mTOR-mediated Protein Synthesis Research Articles

S-adenosylmethionine biosynthesis is a targetable metabolic vulnerability in multiple myeloma.

Multiple myeloma (MM) is the second most prevalent hematologic malignancy and is incurable because of the inevitable development of drug resistance. Methionine adenosyltransferase 2α (MAT2A) is the primary producer of the methyl donor S-adenosylmethionine (SAM) and several studies have documented MAT2A deregulation in different solid cancers. As the role of MAT2A in MM has not been investigated yet, the aim of this study was to clarify the potential role and underlying molecular mechanisms of MAT2A in MM, exploring new therapeutic options to overcome drug resistance. By analyzing publicly available gene expression profiling data, MAT2A was found to be more highly expressed in patient-derived myeloma cells than in normal bone marrow plasma cells. The expression of MAT2A correlated with an unfavorable prognosis in relapsed patients. MAT2A inhibition in MM cells led to a reduction in intracellular SAM levels, which resulted in impaired cell viability and proliferation, and induction of apoptosis. Further mechanistic investigation demonstrated that MAT2A inhibition inactivated the mTOR-4EBP1 pathway, accompanied by a decrease in protein synthesis. MAT2A targeting in vivo with the small molecule compound FIDAS-5 was able to significantly reduce tumor burden in the 5TGM1 model. Finally, we found that MAT2A inhibition can synergistically enhance the anti-MM effect of the standard-of-care agent bortezomib on both MM cell lines and primary human CD138+ MM cells. In summary, we demonstrate that MAT2A inhibition reduces MM cell proliferation and survival by inhibiting mTOR-mediated protein synthesis. Moreover, our findings suggest that the MAT2A inhibitor FIDAS-5 could be a novel compound to improve bortezomib-based treatment of MM.

Inhibition of Oxidative Phosphorylation and Glycolysis Reduces Viability in Multiple Myeloma By Affecting mTOR-Mediated Protein Synthesis

Inhibition of Oxidative Phosphorylation and Glycolysis Reduces Viability in Multiple Myeloma By Affecting mTOR-Mediated Protein Synthesis

Methyltransferase-like 3 promotes the progression of lung cancer via activating PI3K/AKT/mTOR pathway.

Methyltransferase-like 3 (METTL3) catalyses N6-methyladenosine (m6 A) modification on messenger RNA (mRNA) and participates in a wide range of biological functions via epigenetically regulating gene expression. Recent studies suggested that dysregulation of METTL3 is associated with multiple human cancers; however, the role of METTL3 in lung cancer remains unclear. In the present study, through transcriptome analysis of lung cancer patients, we found that METTL3 is overexpressed in lung cancer patients and is associated with poor patient survival. More importantly, combining both in vitro and in vivo models, we revealed that in lung cancer cells, METTL3 overexpression activates PI3K/AKT/mTOR pathway and mTOR-mediated protein synthesis. Mechanistically, METTL3 promotes PI3K expression by introducing m6 A modification in PI3K 3' untranslated region (3' UTR). Elevated PI3K level then activates downstream AKT and mTOR signalling pathway and results in rapid cancer cell proliferation and metastasis. Taken together, our study reveals that METTL3-mediated m6 A methylation promotes lung cancer progression via activating PI3K/AKT/mTOR pathway.

Novelty exposure hinders aversive memory generalization and depends on hippocampal protein synthesis

Fear generalization is defined as the transferring of fear experienced during a traumatic event to safe conditions resembling or not the traumatic event. It has been related to several psychological disorders. Here we set out to determine whether novelty exposure can be effective to avoid fear generalization. We evaluated the effect of a novelty exposure on fear memory generalization using an aversive memory task, the inhibitory avoidance (IA). Male Wistar rats were trained in IA (day 1) and 24 h after (day 2) they were exposed to a new context similar to the original (modified IA - MIA), with some rats being exposed to a novelty just before the exposure to the MIA, while others were not (controls). On day 3, retention tests for IA and MIA contexts were performed. The control rats generalized the memory, expressing aversive behavioral in both contexts whereas rats exposed to novelty only expressed aversion on IA. Furthermore, both anisomycin, an inhibitor of ribosomal protein synthesis, and rapamycin, an inhibitor of mTOR-mediated protein synthesis, injected in the CA1 region of dorsal hippocampus blocked the novelty effect, promoting memory generalization. We conclude that novelty exposure hinders aversive memory generalization depending on hippocampal protein synthesis.

EZH2 Methyltransferase Activity Controls Pten Expression and mTOR Signaling during Fear Memory Reconsolidation.

Memory retrieval induces a transient period of increased transcriptional and translational regulation in neurons called reconsolidation, which is regulated by the protein kinase B (AKT)-mammalian target of rapamycin (mTOR) pathway. However, it is currently unknown how activation of the AKT-mTOR pathway is regulated during the reconsolidation process. Here, we found that in male rats retrieval of a contextual fear memory transiently increased Enhancer of Zeste Homolog 2 (EZH2) levels along with increased histone H3 lysine 27 trimethylation (H3K27me3) levels, which correlated with decreased levels of phosphatase and tensin homolog (PTEN), a potent inhibitor of AKT-mTOR-dependent signaling in the hippocampus. Further experiments found increased H3K27me3 levels and DNA methylation across the Pten promoter and coding regions, indicating transcriptional silencing of the Pten gene. Pten H3K27me3 levels did not change following training or after the retrieval of a remote (old) fear memory, suggesting that this mechanism of Pten repression was specific to the reconsolidation of a new memory. In vivo siRNA-mediated knockdown of Ezh2 in the hippocampus abolished retrieval-induced increases in H3K27me3 and prevented decreases in PTEN levels. Ezh2 knockdown attenuated increases in the phosphorylation of AKT and mTOR following retrieval, which could be restored by simultaneously reducing Pten, suggesting that H3K27me3 regulates AKT-mTOR phosphorylation via repression of Pten Consistent with these results, knockdown of Ezh2 in area CA1 before retrieval impaired memory on later tests. Collectively, these results suggest that EZH2-mediated H3K27me3 plays a critical role in the repression of Pten transcription necessary for AKT-mTOR activation and memory reconsolidation following retrieval.SIGNIFICANCE STATEMENT Understanding how critical translation pathways, like mTOR-mediated protein synthesis, are regulated during the memory storage process is necessary for improving memory impairments. This study tests whether mTOR activation is coupled to epigenetic mechanisms in the hippocampus following the retrieval of a contextual fear memory. Specifically, this study evaluates the role of epigenetic modifications in the form of histone methylation in downstream mTOR translational control during learning-dependent synaptic plasticity in neurons. Considering the broad implications of transcriptional and translational mechanisms in synaptic plasticity, psychiatric, and neurological and neurodegenerative disorders, these data are of interest to the neuroscience community due to the robust and specific regulation of mTOR signaling we found to be dependent on repressive histone methylation.

AMPK and mTOR: sensors and regulators of immunometabolic changes during Salmonella infection in the chicken

Non-typhoidal Salmonella enterica induce an early pro-inflammatory response in chickens, but the response is short-lived, asymptomatic of clinical disease, results in a persistent colonization of the gastrointestinal (GI) tract, and can transmit infections to naïve hosts via fecal shedding of bacteria. The underlying mechanisms that facilitate this persistent colonization of the ceca of chickens by Salmonella are unknown. We have begun to concentrate on the convergence of metabolism and immune function as playing a major role in regulating the host responsiveness to infection. It is now recognized that the immune system monitors the metabolic state of tissues and responds by modulating metabolic function. The aim in this review is to summarize the literature that has defined a series of genotypic and phenotypic alterations in the regulatory host immune-metabolic signaling pathways in the local cecal microenvironment during the first 4 d following infection with Salmonella enterica serovar Enteritidis. Using chicken-specific kinomic immune-metabolism peptide arrays and quantitative real-time-PCR of cecal tissue during the early (4 to 48 h) and late stages (4 to 17 d) of a Salmonella infection in young broiler chickens, the local immunometabolic microenvironment has been ascertained. Distinct immune and metabolic pathways are altered between 2 to 4 d post-infection that dramatically changed the local immunometabolic environment. Thus, the tissue immunometabolic phenotype of the cecum plays a major role in the ability of the bacterium to establish a persistent cecal colonization. In general, our findings show that AMPK and mTOR are key players linking specific extracellular milieu and intracellular metabolism. Phenotypically, the early response (4 to 48 h) to Salmonella infection is pro-inflammatory, fueled by glycolysis and mTOR-mediated protein synthesis, whereas by the later phase (4 to 5 d), the local environment has undergone an immune-metabolic reprogramming to an anti-inflammatory state driven by AMPK-directed oxidative phosphorylation. Therefore, metabolism appears to provide a potential critical control point that can impact infection. Further understanding of metabolic control of immunity during infection should provide crucial information of the development of novel therapeutics based on metabolic modulators that enhance protection or inhibit infection.

LY395756, an mGluR2 agonist and mGluR3 antagonist, enhances NMDA receptor expression and function in the normal adult rat prefrontal cortex, but fails to improve working memory and reverse MK801-induced working memory impairment

Targeting group II metabotropic glutamate receptors (mGluR2/3) has been proposed to correct the dysfunctional glutamatergic system, particularly NMDA receptor (NMDAR) hypofunction, for treatment of schizophrenia. However, how activation of mGluR2/3 affects NMDAR function in adult animals remains elusive. Here we show the effects of LY395756 (LY39), a compound acting as both an mGluR2 agonist and mGluR3 antagonist, on the NMDAR expression and function of normal adult rat prefrontal cortex (PFC) as well as working memory function in the MK801 model of schizophrenia. We found that in vivo administration of LY39 significantly increased the total protein levels of NMDAR subunits and NR2B phosphorylationin the PFC, along with the amplitude of NMDAR-mediated miniature excitatory postsynaptic currents (mEPSC) in the prefrontal cortical neurons. Moreover, LY39 also significantly increased mTOR and pmTOR expression, but not ERK1/2, Akt, and GSK3β, suggesting an activation of mTOR signaling. Indeed, the mTOR inhibitor rapamycin, and actinomycin-D, a transcription inhibitor, blocked the enhanced effects of LY39 on NMDAR-mEPSCs. These results indicate that LY39 regulates NMDAR expression and function through unidentified mTOR-mediated protein synthesis in the normal adult rat PFC. However, this change is insufficient to affect working memory function in normal animals, nor to reverse the MK801-induced working memory deficit. Our data provide the first evidence of an in vivo effect of a novel compound that acts as both an mGluR2 agonist and mGluR3 antagonist on synaptic NMDAR expression and function in the adult rat PFC, although its effect -on PFC-dependent cognitive function remains to be explored.

M12 A769662 Leads to AMPK Activation, Induces Autophagy and has Beneficial Effects IN HD Cells

Background The expansion of a polyglutamine repeat in the HD gene encoding huntingtin (htt) protein causes Huntington disease (HD). Although the exact pathogenesis is not known in its entirety, mutant htt causes disruption of various cellular functions, formation of aggregates and finally cell death. The process of autophagy is the main degradation pathway for mutant htt and various studies demonstrated that the induction of autophagy leads to an improvement of aggregate formation and cell viability. Commonly, the mTor inhibitor rapamycin is used for autophagy induction and has been shown to protect against mutant htt mediated toxicity in several HD models. Aim/method As rapamycin, beside autophagy induction, also suppresses mTor mediated protein synthesis, we aimed to characterise AMPK as a new drug target for autophagy induction. In this regard, we analysed AMPK activation by the AMPK inducer A769662 and its effects on autophagic degradation and mutant htt mediated toxicity. Results Activation of AMPK by A769662 led to an increased expression of the autophagosomal markers LC3 and p62, suggesting an efficient induction of autophagy. The autophagy induction is accompanied by a decrease of mutant htt expression and cytotoxicity. Additionally, mTor mediated protein synthesis, measured by the S6K1 (ribosomal protein S6 kinase) and RPS6 (ribosomal protein S6) expression is not affected. Conclusion Therefore, we identified AMPK as an interesting therapeutic target and A769662 as a potential treatment compound to facilitate the clearance of mutant htt without interfering with mTor mediated protein synthesis.

A molecular brake controls the magnitude of long-term potentiation

Overexpression of suprachiasmatic nucleus circadian oscillatory protein (SCOP), a negative ERK regulator, blocks long-term memory encoding. Inhibition of calpain-mediated SCOP degradation also prevents the formation of long-term memory, suggesting rapid SCOP breakdown is necessary for memory encoding. However, whether SCOP levels also control the magnitude of long-term synaptic plasticity is unknown. Here we show that following synaptic activity-induced SCOP degradation, SCOP is rapidly replaced via mTOR-mediated protein synthesis. We further show that early SCOP degradation is specifically catalyzed by µ–calpain while late SCOP re-synthesis is mediated by m-calpain. We propose that µ–calpain promotes long-term potentiation induction by degrading SCOP and activating ERK, while m-calpain activation limits the magnitude of potentiation by terminating the ERK response via enhanced SCOP synthesis. This unique braking mechanism could account for the advantages of spaced vs. massed training in the formation of long-term memory.

Excitation BolsTORs Motor Neurons in ALS Mice

It is unclear why motor neurons selectively degenerate in amyotrophic lateral sclerosis (ALS). Saxena etal. (2013) demonstrate that excitation of motor neurons can prevent their demise in a mouse model of inherited ALS by a mechanism involving the mTOR pathway.

Rheb REGULATES CRAB MUSCLE ATROPHY

Moulting their shells and misplacing limbs, crabs continually depend on their versatile musculature to reshape their bodies. According to Donald Mykles and colleagues from Colorado State University and the University of Wisconsin – Stevens Point, USA, the massive claw muscle of blackback land crabs withers by as much as 78% before they can slip their shells off. However, Mykles and his colleagues made an interesting discovery in 2010. Instead of shutting down protein synthesis to allow the muscle to wither, the crabs ramp it up 11-fold by shutting down myostatin expression – which usually keeps protein synthesis in check. Knowing that myostatin regulates protein expression in mammals via the mTOR (metazoan target of rapamycin) signalling cascade, which controls mRNA translation, Mykles decided to investigate the mTOR pathway in blackback land crabs to find out whether it also regulates muscle rebuilding in crabs during moulting (p. 590).Stimulating crabs to moult, the team collected muscles from the animals and searched for the mRNA of four key mTOR signalling proteins – Rheb, mTOR, Akt and s6k. The crabs expressed all four genes, confirming that the mTOR pathway is active in moulting crab’s atrophied muscle.Next, the team measured the expression levels of each gene in crabs that had been induced to moult using methods that raise the animals’ moulting hormone (ecdysteroid, which regulates myostatin levels) over different time scales. They found that Rheb levels increased in the moulting crabs’ claws muscles and in the atrophied thoracic muscles of crabs that had lost a limb. Mykles says ‘Rheb is a key activator of mTOR. Its up-regulation by moulting indicates that Rheb plays a role in the stimulation of mTOR-mediated protein synthesis by ecdysteroids’.However, when the team investigated the mechanism by which Rheb is upregulated in the two muscles, there were differences. They found a strong correlation between the levels of Rheb in the crabs’ claws and the ecdysteroid levels, suggesting that ecdysteroid does regulate protein synthesis via mTOR in the claw muscle. Yet, when they compared the ecdysteroid levels and Rheb levels in the thoracic muscles – that wither in response to limb loss – the team found no correlation. Ecdysteroid hormone does not control muscle atrophy in response to limb loss.Analysing the expression patterns of Rheb and myostatin in atrophied claw and thoracic muscles, Mykles and colleagues also saw different expression relationships between the proteins in the two tissues. ‘If blackback land crab myostatin plays a role in both types of atrophy, any unifying mechanism must reconcile the differences between myostatin and Rheb expression in the two muscles’, the team says.Having shown that Rheb is a key component of the signalling pathway that regulates muscle atrophy in crabs, the team says that it is keen to build, ‘a mechanistic understanding of the interactions between ecdysteroids and the signalling pathways that control protein metabolism in crustacean skeletal muscle’.

Presynaptic protein synthesis required for NT-3-induced long-term synaptic modulation

BackgroundNeurotrophins elicit both acute and long-term modulation of synaptic transmission and plasticity. Previously, we demonstrated that the long-term synaptic modulation requires the endocytosis of neurotrophin-receptor complex, the activation of PI3K and Akt, and mTOR mediated protein synthesis. However, it is unclear whether the long-term synaptic modulation by neurotrophins depends on protein synthesis in pre- or post-synaptic cells.ResultsHere we have developed an inducible protein translation blocker, in which the kinase domain of protein kinase R (PKR) is fused with bacterial gyrase B domain (GyrB-PKR), which could be dimerized upon treatment with a cell permeable drug, coumermycin. By genetically targeting GyrB-PKR to specific cell types, we show that NT-3 induced long-term synaptic modulation requires presynaptic, but not postsynaptic protein synthesis.ConclusionsOur results provide mechanistic insights into the cell-specific requirement for protein synthesis in the long-term synaptic modulation by neurotrophins. The GyrB-PKR system may be useful tool to study protein synthesis in a cell-specific manner.

NAD Blocks High Glucose Induced Mesangial Hypertrophy via Activation of the Sirtuins-AMPK-mTOR Pathway

Background/aims-Since the discovery of NAD-dependent deacetylases, Sirtuins, it has been recognized that maintaining intracellular levels of NAD is crucial for the management of stress-response of cells. Here we show that high glucose(HG)-induced mesangial hypertrophy is associated with loss of intracellular levels of NAD. This study was designed to investigate the effect of NAD on HG-induced mesangial hypertrophy. Methods-The rat glomerular mesangial cells (MCs) were incubated in HG medium with or without NAD. Afterwards, NAD⁺/NADH ratio and enzyme activity of Sirtuins was determined. In addition, the expression analyses of AMPK-mTOR signaling were evaluated by Western blot analysis. Results-We showed that HG induced the NAD⁺/NADH ratio and the levels of SIRT1 and SIRT3 activity decreased as well as mesangial hypertrophy, but NAD was capable of maintaining intracellular NAD⁺/NADH ratio and levels of SIRT1 and SIRT3 activity as well as of blocking the HG-induced mesangial hypertrophy in vitro. Activating Sirtuins by NAD blocked the activation of pro-hypertrophic Akt signaling, and augmented the activity of the antihypertrophic AMPK signaling in MCs, which prevented the subsequent induction of mTOR-mediated protein synthesis. By AMPK knockdown, we showed it upregulated phosphorylation of mTOR. In such, the NAD inhibited HG-induced mesangial hypertrophy whereas NAD lost its inhibitory effect in the presence of AMPK siRNA. Conclusion-These results reveal a novel role of NAD as an inhibitor of mesangial hypertrophic signaling, and suggest that prevention of NAD depletion may be critical in the treatment of mesangial hypertrophy.

Exogenous NAD Blocks Cardiac Hypertrophic Response via Activation of the SIRT3-LKB1-AMP-activated Kinase Pathway

Since the discovery of NAD-dependent deacetylases, sirtuins, it has been recognized that maintaining intracellular levels of NAD is crucial for the management of stress response of cells. Here we show that agonist-induced cardiac hypertrophy is associated with loss of intracellular levels of NAD, but not exercise-induced physiologic hypertrophy. Exogenous addition of NAD was capable of maintaining intracellular levels of NAD and blocking the agonist-induced cardiac hypertrophic response in vitro as well as in vivo. NAD treatment blocked the activation of pro-hypertrophic Akt1 signaling, and augmented the activity of anti-hypertrophic LKB1-AMPK signaling in the heart, which prevented subsequent induction of mTOR-mediated protein synthesis. By using gene knock-out and transgenic mouse models of SIRT3 and SIRT1, we showed that the anti-hypertrophic effects of exogenous NAD are mediated through activation of SIRT3, but not SIRT1. SIRT3 deacetylates and activates LKB1, thus augmenting the activity of the LKB1-AMPK pathway. These results reveal a novel role of NAD as an inhibitor of cardiac hypertrophic signaling, and suggest that prevention of NAD depletion may be critical in the treatment of cardiac hypertrophy and heart failure.

Activation of polyamine catabolism by N1, N11-diethylnorspermine alters the cellular localization of mTOR and downregulates mTOR protein level in glioblastoma cells

N1, N11-Diethylnorspermine (DENSPM) is a spermine analog and prototype anti-cancer drug that depletes cellular polyamine, increases cellular oxidative stress through the generation of H2O2, and induces the death of multiple types of cancer cells. However, the survival pathways perturbed by DENSPM are uncertain. To identify these pathways, we examined a series of proteins in the phosphoinositide 3-kinase /AKT/mammalian target of rapamycin (PI3K/AKT/mTOR) pathways in glioblastoma cell lines before and after treatment with DENSPM. We found that DENSPM did not change the protein levels of PI3K but did reduce the levels of AKT, phosphorylated AKT, mTOR, phosphorylated mTOR, p70S6K, phosphorylated p70S6K, 4E-BP1, phosphorylated 4E-BP1, and eIF-4B proteins. From this it appears that DENSPM directly targets the mTOR protein level in these glioblastoma cells by inhibiting mTOR-mediated protein synthesis. Immunofluorescence analysis of mTOR showed that DENSPM sequestered mTOR in the perinuclear region of the cells. We also detected a marked collapse of microtubules in U87 cells and a detachment of cells in a process resembling anoikis. We further showed that the levels of many proteins regulating cell growth and cell adhesion were down-regulated, suggesting a broad effect of DENSPM on mTOR-mediated protein synthesis. We conclude that the activation of polyamine catabolism alters the cellular location of mTOR, thus negatively affecting mTOR-mediated protein synthesis and leading to the death of glioblastoma cells.

Mitogen-Activated Protein Kinase Upregulates the Dendritic Translation Machinery in Long-Term Potentiation by Controlling the Mammalian Target of Rapamycin Pathway

Protein synthesis is required for persistent forms of synaptic plasticity, including long-term potentiation (LTP). A key regulator of LTP-related protein synthesis is mammalian target of rapamycin (mTOR), which is thought to modulate translational capacity by facilitating the synthesis of particular components of the protein synthesis machinery. Recently, extracellularly regulated kinase (ERK) also was shown to mediate plasticity-related translation, an effect that may involve regulation of the mTOR pathway. We studied the interaction between the mTOR and ERK pathways in hippocampal LTP induced at CA3-CA1 synapses by high-frequency synaptic stimulation (HFS). Within minutes after HFS, the expression of multiple translational proteins, the synthesis of which is under the control of mTOR, increased in area CA1 stratum radiatum. This upregulation was detected in pyramidal cell dendrites and was blocked by inhibitors of the ERK pathway. In addition, ERK mediated the stimulation of mTOR by HFS. The possibility that ERK regulates mTOR by acting at a component further upstream in the phosphatidylinositide 3-kinase (PI3K)-mTOR pathway was tested by probing the phosphorylation of p90-S6 kinase, phosphoinositide-dependent kinase 1 (PDK1), and Akt. ERK inhibitors blocked HFS-induced phosphorylation of all three proteins at sites implicated in the regulation of mTOR. Moreover, a component of basal and HFS-induced ERK activity depended on PI3K, indicating that mTOR-mediated protein synthesis in LTP requires coincident and mutually dependent activity in the PI3K and ERK pathways. The role of ERK in regulating PDK1 and Akt, with their extensive effects on cellular function, has important implications for the coordinated response of the neuron to LTP-inducing stimulation.

Heterodimerization of Insulin-like Growth Factor Receptor/Epidermal Growth Factor Receptor and Induction of Survivin Expression Counteract the Antitumor Action of Erlotinib

Epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs) have been used to treat non-small cell lung cancer (NSCLC). However, the overall response rate to EGFR TKIs is limited, and the mechanisms mediating resistance to the drugs are poorly understood. Here, we report that insulin-like growth factor-I receptor (IGF-IR) activation interferes with the antitumor activity of erlotinib, an EGFR TKI. Treatment with erlotinib increased the levels of EGFR/IGF-IR heterodimer localized on cell membrane, activated IGF-IR and its downstream signaling mediators, and stimulated mammalian target of rapamycin (mTOR)-mediated de novo protein synthesis of EGFR and survivin in NSCLC cells. Inhibition of IGF-IR activation, suppression of mTOR-mediated protein synthesis, or knockdown of survivin expression abolished resistance to erlotinib and induced apoptosis in NSCLC cells in vitro and in vivo. Our data suggest that enhanced synthesis of survivin protein mediated by the IGFR/EGFR heterodimer counteracts the antitumor action of erlotinib, indicating the needs of integration of IGF-IR-targeted agents to the treatment regimens with EGFR TKI for patients with lung cancer.