research.

Protein Kinase Inhibitor H89 Enhances the Activity of Pseudomonas Exotoxin A-Based Immunotoxins

HA22 (Moxetumomab pasudotox) is a recombinant immunotoxin (RIT), composed of an anti-CD22 Fv fused to a truncated portion of Pseudomonas exotoxin A. HA22 is in clinical trials to treat patients with hairy cell leukemia and acute lymphoblastic leukemia (ALL). LMB-11 is an improved variant of HA22 with reduced immunogenicity, has a longer half-life in the blood and high activity in vitro and in a Burkitt lymphoma model in vivo Searching for RIT enhancing combination therapies, we found the protein kinase A inhibitor H89 to enhance LMB-11 and HA22 activity 5- to 10-fold on ALL cell lines and on patient-derived ALL samples. In addition, H89 increased the activity of mesothelin-targeting RITs SS1P (38-fold) and RG7787 (7-fold) against the cervical cancer cell line KB31. Unexpectedly we found that the enhancement by H89 was not because of inhibition of protein kinase A; it was partially recapitulated by inhibition of S6K1, which led to inactivation of its downstream targets rpS6 and GSK3β, resulting in a fall in MCL1 levels. H89 increased the rate of ADP-ribosylation of eukaryotic elongation factor 2, enhancing the arrest of protein synthesis and the reduction of MCL1 in synergy with the RIT. In summary, H89 increased RIT activity by enhancing the two key events: ADP-ribosylation of eEF2 and reduction of MCL1 levels. Significant enhancement was seen with both CD22- and mesothelin-targeting RITs, indicating that H89 might be a potent addition to RIT treatment of CD22-positive ALL and mesothelin-expressing solid tumors. Mol Cancer Ther; 15(5); 1053-62. ©2016 AACR.

Metformin promotes tau aggregation and exacerbates abnormal behavior in a mouse model of tauopathy

Background

Alzheimer disease (AD) and other tauopathies develop cerebral intracellular inclusions of hyperphosphorylated tau. Epidemiological and experimental evidence suggests a clear link between type 2 diabetes mellitus and AD. In AD animal models, tau pathology is exacerbated by metabolic comorbidities, such as insulin resistance and diabetes. Within this context, anitidiabetic drugs, including the widely-prescribed insulin-sensitizing drug metformin, are currently being investigated for AD therapy. However, their efficacy for tauopathy in vivo has not been tested.

Results

Here, we report that in the P301S mutant human tau (P301S) transgenic mouse model of tauopathy, chronic administration of metformin exerts paradoxical effects on tau pathology. Despite reducing tau phosphorylation in the cortex and hippocampus via AMPK/mTOR and PP2A, metformin increases insoluble tau species (including tau oligomers) and the number of inclusions with β-sheet aggregates in the brain of P301S mice. In addition, metformin exacerbates hindlimb atrophy, increases P301S hyperactive behavior, induces tau cleavage by caspase 3 and disrupts synaptic structures.

Conclusions

These findings indicate that metformin pro-aggregation effects mitigate the potential benefits arising from its dephosphorylating action, possibly leading to an overall increase of the risk of tauopathy in elderly diabetic patients.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-016-0082-7) contains supplementary material, which is available to authorized users.

Tsc1 deficiency impairs mammary development in mice by suppression of AKT, nuclear ERα, and cell-cycle-driving proteins

Loss of Tsc1/Tsc2 results in excess cell growth that eventually forms hamartoma in multiple organs. Our study using a mouse model with Tsc1 conditionally knockout in mammary epithelium showed that Tsc1 deficiency impaired mammary development. Phosphorylated S6 was up-regulated in Tsc1^−/− mammary epithelium, which could be reversed by rapamycin, suggesting that mTORC1 was hyperactivated in Tsc1^−/− mammary epithelium. The mTORC1 inhibitor rapamycin restored the development of Tsc1^−/− mammary glands whereas suppressed the development of Tsc1^wt/wt mammary glands, indicating that a modest activation of mTORC1 is critical for mammary development. Phosphorylated PDK1 and AKT, nuclear ERα, nuclear IRS-1, SGK3, and cell cycle regulators such as Cyclin D1, Cyclin E, CDK2, CDK4 and their target pRB were all apparently down-regulated in Tsc1^−/− mammary glands, which could be reversed by rapamycin, suggesting that suppression of AKT by hyperactivation of mTORC1, inhibition on nuclear ERα signaling, and down-regulation of cell-cycle-driving proteins play important roles in the retarded mammary development induced by Tsc1 deletion. This study demonstrated for the first time the in vivo role of Tsc1 in pubertal mammary development of mice, and revealed that loss of Tsc1 does not necessarily lead to tissue hyperplasia.

Comparative effects of whey protein versus L-leucine on skeletal muscle protein synthesis and markers of ribosome biogenesis following resistance exercise

We compared immediate post-exercise whey protein (WP, 500 mg) versus L-leucine (LEU, 54 mg) feedings on skeletal muscle protein synthesis (MPS) mechanisms and ribosome biogenesis markers 3 h following unilateral plantarflexor resistance exercise in male, Wistar rats (~250 g). Additionally, in vitro experiments were performed on differentiated C2C12 myotubes to compare nutrient (i.e., WP, LEU) and 'exercise-like' treatments (i.e., caffeine, hydrogen peroxide, and AICAR) on ribosome biogenesis markers. LEU and WP significantly increased phosphorylated-rpS6 (Ser235/236) in the exercised (EX) leg 2.4-fold (P < 0.01) and 2.7-fold (P < 0.001) compared to the non-EX leg, respectively, whereas vehicle-fed control (CTL) did not (+65 %, P > 0.05). Compared to the non-EX leg, MPS levels increased 32 % and 52 % in the EX leg of CTL (P < 0.01) and WP rats (P < 0.001), respectively, but not in LEU rats (+15 %, P > 0.05). Several genes associated with ribosome biogenesis robustly increased in the EX versus non-EX legs of all treatments; specifically, c-Myc mRNA, Nop56 mRNA, Bop1 mRNA, Ncl mRNA, Npm1 mRNA, Fb1 mRNA, and Xpo-5 mRNA. However, only LEU significantly increased 45S pre-rRNA levels in the EX leg (63 %, P < 0.001). In vitro findings confirmed that 'exercise-like' treatments similarly altered markers of ribosome biogenesis, but only LEU increased 47S pre-rRNA levels (P < 0.01). Collectively, our data suggests that resistance exercise, as well as 'exercise-like' signals in vitro, acutely increase the expression of genes associated with ribosome biogenesis independent of nutrient provision. Moreover, while EX with or without WP appears superior for enhancing translational efficiency (i.e., increasing MPS per unit of RNA), LEU administration (or co-administration) may further enhance ribosome biogenesis over prolonged periods with resistance exercise.

da Silva AS. Downhill Running-Based Overtraining Protocol Improves Hepatic Insulin Signaling Pathway without Concomitant Decrease of Inflammatory Proteins

The purpose of this study was to verify the effects of overtraining (OT) on insulin, inflammatory and gluconeogenesis signaling pathways in the livers of mice. Rodents were divided into control (CT), overtrained by downhill running (OTR/down), overtrained by uphill running (OTR/up) and overtrained by running without inclination (OTR). Rotarod, incremental load, exhaustive and grip force tests were used to evaluate performance. Thirty-six hours after a grip force test, the livers were extracted for subsequent protein analyses. The phosphorylation of insulin receptor beta (pIRbeta), glycogen synthase kinase 3 beta (pGSK3beta) and forkhead box O1 (pFoxo1) increased in OTR/down versus CT. pGSK3beta was higher in OTR/up versus CT, and pFoxo1 was higher in OTR/up and OTR versus CT. Phosphorylation of protein kinase B (pAkt) and insulin receptor substrate 1 (pIRS–1) were higher in OTR/up versus CT and OTR/down. The phosphorylation of IκB kinase alpha and beta (pIKKalpha/beta) was higher in all OT protocols versus CT, and the phosphorylation of stress-activated protein kinases/Jun amino-terminal kinases (pSAPK-JNK) was higher in OTR/down versus CT. Protein levels of peroxisome proliferator-activated receptor-gamma coactivator 1alpha (PGC-1alpha) and hepatocyte nuclear factor 4alpha (HNF-4alpha) were higher in OTR versus CT. In summary, OTR/down improved the major proteins of insulin signaling pathway but up-regulated TRB3, an Akt inhibitor, and its association with Akt.

Activating the translational repressor 4E-BP or reducing S6K-GSK3β activity prevents accelerated axon growth induced by hyperactive mTOR in vivo

Abnormal axonal connectivity and hyperactive mTOR complex 1 (mTORC1) are shared features of several neurological disorders. Hyperactive mTORC1 alters axon length and polarity of hippocampal neurons in vitro, but the impact of hyperactive mTORC1 on axon growth in vivo and the mechanisms underlying those effects remain unclear. Using in utero electroporation during corticogenesis, we show that increasing mTORC1 activity accelerates axon growth without multiple axon formation. This was prevented by counteracting mTORC1 signaling through p70S6Ks (S6K1/2) or eukaryotic initiation factor 4E-binding protein (4E-BP1/2), which both regulate translation. In addition to regulating translational targets, S6K1 indirectly signals through GSK3β, a regulator of axogenesis. Although blocking GSK3β activity did not alter axon growth under physiological conditions in vivo, blocking it using a dominant-negative mutant or lithium chloride prevented mTORC1-induced accelerated axon growth. These data reveal the contribution of translational and non-translational downstream effectors such as GSK3β to abnormal axon growth in neurodevelopmental mTORopathies and open new therapeutic options for restoring long-range connectivity.

Phosphorylation of GSK3α/β correlates with activation of AKT and is prognostic for poor overall survival in acute myeloid leukemia patients

Background

Acute myeloid leukemia (AML) patients with highly active AKT tend to do poorly. Cell cycle arrest and apoptosis are tightly regulated by AKT via phosphorylation of GSK3α and β isoforms which inactivates these kinases. In the current study we examine the prognostic role of AKT mediated GSK3 phosphorylation in AML.

Methods

We analyzed GSK3α/β phosphorylation by reverse phase protein analysis (RPPA) in a cohort of 511 acute myeloid leukemia (AML) patients. Levels of phosphorylated GSK3 were correlated with patient characteristics including survival and with expression of other proteins important in AML cell survival.

Results

High levels of p-GSK3α/β correlated with adverse overall survival and a lower incidence of complete remission duration in patients with intermediate cytogenetics, but not in those with unfavorable cytogenetics. Intermediate cytogenetic patients with FLT3 mutation also fared better respectively when p-GSK3α/β levels were lower. Phosphorylated GSK3α/β expression was compared and contrasted with that of 229 related cell cycle arrest and/or apoptosis proteins. Consistent with p-GSK3α/β as an indicator of AKT activation, RPPA revealed that p-GSK3α/β positively correlated with phosphorylation of AKT, BAD, and P70S6K, and negatively correlated with β-catenin and FOXO3A. PKCδ also positively correlated with p-GSK3α/β expression, suggesting crosstalk between the AKT and PKC signaling pathways in AML cells.

Conclusions

These findings suggest that AKT-mediated phosphorylation of GSK3α/β may be beneficial to AML cell survival, and hence detrimental to the overall survival of AML patients. Intrinsically, p-GSK3α/β may serve as an important adverse prognostic factor for a subset of AML patients.

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Phospho-GSK3 is prognostic for poor survival in a subset of AML patients.

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Phospho-GSK3 is a biomarker for active AKT in AML.

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AKT is a PKCδ kinase in AML cells.

Regulation of mTOR Signaling by Semaphorin 3F-Neuropilin 2 Interactions In Vitro and In Vivo

Semaphorin 3F (SEMA3F) provides neuronal guidance cues via its ability to bind neuropilin 2 (NRP2) and Plexin A family molecules. Recent studies indicate that SEMA3F has biological effects in other cell types, however its mechanism(s) of function is poorly understood. Here, we analyze SEMA3F-NRP2 signaling responses in human endothelial, T cell and tumor cells using phosphokinase arrays, immunoprecipitation and Western blot analyses. Consistently, SEMA3F inhibits PI-3K and Akt activity, and responses are associated with the disruption of mTOR/rictor assembly and mTOR-dependent activation of the RhoA GTPase. We also find that the expression of vascular endothelial growth factor, as well as mTOR-inducible cellular activation responses and cytoskeleton stability are inhibited by SEMA3F-NRP2 interactions in vitro. In vivo, local and systemic overproduction of SEMA3F reduces tumor growth in NRP2-expressing xenografts. Taken together, SEMA3F regulates mTOR signaling in diverse human cell types, suggesting that it has broad therapeutic implications.

The Yeast GSK-3 Homologue Mck1 Is a Key Controller of Quiescence Entry and Chronological Lifespan

Upon starvation for glucose or any other core nutrient, yeast cells exit from the mitotic cell cycle and acquire a set of G0-specific characteristics to ensure long-term survival. It is not well understood whether or how cell cycle progression is coordinated with the acquisition of different G0-related features during the transition to stationary phase (SP). Here, we identify the yeast GSK-3 homologue Mck1 as a key regulator of G0 entry and reveal that Mck1 acts in parallel to Rim15 to activate starvation-induced gene expression, the acquisition of stress resistance, the accumulation of storage carbohydrates, the ability of early SP cells to exit from quiescence, and their chronological lifespan. FACS and microscopy imaging analyses indicate that Mck1 promotes mother-daughter cell separation and together with Rim15, modulates cell size. This indicates that the two kinases coordinate the transition-phase cell cycle, cell size and the acquisition of different G0-specific features. Epistasis experiments place MCK1, like RIM15, downstream of RAS2 in antagonising cell growth and activating stress resistance and glycogen accumulation. Remarkably, in the ras2∆ cells, deletion of MCK1 and RIM15 together, compared to removal of either of them alone, compromises respiratory growth and enhances heat tolerance and glycogen accumulation. Our data indicate that the nutrient sensor Ras2 may prevent the acquisition of G0-specific features via at least two pathways. One involves the negative regulation of the effectors of G0 entry such as Mck1 and Rim15, while the other likely to involve its functions in promoting respiratory growth, a phenotype also contributed by Mck1 and Rim15.

Glycogen synthase kinase-3 beta (GSK-3β) signaling: Implications for Parkinson's disease

Glycogen synthase kinase 3 (GSK-3) dysregulation plays an important role in the pathogenesis of numerous disorders, affecting the central nervous system (CNS) encompassing both neuroinflammation and neurodegenerative diseases. Several lines of evidence have illustrated a key role of the GSK-3 and its cellular and molecular signaling cascades in the control of neuroinflammation. Glycogen synthase kinase 3 beta (GSK-3β), one of the GSK-3 isomers, plays a major role in neuronal apoptosis and its inhibition decreases expression of alpha-Synuclein (α-Synuclein), which make this kinase an attractive therapeutic target for neurodegenerative disorders. Parkinson's disease (PD) is a chronic neurodegenerative movement disorder characterized by the progressive and massive loss of dopaminergic neurons by neuronal apoptosis in the substantia nigra pars compacta and depletion of dopamine in the striatum, which lead to pathological and clinical abnormalities. Thus, understanding the role of GSK-3β in PD will enhance our knowledge of the basic mechanisms underlying the pathogenesis of this disorder and facilitate the identification of new therapeutic avenues. In recent years, GSK-3β has been shown to play essential roles in modulating a variety of cellular functions, which have prompted efforts to develop GSK-3β inhibitors as therapeutics. In this review, we summarize GSK-3 signaling pathways and its association with neuroinflammation. Moreover, we highlight the interaction between GSK-3β and several cellular processes involved in the pathogenesis of PD, including the accumulation of α-Synuclein aggregates, oxidative stress and mitochondrial dysfunction. Finally, we discuss about GSK-3β inhibitors as a potential therapeutic strategy in PD.

4E-BPs Control Fat Storage by Regulating the Expression of Egr1 and ATGL

Early growth response transcription factor Egr1 controls multiple aspects of cell physiology and metabolism. In particular, Egr1 suppresses lipolysis and promotes fat accumulation in adipocytes by inhibiting the expression of adipose triglyceride lipase. According to current dogma, regulation of the Egr1 expression takes place primarily at the level of transcription. Correspondingly, treatment of cultured adipocytes with insulin stimulates expression of Egr1 mRNA and protein. Unexpectedly, the MEK inhibitor PD98059 completely blocks insulin-stimulated increase in the Egr1 mRNA but has only a moderate effect on the Egr1 protein. At the same time, mTORC1 inhibitors rapamycin and PP242 suppress expression of the Egr1 protein and have an opposite effect on the Egr1 mRNA. Mouse embryonic fibroblasts with genetic ablations of TSC2 or 4E-BP1/2 express less Egr1 mRNA but more Egr1 protein than wild type controls. (35)S-labeling has confirmed that translation of the Egr1 mRNA is much more effective in 4E-BP1/2-null cells than in control. A selective agonist of the CB1 receptors, ACEA, up-regulates Egr1 mRNA, but does not activate mTORC1 and does not increase Egr1 protein in adipocytes. These data suggest that although insulin activates both the Erk and the mTORC1 signaling pathways in adipocytes, regulation of the Egr1 expression takes place predominantly via the mTORC1/4E-BP-mediated axis. In confirmation of this model, we show that 4E-BP1/2-null MEFs express less ATGL and accumulate more fat than control cells, while knock down of Egr1 in 4E-BP1/2-null MEFs increases ATGL expression and decreases fat storage.

The S6K protein family in health and disease

The S6K proteins are mTOR pathway effectors and accumulative evidence suggest that mTOR/S6K signaling contributes to several pathological conditions, such as diabetes, cancer and obesity. The activation of the mTOR/S6K axis stimulates protein synthesis and cell growth. S6K1 has two well-known isoforms, p70-S6K1 and p85-S6K1, generated by alternative translation initiation sites. A third isoform, named p31-S6K1, has been characterized as a truncated type of the protein due to alternative splicing, and reports have shown its important role in cancer. Studies involving S6K2 are scarce. This article aims to review what is new in the literature about these kinases and establish differences regarding their interacting proteins, activation and function, connecting their roles in the homeostasis of the cell and in pathological conditions.

Adenovirus entry from the apical surface of polarized epithelia is facilitated by the host innate immune response

Prevention of viral-induced respiratory disease begins with an understanding of the factors that increase or decrease susceptibility to viral infection. The primary receptor for most adenoviruses is the coxsackievirus and adenovirus receptor (CAR), a cell-cell adhesion protein normally localized at the basolateral surface of polarized epithelia and involved in neutrophil transepithelial migration. Recently, an alternate isoform of CAR, CAREx8, has been identified at the apical surface of polarized airway epithelia and is implicated in viral infection from the apical surface. We hypothesized that the endogenous role of CAREx8 may be to facilitate host innate immunity. We show that IL-8, a proinflammatory cytokine and a neutrophil chemoattractant, stimulates the protein expression and apical localization of CAREx8 via activation of AKT/S6K and inhibition of GSK3β. Apical CAREx8 tethers infiltrating neutrophils at the apical surface of a polarized epithelium. Moreover, neutrophils present on the apical-epithelial surface enhance adenovirus entry into the epithelium. These findings suggest that adenovirus evolved to co-opt an innate immune response pathway that stimulates the expression of its primary receptor, apical CAREx8, to allow the initial infection the intact epithelium. In addition, CAREx8 is a new target for the development of novel therapeutics for both respiratory inflammatory disease and adenoviral infection.

GSK-3 modulates cellular responses to a broad spectrum of kinase inhibitors

A fundamental challenge in treating disease is identifying molecular states that affect cellular responses to drugs. Here, we focus on glycogen synthase kinase 3 (GSK-3), a key regulator for many of the hallmark behaviors of cancer cells. We alter GSK-3 activity in colon epithelial cells to test its role in modulating drug response. We find that GSK-3 activity broadly affects the cellular sensitivities to a panel of oncology drugs and kinase inhibitors. Specifically, inhibition of GSK-3 activity can strongly desensitize or sensitize cells to kinase inhibitors (for example, mTOR or PLK1 inhibitors, respectively). Additionally, colorectal cancer cell lines, in which GSK-3 function is commonly suppressed, are resistant to mTOR inhibitors and yet highly sensitive to PLK1 inhibitors, and this is further exacerbated by additional GSK-3 inhibition. Finally, by conducting a kinome-wide RNAi screen, we find that GSK-3 modulates the cell proliferative phenotype of a large fraction (∼35%) of the kinome, which includes ∼50% of current, clinically relevant kinase-targeted drugs. Our results highlight an underappreciated interplay of GSK-3 with therapeutically important kinases and suggest strategies for identifying disease-specific molecular profiles that can guide optimal selection of drug treatment.

The mTORC1 effectors S6K1 and 4E-BP play different roles in CNS axon regeneration

Using mouse optic nerve (ON) crush as a CNS injury model, we and others have found that activation of the mammalian target of rapamycin complex 1 (mTORC1) in mature retinal ganglion cells by deletion of the negative regulators, phosphatase and tensin homolog (PTEN) and tuberous sclerosis 1, promotes ON regeneration. mTORC1 activation inhibits eukaryotic translation initiation factor 4E-binding protein (4E-BP) and activates ribosomal protein S6 kinase 1 (S6K1), both of which stimulate translation. We reasoned that mTORC1’s regeneration-promoting effects might be separable from its deleterious effects by differential manipulation of its downstream effectors. Here we show that S6K1 activation, but not 4E-BP inhibition, is sufficient to promote axon regeneration. However, inhibition of 4E-BP is required for PTEN deletion-induced axon regeneration. Both activation and inhibition of S6K1 decrease the effect of PTEN deletion on axon regeneration, implicating a dual role of S6K1 in regulating axon growth.

Macrophage mTORC1 disruption reduces inflammation and insulin resistance in obese mice

AIMS/HYPOTHESIS

Inflammatory factors secreted by macrophages play an important role in obesity-related insulin resistance. Being at the crossroads of a nutrient-hormonal signalling network, the mammalian target of rapamycin complex 1 (mTORC1) controls important functions in the regulation of energy balance and peripheral metabolism. However, the role of macrophage mTORC1 in insulin resistance is still unclear. In the current study, we investigated the physiological role of macrophage mTORC1 in regulating inflammation and insulin sensitivity.

METHODS

We generated mice deficient in the regulatory associated protein of mTOR (Raptor) in macrophages, by crossing Raptor (also known as Rptor) floxed mice (Raptor (flox/flox)) with mice expressing Cre recombinase under the control of the Lysm-Cre promoter (Mac-Raptor (KO)). We fed mice chow or high-fat diet (HFD) and assessed insulin sensitivity in liver, muscle and adipose tissue. Subsequently, we measured inflammatory gene expression in liver and adipose tissue and investigated the role of Raptor deficiency in the regulation of inflammatory responses in peritoneal macrophages from HFD-fed mice or in palmitic acid-stimulated bone marrow-derived macrophages (BMDMs).

RESULTS

Mac-Raptor (KO) mice fed HFD had improved systemic insulin sensitivity compared with Raptor (flox/flox) mice. Macrophage Raptor deficiency reduced inflammatory gene expression in liver and adipose tissue, fatty liver and adipose tissue macrophage content in response to HFD. In peritoneal macrophages from mice fed with an HFD for 12 weeks, macrophage Raptor deficiency decreased inflammatory gene expression, through attenuation of the inactivation of Akt and subsequent inhibition of the inositol-requiring element 1α/clun NH2-terminal kinase-nuclear factor kappa-light-chain-enhancer of activated B cells (IRE1α/JNK/NFκB) pathways. Similarly, mTOR inhibition as a result of Raptor deficiency or rapamycin treatment decreased palmitic acid-induced inflammatory gene expression in BMDMs in vitro.

CONCLUSIONS/INTERPRETATION

The disruption of mTORC1 signalling in macrophages protects mice against inflammation and insulin resistance potentially by inhibiting HFD- and palmitic acid-induced IRE1α/JNK/NFκB pathway activation.

Activation of glycogen synthase kinase 3β ameliorates diabetes-induced kidney injury

Increase in protein synthesis contributes to kidney hypertrophy and matrix protein accumulation in diabetes. We have previously shown that high glucose-induced matrix protein synthesis is associated with inactivation of glycogen synthase kinase 3β (GSK3β) in renal cells and in the kidneys of diabetic mice. We tested whether activation of GSK3β by sodium nitroprusside (SNP) mitigates kidney injury in diabetes. Studies in kidney-proximal tubular epithelial cells showed that SNP abrogated high glucose-induced laminin increment by stimulating GSK3β and inhibiting Akt, mTORC1, and events in mRNA translation regulated by mTORC1 and ERK. NONOate, an NO donor, also activated GSK3β, indicating that NO may mediate SNP stimulation of GSK3β. SNP administered for 3 weeks to mice with streptozotocin-induced type 1 diabetes ameliorated kidney hypertrophy, accumulation of matrix proteins, and albuminuria without changing blood glucose levels. Signaling studies showed that diabetes caused inactivation of GSK3β by activation of Src, Pyk2, Akt, and ERK; GSK3β inhibition activated mTORC1 and downstream events in mRNA translation in the kidney cortex. These reactions were abrogated by SNP. We conclude that activation of GSK3β by SNP ameliorates kidney injury induced by diabetes.

The mTORC1/S6K1 pathway regulates glutamine metabolism through the eIF4B-dependent control of c-Myc translation

Growth-promoting signaling molecules, including the mammalian target of rapamycin complex 1 (mTORC1), drive the metabolic reprogramming of cancer cells required to support their biosynthetic needs for rapid growth and proliferation. Glutamine is catabolyzed to α-ketoglutarate (αKG), a tricarboxylic acid (TCA) cycle intermediate, through two deamination reactions, the first requiring glutaminase (GLS) to generate glutamate and the second occurring via glutamate dehydrogenase (GDH) or transaminases. Activation of the mTORC1 pathway has been shown previously to promote the anaplerotic entry of glutamine to the TCA cycle via GDH. Moreover, mTORC1 activation also stimulates the uptake of glutamine, but the mechanism is unknown. It is generally thought that rates of glutamine utilization are limited by mitochondrial uptake via GLS, suggesting that, in addition to GDH, mTORC1 could regulate GLS. Here we demonstrate that mTORC1 positively regulates GLS and glutamine flux through this enzyme. We show that mTORC1 controls GLS levels through the S6K1-dependent regulation of c-Myc (Myc). Molecularly, S6K1 enhances Myc translation efficiency by modulating the phosphorylation of eukaryotic initiation factor eIF4B, which is critical to unwind its structured 5' untranslated region (5'UTR). Finally, our data show that the pharmacological inhibition of GLS is a promising target in pancreatic cancers expressing low levels of PTEN.

Glycogen synthase kinase 3 regulates cell death and survival signaling in tumor cells under redox stress

Targeting tumor-specific metabolic adaptations is a promising anticancer strategy when tumor defense mechanisms are restrained. Here, we show that redox-modulating drugs including the retinoid N-(4-hydroxyphenyl)retinamide (4HPR), the synthetic triterpenoid bardoxolone (2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oic acid methyl ester), arsenic trioxide (As2O3), and phenylethyl isothiocyanate (PEITC), while affecting tumor cell viability, induce sustained Ser9 phosphorylation of the multifunctional kinase glycogen synthase kinase 3β (GSK3β). The antioxidant N-acetylcysteine decreased GSK3β phosphorylation and poly(ADP-ribose) polymerase cleavage induced by 4HPR, As2O3, and PEITC, implicating oxidative stress in these effects. GSK3β phosphorylation was associated with up-regulation of antioxidant enzymes, in particular heme oxygenase-1 (HO-1), and transient elevation of intracellular glutathione (GSH) in cells surviving acute stress, before occurrence of irreversible damage and death. Genetic inactivation of GSK3β or transfection with the non-phosphorylatable GSK3β-S9A mutant inhibited HO-1 induction under redox stress, while tumor cells resistant to 4HPR exhibited increased GSK3β phosphorylation, HO-1 expression, and GSH levels. The above-listed findings are consistent with a role for sustained GSK3β phosphorylation in a signaling network activating antioxidant effector mechanisms during oxidoreductive stress. These data underlie the importance of combination regimens of antitumor redox drugs with inhibitors of survival signaling to improve control of tumor development and progression and overcome chemoresistance.

The serine-threonine kinase p90RSK is a new target of enzastaurin in follicular lymphoma cells

BACKGROUND AND PURPOSE

Follicular lymphoma is the second most common non-Hodgkin's lymphoma and, despite the introduction of rituximab for its treatment, this disease is still considered incurable. Besides genetic alterations involving Bcl-2, Bcl-6 or c-Myc, follicular lymphoma cells often display altered B-cell receptor signalling pathways including overactive PKC and PI3K/Akt systems.

EXPERIMENTAL APPROACH

The effect of enzastaurin, an inhibitor of PKC, was evaluated both in vitro on follicular lymphoma cell lines and in vivo on a xenograft murine model. Using pharmacological inhibitors and siRNA transfection, we determined the different signalling pathways after enzastaurin treatment.

KEY RESULTS

Enzastaurin inhibited the serine-threonine kinase p90RSK which has downstream effects on GSK3β. Bad and p70S6K. These signalling proteins control follicular lymphoma cell survival and apoptosis; which accounted for the inhibition by enzastaurin of cell survival and its induction of apoptosis of follicular lymphoma cell lines in vitro. Importantly, these results were replicated in vivo where enzastaurin inhibited the growth of follicular lymphoma xenografts in mice.

CONCLUSIONS AND IMPLICATIONS

The targeting of p90RSK by enzastaurin represents a new therapeutic option for the treatment of follicular lymphoma.

Phytohormone regulation of legume-rhizobia interactions

The symbiosis between legumes and nitrogen fixing bacteria called rhizobia leads to the formation of root nodules. Nodules are highly organized root organs that form in response to Nod factors produced by rhizobia, and they provide rhizobia with a specialized niche to optimize nutrient exchange and nitrogen fixation. Nodule development and invasion by rhizobia is locally controlled by feedback between rhizobia and the plant host. In addition, the total number of nodules on a root system is controlled by a systemic mechanism termed 'autoregulation of nodulation'. Both the local and the systemic control of nodulation are regulated by phytohormones. There are two mechanisms by which phytohormone signalling is altered during nodulation: through direct synthesis by rhizobia and through indirect manipulation of the phytohormone balance in the plant, triggered by bacterial Nod factors. Recent genetic and physiological evidence points to a crucial role of Nod factor-induced changes in the host phytohormone balance as a prerequisite for successful nodule formation. Phytohormones synthesized by rhizobia enhance symbiosis effectiveness but do not appear to be necessary for nodule formation. This review provides an overview of recent advances in our understanding of the roles and interactions of phytohormones and signalling peptides in the regulation of nodule infection, initiation, positioning, development, and autoregulation. Future challenges remain to unify hormone-related findings across different legumes and to test whether hormone perception, response, or transport differences among different legumes could explain the variety of nodules types and the predisposition for nodule formation in this plant family. In addition, the molecular studies carried out under controlled conditions will need to be extended into the field to test whether and how phytohormone contributions by host and rhizobial partners affect the long term fitness of the host and the survival and competition of rhizobia in the soil. It also will be interesting to explore the interaction of hormonal signalling pathways between rhizobia and plant pathogens.

The TSC-mTOR pathway regulates macrophage polarization

Macrophages are able to polarize to proinflammatory M1 or alternative M2 states with distinct phenotypes and physiological functions. How metabolic status regulates macrophage polarization remains not well understood, and here we examine the role of mTOR (Mechanistic Target of Rapamycin), a central metabolic pathway that couples nutrient sensing to regulation of metabolic processes. Using a mouse model in which myeloid lineage specific deletion of Tsc1 (Tsc1^Δ/Δ) leads to constitutive mTOR Complex 1 (mTORC1) activation, we find that Tsc1^Δ/Δ macrophages are refractory to IL-4 induced M2 polarization, but produce increased inflammatory responses to proinflammatory stimuli. Moreover, mTORC1-mediated downregulation of Akt signaling critically contributes to defective polarization. These findings highlight a key role for the mTOR pathway in regulating macrophage polarization, and suggest how nutrient sensing and metabolic status could be “hard-wired” to control of macrophage function, with broad implications for regulation of Type 2 immunity, inflammation, and allergy.

Berberine-induced apoptotic and autophagic death of HepG2 cells requires AMPK activation

Background

Hepatocellular carcinoma (HCC), the primary liver cancer, is one of the most malignant human tumors with extremely poor prognosis. The aim of this study was to investigate the anti-cancer effect of berberine in a human hepatocellular carcinoma cell line (HepG2), and to study the underlying mechanisms by focusing on the AMP-activated protein kinase (AMPK) signaling cascade.

Results

We found that berberine induced both apoptotic and autophagic death of HepG2 cells, which was associated with a significant activation of AMPK and an increased expression of the inactive form of acetyl-CoA carboxylase (ACC). Inhibition of AMPK by RNA interference (RNAi) or by its inhibitor compound C suppressed berberine-induced caspase-3 cleavage, apoptosis and autophagy in HepG2 cells, while AICAR, the AMPK activator, possessed strong cytotoxic effects. In HepG2 cells, mammalian target of rapamycin complex 1 (mTORC1) activation was important for cell survival, and berberine inhibited mTORC1 via AMPK activation.

Conclusions

Together, these results suggested that berberine-induced both apoptotic and autophagic death requires AMPK activation in HepG2 cells.

GSK-3 – at the crossroads of cell death and survival

Glycogen synthase kinase 3 (GSK-3) is involved in various signaling pathways controlling metabolism, differentiation and immunity, as well as cell death and survival. GSK-3 targets transcription factors, regulates the activity of metabolic and signaling enzymes, and controls the half-life of proteins by earmarking them for degradation. GSK-3 is unique in its mode of substrate recognition and the regulation of its kinase activity, which is repressed by pro-survival phosphoinositide 3-kinase (PI3K)–AKT signaling. In turn, GSK-3 exhibits pro-apoptotic functions when the PI3K–AKT pathway is inactive. Nevertheless, as GSK-3 is crucially involved in many signaling pathways, its role in cell death regulation is not uniform, and in some situations it promotes cell survival. In this Commentary, we focus on the various aspects of GSK-3 in the regulation of cell death and survival. We discuss the effects of GSK-3 on the regulation of proteins of the BCL-2 family, through which GSK-3 exhibits pro-apoptotic activity. We also highlight the pro-survival activities of GSK-3, which are observed in the context of nuclear factor κB (NFκB) signaling, and we discuss how GSK-3, by impacting on cell death and survival, might play a role in diseases such as cancer.

Innate PI3K p110δ regulates Th1/Th17 development and microbiota-dependent colitis

The p110δ subunit of class IA PI3K modulates signaling in innate immune cells. We previously demonstrated that mice harboring a kinase-dead p110δ subunit (p110δ(KD)) develop spontaneous colitis. Macrophages contributed to the Th1/Th17 cytokine bias in p110δ(KD) mice through increased IL-12 and IL-23 expression. In this study, we show that the enteric microbiota is required for colitis development in germfree p110δ(KD) mice. Colonic tissue and macrophages from p110δ(KD) mice produce significantly less IL-10 compared with wild-type mice. p110δ(KD) APCs cocultured with naive CD4+ Ag-specific T cells also produce significantly less IL-10 and induce more IFN-γ- and IL-17A-producing CD4+ T cells compared with wild-type APCs. Illustrating the importance of APC-T cell interactions in colitis pathogenesis in vivo, Rag1(-/-)/p110δ(KD) mice develop mild colonic inflammation and produced more colonic IL-12p40 compared with Rag1(-/-) mice. However, CD4+ CD45RB(high/low) T cell Rag1(-/-)/p110δ(KD) recipient mice develop severe colitis with increased percentages of IFN-γ- and IL-17A-producing lamina propria CD3+D4+ T cells compared with Rag1(-/-) recipient mice. Intestinal tissue samples from patients with Crohn's disease reveal significantly lower expression of PIK3CD compared with intestinal samples from non-inflammatory bowel disease control subjects (p < 0.05). PIK3CD expression inversely correlates with the ratio of IL12B:IL10 expression. In conclusion, the PI3K subunit p110δ controls homeostatic APC-T cell interactions by altering the balance between IL-10 and IL-12/23. Defects in p110δ expression and/or function may underlie the pathogenesis of human inflammatory bowel disease and lead to new therapeutic strategies.

Regulation of Wnt/β-catenin signaling by posttranslational modifications

The canonical Wnt signaling pathway (or Wnt/β-catenin pathway) plays a pivotal role in embryonic development and adult homeostasis; deregulation of the Wnt pathway contributes to the initiation and progression of human diseases including cancer. Despite its importance in human biology and disease, how regulation of the Wnt/β-catenin pathway is achieved remains largely undefined. Increasing evidence suggests that post-translational modifications (PTMs) of Wnt pathway components are essential for the activation of the Wnt/β-catenin pathway. PTMs create a highly dynamic relay system that responds to Wnt stimulation without requiring de novo protein synthesis and offer a platform for non-Wnt pathway components to be involved in the regulation of Wnt signaling, hence providing alternative opportunities for targeting the Wnt pathway. This review highlights the current status of PTM-mediated regulation of the Wnt/β-catenin pathway with a focus on factors involved in Wnt-mediated stabilization of β-catenin.

Tsc2 is a molecular checkpoint controlling osteoblast development and glucose homeostasis

Insulin signaling in osteoblasts regulates global energy balance by stimulating the production of osteocalcin, a bone-derived protein that promotes insulin production and action. To identify the signaling pathways in osteoblasts that mediate insulin's effects on bone and energy metabolism, we examined the function of the tuberous sclerosis 2 (Tsc2) protein, a key target important in coordinating nutrient signaling. Here, we show that loss of Tsc2 in osteoblasts constitutively activates mTOR and destabilizes Irs1, causing osteoblasts to differentiate poorly and become resistant to insulin. Young Tsc2 mutant mice demonstrate hypoglycemia with increased levels of insulin and undercarboxylated osteocalcin. However, with age, Tsc2 mutants develop metabolic features similar to mice lacking the insulin receptor in the osteoblast, including peripheral adiposity, hyperglycemia, and decreased pancreatic β cell mass. These metabolic abnormalities appear to result from chronic elevations in undercarboxylated osteocalcin that lead to downregulation of the osteocalcin receptor and desensitization of the β cell to this hormone. Removal of a single mTOR allele from the Tsc2 mutant mice largely normalizes the bone and metabolic abnormalities. Together, these findings suggest that Tsc2 serves as a key checkpoint in the osteoblast that is required for proper insulin signaling and acts to ensure normal bone acquisition and energy homeostasis.

GSK3β, but not GSK3α, inhibits the neuronal differentiation of neural progenitor cells as a downstream target of mammalian target of rapamycin complex1

Glycogen synthase kinase 3 (GSK3) acts as an important regulator during the proliferation and differentiation of neural progenitor cells (NPCs), but the roles of the isoforms of this molecule (GSK3α and GSK3β) have not been clearly defined. In this study, we investigated the functions of GSK3α and GSK3β in the context of neuronal differentiation of murine NPCs. Treatment of primary NPCs with a GSK3 inhibitor (SB216763) resulted in an increase in the percentage of TuJ1-positive immature neurons, suggesting an inhibitory role of GSK3 in embryonic neurogenesis. Downregulation of GSK3β expression increased the percentage of TuJ1-positive cells, while knock-down of GSK3α seemed to have no effect. When primary NPCs were engineered to stably express either isoform of GSK3 using retroviral vectors, GSK3β, but not GSK3α, inhibited neuronal differentiation and helped the cells to maintain the characteristics of NPCs. Mutant GSK3β (Y216F) failed to suppress neuronal differentiation, indicating that the kinase activity of GSK3β is important for this regulatory function. Similar results were obtained in vivo when a retroviral vector expressing GSK3β was delivered to E9.5 mouse brains using the ultrasound image-guided gene delivery technique. In addition, SB216763 was found to block the rapamycin-mediated inhibition of neuronal differentiation of NPCs. Taken together, our results demonstrate that GSK3β, but not GSK3α, negatively controls the neuronal differentiation of progenitor cells and that GSK3β may act downstream of the mammalian target of rapamycin complex1 signaling pathway.

Suppression of the mTORC1/STAT3/Notch1 pathway by activated AMPK prevents hepatic insulin resistance induced by excess amino acids

Nutrient overload is associated with the development of obesity, insulin resistance, and type 2 diabetes. However, the underlying mechanisms for developing insulin resistance in the presence of excess nutrients are incompletely understood. We investigated whether activation of AMP-activated protein kinase (AMPK) prevents the hepatic insulin resistance that is induced by the consumption of a high-protein diet (HPD) and the presence of excess amino acids. Exposure of HepG2 cells to excess amino acids reduced AMPK phosphorylation, upregulated Notch1 expression, and impaired the insulin-stimulated phosphorylation of Akt Ser(473) and insulin receptor substrate-1 (IRS-1) Tyr(612). Inhibition of Notch1 prevented amino acid-induced insulin resistance, which was accompanied by reduced expression of Rbp-Jk, hairy and enhancer of split-1, and forkhead box O1. Mechanistically, mTORC1 signaling was activated by excess amino acids, which then positively regulated Notch1 expression through the activation of the signal transducer and activator of transcription 3 (STAT3). Activation of AMPK by metformin inhibited mTORC1-STAT3 signaling, thereby preventing excess amino acid-impaired insulin signaling. Finally, HPD feeding suppressed AMPK activity, activated mTORC1/STAT3/Notch1 signaling, and induced insulin resistance. Chronic administration of either metformin or rapamycin inhibited the HPD-activated mTORC1/STAT3/Notch1 signaling pathway and prevented hepatic insulin resistance. We conclude that the upregulation of Notch1 expression by hyperactive mTORC1 signaling is an essential event in the development of hepatic insulin resistance in the presence of excess amino acids. Activation of AMPK prevents amino acid-induced insulin resistance through the suppression of the mTORC1/STAT3/Notch1 signaling pathway.

Rapamycin ameliorates inflammation and fibrosis in the early phase of cirrhotic portal hypertension in rats through inhibition of mTORC1 but not mTORC2

Objective

Hepatic stellate cells (HSCs) transdifferentiation and subsequent inflammation are important pathological processes involved in the formation of cirrhotic portal hypertension. This study characterizes the pathogenetic mechanisms leading to cholestatic liver fibrosis and portal hypertension, and focuses on mammalian target of rapamycin (mTOR) pathway as a potential modulator in the early phase of cirrhotic portal hypertension.

Methods

Early cirrhotic portal hypertension was induced by bile duct ligation (BDL) for three weeks. One week after operation, sham-operated (SHAM) and BDL rats received rapamycin (2 mg/kg/day) by intraperitoneal injection for fourteen days. Vehicle-treated SHAM and BDL rats served as controls. Fibrosis, inflammation, and portal pressure were evaluated by histology, morphometry, and hemodynamics. Expressions of pro-fibrogenic and pro-inflammatory genes in liver were measured by RT-PCR; alpha smooth muscle actin (α-SMA) and antigen Ki67 were detected by immunohistochemistry; expressions of AKT/mTOR signaling molecules, extracellular-signal-regulated kinase 1/2 (ERK1/2), p-ERK1/2, and interleukin-1 beta (IL-1β) were assessed by western blot.

Results

The AKT/mTOR signaling pathway was markedly activated in the early phase of cirrhotic portal hypertension induced by BDL in rats. mTOR blockade by rapamycin profoundly improved liver function by limiting inflammation, fibrosis and portal pressure. Rapamycin significantly inhibited the expressions of phosphorylated 70KD ribosomal protein S6 kinase (p-P70S6K) and phosphorylated ribosomal protein S6 (p-S6) but not p-AKT Ser473 relative to their total proteins in BDL-Ra rats. Those results suggested that mTOR Complex 1 (mTORC1) rather than mTORC2 was inhibited by rapamycin. Interestingly, we also found that the level of p-ERK1/2 to ERK1/2 was significantly increased in BDL rats, which was little affected by rapamycin.

Conclusions

The AKT/mTOR signaling pathway played an important role in the early phase of cirrhotic portal hypertension in rats, which could be a potential target for therapeutic intervention in the early phase of such pathophysiological progress.

NCYM, a Cis-antisense gene of MYCN, encodes a de novo evolved protein that inhibits GSK3β resulting in the stabilization of MYCN in human neuroblastomas

The rearrangement of pre-existing genes has long been thought of as the major mode of new gene generation. Recently, de novo gene birth from non-genic DNA was found to be an alternative mechanism to generate novel protein-coding genes. However, its functional role in human disease remains largely unknown. Here we show that NCYM, a cis-antisense gene of the MYCN oncogene, initially thought to be a large non-coding RNA, encodes a de novo evolved protein regulating the pathogenesis of human cancers, particularly neuroblastoma. The NCYM gene is evolutionally conserved only in the taxonomic group containing humans and chimpanzees. In primary human neuroblastomas, NCYM is 100% co-amplified and co-expressed with MYCN, and NCYM mRNA expression is associated with poor clinical outcome. MYCN directly transactivates both NCYM and MYCN mRNA, whereas NCYM stabilizes MYCN protein by inhibiting the activity of GSK3β, a kinase that promotes MYCN degradation. In contrast to MYCN transgenic mice, neuroblastomas in MYCN/NCYM double transgenic mice were frequently accompanied by distant metastases, behavior reminiscent of human neuroblastomas with MYCN amplification. The NCYM protein also interacts with GSK3β, thereby stabilizing the MYCN protein in the tumors of the MYCN/NCYM double transgenic mice. Thus, these results suggest that GSK3β inhibition by NCYM stabilizes the MYCN protein both in vitro and in vivo. Furthermore, the survival of MYCN transgenic mice bearing neuroblastoma was improved by treatment with NVP-BEZ235, a dual PI3K/mTOR inhibitor shown to destabilize MYCN via GSK3β activation. In contrast, tumors caused in MYCN/NCYM double transgenic mice showed chemo-resistance to the drug. Collectively, our results show that NCYM is the first de novo evolved protein known to act as an oncopromoting factor in human cancer, and suggest that de novo evolved proteins may functionally characterize human disease.

The MYCN oncogene has a central role in the genesis and progression of neuroblastomas, and its amplification is associated with an unfavorable prognosis. We have found that NCYM, a MYCN cis-antisense RNA, is translated in humans to a de novo evolved protein. NCYM inhibits GSK3β to stabilize MYCN, whereas MYCN induces NCYM transcription. The positive feedback regulation formed in the MYCN/NCYM-amplified tumors promotes the aggressive nature of human neuroblastoma. MYCN transgenic mice, which express human MYCN in sympathoadrenal tissues, spontaneously develop neuroblastomas. However, unlike human neuroblastoma, distant metastasis is infrequent. We established MYCN/NCYM double transgenic mice as a new animal model for studying neuroblastoma pathogenesis. We found that NCYM expression promoted both the metastasis and chemo-resistance of the neuroblastomas formed in the double transgenic mice. These results demonstrate that NCYM may be a potential target for developing novel therapeutic tools against high-risk neuroblastomas in humans, and that the MYCN/NCYM double transgenic mouse may be a suitable model for the screening of these new drugs.

O-GlcNAc: A Bittersweet Switch in Liver

The liver is a vital organ responsible for maintaining nutrient homeostasis. After a meal, insulin stimulates glycogen and lipid synthesis in the liver; in the fasted state, glucagon induces gluconeogenesis and ketogenesis, which produce glucose and ketone bodies for other tissues to use as energy sources. These metabolic changes involve spatiotemporally co-ordinated signaling cascades. O-linked β-N-acetylglucosamine (O-GlcNAc) modification has been recognized as a nutrient sensor and regulatory molecular switch. This review highlights mechanistic insights into spatiotemporal regulation of liver metabolism by O-GlcNAc modification and discusses its pathophysiological implications in insulin resistance, non-alcoholic fatty liver disease, and fibrosis.

Akt mediates GSK-3β phosphorylation in the rat prefrontal cortex during the process of ketamine exerting rapid antidepressant actions

Ketamine may produce rapid and sustained antidepressant effects. Despite the fact that the detailed underlying mechanism remains unknown, recent studies have suggested the involvement of the mammalian target of rapamycin (mTOR) pathway and glycogen synthase kinase-3 (GSK-3) signal, respectively, in the process of ketamine exerting antidepressant actions. This study aimed to investigate the mechanism by which ketamine phosphorylates GSK-3β in the rat prefrontal cortex (PFC) via applying vehicle or the antagonists of mTOR signalling pathway proteins including PI3K/Akt, mTOR and p70S6 kinase to the rats in the forced swimming test (FST) prior to ketamine administration, and subsequently observing the levels of phosphorylated GSK-3β, mTOR and p70S6K in rat PFC as well as the immobility time of rats in the FST. Our results revealed that compared to treatment with vehicle, ketamine increased the levels of phosphorylated GSK-3β in rat PFC (p < 0.05), which was attenuated by PI3K/Akt antagonist pretreatment (p < 0.05), but could not be affected by mTOR antagonist or p70S6K antagonist pretreatment. In addition, all the antagonists reversed the ketamine-induced increases in the phosphorylation of mTOR and p70S6K (p < 0.05). They also all abolished the rapid-acting antidepressant actions of ketamine demonstrated by the increased immobility time of rats in the FST. In conclusion, Akt mediates the phosphorylation of GSK-3β in rat PFC during the process of ketamine exerting rapid antidepressant actions.

TORC1 inhibits GSK3-mediated Elo2 phosphorylation to regulate very long chain fatty acid synthesis and autophagy

Very long chain fatty acids (VLCFAs) are essential fatty acids with multiple functions, including ceramide synthesis. Although the components of the VLCFA biosynthetic machinery have been elucidated, how their activity is regulated to meet the cell's metabolic demand remains unknown. The goal of this study was to identify mechanisms that regulate the rate of VLCFA synthesis, and we discovered that the fatty acid elongase Elo2 is regulated by phosphorylation. Elo2 phosphorylation is induced upon inhibition of TORC1 and requires GSK3. Expression of nonphosphorylatable Elo2 profoundly alters the ceramide spectrum, reflecting aberrant VLCFA synthesis. Furthermore, VLCFA depletion results in constitutive activation of autophagy, which requires sphingoid base phosphorylation. This constitutive activation of autophagy diminishes cell survival, indicating that VLCFAs serve to dampen the amplitude of autophagy. Together, our data reveal a function for TORC1 and GSK3 in the regulation of VLCFA synthesis that has important implications for autophagy and cell homeostasis.

Impact of mTORC1 inhibition on keratinocyte proliferation during skin tumor promotion in wild-type and BK5.AktWT mice

In this study, we examined the impact of rapamycin on mTORC1 signaling during 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced keratinocyte proliferation and skin tumor promotion in both wild-type (FVB/N) and BK5.Akt(WT) mice. TPA activated mTORC1 signaling in a time-dependent manner in cultured primary mouse keratinocytes and a mouse keratinocyte cell line. Early activation (15-30 min) of mTORC1 signaling induced by TPA was mediated in part by PKC activation, whereas later activation (2-4 h) was mediated by activation of EGFR and Akt. BK5.Akt(WT) transgenic mice, where Akt1 is overexpressed in basal epidermis, are highly sensitive to TPA-induced epidermal proliferation and two-stage skin carcinogenesis. Targeting mTORC1 with rapamycin effectively inhibited TPA-induced epidermal hyperplasia and hyperproliferation as well as tumor promotion in a dose-dependent manner in both wild-type and BK5.Akt(WT) mice. A significant expansion (∼threefold) of the label retaining cell (LRC) population per hair follicle was observed in BK5.Akt(WT) mice compared to FVB/N mice. There was also a significant increase in K15 expressing cells in the hair follicle of transgenic mice that coincided with expression of phospho-Akt, phospho-S6K, and phospho-PRAS40, suggesting an important role of mTORC1 signaling in bulge-region keratinocyte stem cell (KSC) homeostasis. After 2 weeks of TPA treatment, LRCs had moved upward into the interfollicular epidermis from the bulge region of both wild-type and BK5.Akt(WT) mice. TPA-mediated LRC proliferation and migration was significantly inhibited by rapamycin. Collectively, the current data indicate that signaling through mTORC1 contributes significantly to the process of skin tumor promotion through effects on proliferation of the target cells for tumor development.

AMPKα1 deficiency amplifies proinflammatory myeloid APC activity and CD40 signaling

AMPK is a serine/threonine kinase that regulates energy homeostasis and metabolic stress in eukaryotes. Previous work from our laboratory, as well as by others, has provided evidence that AMPKα1 acts as a negative regulator of TLR-induced inflammatory function. Herein, we demonstrate that AMPKα1-deficient macrophages and DCs exhibit heightened inflammatory function and an enhanced capacity for antigen presentation favoring the promotion of Th1 and Th17 responses. Macrophages and DCs generated from AMPKα1-deficient mice produced higher levels of proinflammatory cytokines and decreased production of the anti-inflammatory cytokine IL-10 in response to TLR and CD40 stimulation as compared with WT cells. In assays of antigen presentation, AMPKα1 deficiency in the myeloid APC and T cell populations contributed to enhanced IL-17 and IFN-γ production. Focusing on the CD154-CD40 interaction, we found that CD40 stimulation resulted in increased phosphorylation of ERK1/2, p38, and NF-κB p65 and decreased activation of the anti-inflammatory Akt -GSK3β-CREB pathway in DCs deficient for AMPKα1. Our data demonstrate that AMPKα1 serves to attenuate LPS and CD40-mediated proinflammatory activity of myeloid APCs and that AMPKα1 activity in both APC and T cells contributes to T cell functional polarization during antigen presentation.

The Paradox of Akt-mTOR Interactions

The serine threonine protein kinase, Akt, is at the central hub of signaling pathways that regulates cell growth, differentiation, and survival. The reciprocal relation that exists between the two activating phosphorylation sites of Akt, T308 and S473, and the two mTOR complexes, C1 and C2, forms the central controlling hub that regulates these cellular functions. In our previous review “PI3Kinase (PI3K)-AKT-mTOR and Wnt signaling pathways in cell cycle” we discussed the reciprocal relation between mTORC1 and C2 complexes in regulating cell metabolism and cell cycle progression in cancer cells. We present in this article, a hypothesis that activation of Akt-T308 phosphorylation in the presence of high ATP:AMP ratio promotes the stability of its phosphorylations and activates mTORC1 and the energy consuming biosynthetic processes. Depletion of energy leads to inactivation of mTORC1, activation of AMPK, FoxO, and promotes constitution of mTORC2 that leads to phosphorylation of Akt S473. Akt can also be activated independent of PI3K; this appears to have an advantage under situations like dietary restrictions, where insulin/insulin growth factor signaling could be a casualty.

AKT-ing out: SGK kinases come to the fore

The success of targeted therapies in treating cancer over the last decade has been tempered by acquired drug resistance that follows long-term treatment. There is also emerging evidence for innate mechanisms of cancer cell resistance to targeted therapy that pre-exist as parallel signalling pathways. This aspect is explored by the Alessi group and collaborators from AstraZeneca in this issue of the Biochemical Journal, who identify a subset of breast cancer cell lines that are intrinsically resistant to Akt inhibition through constitutive up-regulation of the related AGC serine/threonine kinase SGK1 (serum- and glucocorticoid-regulated kinase 1). The study could help to profile tumours for sensitivity to Akt inhibitors and once more highlights the therapeutic complexity of cancer and the importance of exploring combination therapies in the clinic.

High expression of PP2A-Aα is associated with diapause induction during the photoperiod-sensitive stage of the cotton bollworm, Helicoverpa armigera

Protein phosphatase 2A (PP2A) is a major serine-threonine protein phosphatase which regulates metabolism, transcription, RNA splicing, translation, differentiation, cell cycle, oncogenic transformation and signal transduction. PP2A-Aα, an isoform of PP2A-A, is a structural subunit of the PP2A complex. We identified the photoperiod-sensitive stage for pupal diapause induction to be from the fifth instar to the early sixth instar larvae in the cotton bollworm, Helicoverpa armigera. PP2A-Aα cDNA from brains of diapause-destined fifth instar larvae was obtained by suppressive subtractive hybridization using nondiapause-destined larval brains as a control. Developmental expression of PP2A-Aα mRNA during the photoperiod-sensitive stage was higher in brains of diapause-destined larvae, and the PP2A-Aα protein showed a similar expression pattern as the mRNA. When larvae were transferred from diapause-inducing short days to long days during the diapause-sensitive stage, both PP2A-Aα mRNA and protein decreased significantly, and diapause incidence was also reduced. Thus, high PP2A-Aα expression during the diapause-sensitive stage may play a crucial role in photoperiodic induction of diapause, suggesting that it may be a new player involved in the molecular mechanism for diapause induction.

dMyc expression in the fat body affects DILP2 release and increases the expression of the fat desaturase Desat1 resulting in organismal growth

Drosophila dMyc (dMyc) is known for its role in cell-autonomous regulation of growth. Here we address its role in the fat body (FB), a metabolic tissue that functions as a sensor of circulating nutrients to control the release of Drosophila Insulin-like peptides (Dilps) from the brain influencing growth and development. Our results show that expression of dMyc in the FB affects development and animal size. Expression of dMyc, but not of CycD/cdk4 or Rheb, in the FB diminishes the ability to retain Drosophila Insulin-like peptide-2 (DILP2) in the brain during starvation, suggesting that expression of dMyc mimics the signal that remotely controls the release of Dilps into the hemolymph. dMyc also affects glucose metabolism and increases the transcription of Glucose-transporter-1 mRNA, and of Hexokinase and Pyruvate-Kinase mRNAs, key regulators of glycolysis. These animals are able to counteract the increased levels of circulating trehalose induced by a high sugar diet leading to the conclusion that dMyc activity in the FB promotes glucose disposal. dMyc expression induces cell autonomous accumulation of triglycerides, which correlates with increased levels of Fatty Acid Synthase and Acetyl CoA Carboxylase mRNAs, enzymes responsible for lipid synthesis. We also found the expression of Stearoyl-CoA desaturase, Desat1 mRNA significantly higher in FB overexpressing dMyc. Desat1 is an enzyme that is necessary for monosaturation and production of fatty acids, and its reduction affects dMyc's ability to induce fat storage and resistance to animal survival. In conclusion, here we present novel evidences for dMyc function in the Drosophila FB in controlling systemic growth. We discovered that dMyc expression triggers cell autonomous mechanisms that control glucose and lipid metabolism to favor the storage of nutrients (lipids and sugars). In addition, the regulation of Desat1 controls the synthesis of triglycerides in FB and this may affect the humoral signal that controls DILP2 release in the brain.