A role for Raptor phosphorylation in the mechanical activation of mTOR signaling

Molecular mechanism of microRNA-mediated hypoglycemic effect of whole grain highland barley

As a non-coding RNA, microRNA (miRNA) has been proven to play an important role in the development and progression of type 2 diabetes mellitus (T2DM). Highland barley is a whole grain from the Tibetan areas of China. Our previous studies have demonstrated its hypoglycemic effect. To further explore the underlining molecular mechanism, we investigated the effect of highland barley intervention on liver miRNA expression profiles in diabetic mice. Our results showed that ten differentially expressed miRNA among different groups were identified and their target genes were predicted. Remarkably, many glycometabolism-associated genes, including Foxo3, Nras, Rptor, Igf1r, Tsc2 and Braf, were negatively regulated by miR-122-5p, miR-503-5p, miR-455-5p and miR-210-3p, respectively. Pathway enrichment analysis revealed these target genes were mainly involved in AMPK, MAPK and FOXO signaling pathways. Thereby, these miRNA and mRNA were validated using qRT-PCR, and the results were consistent with the small RNA-seq and expectations. Highland barley could regulate the MAPK, AMPK, and FOXO signaling pathways by regulating critical miRNA-mRNA pairs, e.x. miR-210-3p-Tsc2/Braf, miR-122-5p-Foxo3, and miR-455-5p-Igf1r, thereby improving blood glucose metabolism in diabetic mice. The present study preliminarily explored the hypoglycaemic effects of highland barley based on transcriptomics, and more detailed and in-depth studies on this topic are needed in the future.

Mechanistic target of rapamycin in regulating macrophage function in inflammatory cardiovascular diseases

The mechanistic target of rapamycin (mTOR) is evolutionarily conserved from yeast to humans and is one of the most fundamental pathways of living organisms. Since its discovery three decades ago, mTOR has been recognized as the center of nutrient sensing and growth, homeostasis, metabolism, life span, and aging. The role of dysregulated mTOR in common diseases, especially cancer, has been extensively studied and reported. Emerging evidence supports that mTOR critically regulates innate immune responses that govern the pathogenesis of various cardiovascular diseases. This review discusses the regulatory role of mTOR in macrophage functions in acute inflammation triggered by ischemia and in atherosclerotic cardiovascular disease (ASCVD) and heart failure with preserved ejection fraction (HFpEF), in which chronic inflammation plays critical roles. Specifically, we discuss the role of mTOR in trained immunity, immune senescence, and clonal hematopoiesis. In addition, this review includes a discussion on the architecture of mTOR, the function of its regulatory complexes, and the dual-arm signals required for mTOR activation to reflect the current knowledge state. We emphasize future research directions necessary to understand better the powerful pathway to take advantage of the mTOR inhibitors for innovative applications in patients with cardiovascular diseases associated with aging and inflammation.

Mechanism of the Mitogen-Activated Protein Kinases/Mammalian Target of Rapamycin Pathway in the Process of Cartilage Endplate Stem Cell Degeneration Induced by Tension Load

STUDY DESIGN

Basic Research.

OBJECTIVE

Intervertebral disc degeneration (IVDD) is caused by the cartilage endplate (CEP). Cartilage endplate stem cell (CESC) is involved in the recovery of CEP degeneration. Tension load (TL) contributes a lot to the initiation and progression of IVDD. This study aims to investigate the regulatory mechanism of the Mitogen-activated protein kinases/Mammalian target of rapamycin (MAPK/mTOR) pathway during TL-induced CESC degeneration.

METHODS

CESCs were isolated from New Zealand big-eared white female rabbits (6 months old). FX-4000T cell stress loading system was applied to establish a TL-induced degeneration model of CESCs. Western blotting was used to detect the level of mTOR pathway-related proteins and autophagy markers LC3-Ⅱ, Beclin-1, and p62 in degenerative CESCs. The expression of MAPK pathway-related proteins JNK and extracellular signal-regulated kinases (ERK) in degenerated CESCs was inhibited by cell transfection to explore whether JNK and ERK play a regulatory role in TL-induced autophagy in CESCs.

RESULTS

In the CESC degeneration model, the mTOR pathway was activated. After inhibition of mTOR, the autophagy level of CESCs was increased, and the degeneration of CESCs was alleviated. The MAPK pathway was also activated in the CESC degeneration model. Inhibition of JNK expression may alleviate TL-induced CEP degeneration by inhibiting Raptor phosphorylation and activating autophagy. Inhibition of ERK expression may alleviate TL-induced CEP degeneration by inhibiting mTOR phosphorylation and activating autophagy.

CONCLUSION

Inhibition of JNK and ERK in the MAPK signaling family alleviated TL-induced CESC degeneration by inhibiting the phosphorylation of Raptor and mTOR in the mTOR pathway.

Different Resistance Exercise Loading Paradigms Similarly Affect Skeletal Muscle Gene Expression Patterns of Myostatin-Related Targets and mTORC1 Signaling Markers

Although transcriptome profiling has been used in several resistance training studies, the associated analytical approaches seldom provide in-depth information on individual genes linked to skeletal muscle hypertrophy. Therefore, a secondary analysis was performed herein on a muscle transcriptomic dataset we previously published involving trained college-aged men (n = 11) performing two resistance exercise bouts in a randomized and crossover fashion. The lower-load bout (30 Fail) consisted of 8 sets of lower body exercises to volitional fatigue using 30% one-repetition maximum (1 RM) loads, whereas the higher-load bout (80 Fail) consisted of the same exercises using 80% 1 RM loads. Vastus lateralis muscle biopsies were collected prior to (PRE), 3 h, and 6 h after each exercise bout, and 58 genes associated with skeletal muscle hypertrophy were manually interrogated from our prior microarray data. Select targets were further interrogated for associated protein expression and phosphorylation induced-signaling events. Although none of the 58 gene targets demonstrated significant bout x time interactions, ~57% (32 genes) showed a significant main effect of time from PRE to 3 h (15↑ and 17↓, p < 0.01), and ~26% (17 genes) showed a significant main effect of time from PRE to 6 h (8↑ and 9↓, p < 0.01). Notably, genes associated with the myostatin (9 genes) and mammalian target of rapamycin complex 1 (mTORC1) (9 genes) signaling pathways were most represented. Compared to mTORC1 signaling mRNAs, more MSTN signaling-related mRNAs (7 of 9) were altered post-exercise, regardless of the bout, and RHEB was the only mTORC1-associated mRNA that was upregulated following exercise. Phosphorylated (phospho-) p70S6K (Thr389) (p = 0.001; PRE to 3 h) and follistatin protein levels (p = 0.021; PRE to 6 h) increased post-exercise, regardless of the bout, whereas phospho-AKT (Thr389), phospho-mTOR (Ser2448), and myostatin protein levels remained unaltered. These data continue to suggest that performing resistance exercise to volitional fatigue, regardless of load selection, elicits similar transient mRNA and signaling responses in skeletal muscle. Moreover, these data provide further evidence that the transcriptional regulation of myostatin signaling is an involved mechanism in response to resistance exercise.

DNA-dependent protein kinase catalytic subunit (DNA-PKcs) drives chronic kidney disease progression in male mice

Kidney injury initiates epithelial dedifferentiation and myofibroblast activation during the progression of chronic kidney disease. Herein, we find that the expression of DNA-PKcs is significantly increased in the kidney tissues of both chronic kidney disease patients and male mice induced by unilateral ureteral obstruction and unilateral ischemia-reperfusion injury. In vivo, knockout of DNA-PKcs or treatment with its specific inhibitor NU7441 hampers the development of chronic kidney disease in male mice. In vitro, DNA-PKcs deficiency preserves epithelial cell phenotype and inhibits fibroblast activation induced by transforming growth factor-beta 1. Additionally, our results show that TAF7, as a possible substrate of DNA-PKcs, enhances mTORC1 activation by upregulating RAPTOR expression, which subsequently promotes metabolic reprogramming in injured epithelial cells and myofibroblasts. Taken together, DNA-PKcs can be inhibited to correct metabolic reprogramming via the TAF7/mTORC1 signaling in chronic kidney disease, and serve as a potential target for treating chronic kidney disease.

Effective-compounds of Jinshui Huanxian formula ameliorates fibroblast activation in pulmonary fibrosis by inhibiting the activation of mTOR signaling

BACKGROUND

Jinshui Huanxian formula (JHF) ameliorates idiopathic pulmonary fibrosis patients. Active compounds, including icariin, isoliquiritigenin, nobiletin, peimine, and paeoniflorin, deriving from JHF were combined as effective-component compatibility ECC of JHF II (ECC-JHF II), which is an effective therapeutic strategy for pulmonary fibrosis (PF) induced by bleomycin (BLM) in rats.

PURPOSE

This study aimed to explore the underlying mechanism of ECC-JHF II on pulmonary fibrosis.

METHODS

A model of PF in rats was established through intratracheal instillation of BLM. Pulmonary function, pathological changes, and collagen deposition were examined. The gene and protein expressions in fibroblast activation were detected by quantitative real-time PCR and western blotting respectively.

RESULTS

ECC-JHF II significantly improved BLM-induced PF in rats, manifested as decreased collagen deposition, reduced pathological damage and improved pulmonary function. Furthermore, ECC-JHF II inhibited fibroblast activation by reducing the expression of α-smooth muscle actin (α-SMA) and fibronectin. We analyzed the targets of ECC-JHF II and differentially expressed genes (DEGs) of fibroblast activation induced by transforming growth factor-β1 (TGF-β1) and found that ECC-JHF II might regulate fibroblast activation by EGFR, PI3K-Akt or mTOR signaling pathway. In vitro experiments, we also found that ECC-JHF II suppressed the mTOR pathway, such as downregulating the phosphorylation levels of p70S6K in fibroblast activation induced by TGF-β1. After activating mTOR signaling, the inhibition of ECC-JHF II on fibroblast activation was blocked. These results suggested that ECC-JHF II potently ameliorated pulmonary fibrosis in rats and effectively suppressed fibroblast activation by interfering with mTOR signaling.

CONCLUSION

We combined transcriptomics with the network analysis to predict the mechanism underlying ECC-JHF II suppression of fibroblast activation. In summary, ECC-JHF II improved BLM-induced pulmonary fibrosis, which might be associated with the suppression of fibroblast activation by inhibiting the mTOR signaling.

Mammalian Target of Rapamycin (mTOR) Signaling at the Crossroad of Muscle Fiber Fate in Sarcopenia

The mammalian target of rapamycin (mTOR) is a major regulator of skeletal myocyte viability. The signaling pathways triggered by mTOR vary according to the type of endogenous and exogenous factors (e.g., redox balance, nutrient availability, physical activity) as well as organismal age. Here, we provide an overview of mTOR signaling in skeletal muscle, with a special focus on the role played by mTOR in the development of sarcopenia. Intervention strategies targeting mTOR in sarcopenia (e.g., supplementation of plant extracts, hormones, inorganic ions, calorie restriction, and exercise) have also been discussed.

ZNF143 regulates autophagic flux to alleviate myocardial ischemia/reperfusion injury through Raptor

The exact role of autophagy in myocardial ischemia/reperfusion (I/R) injury is still controversial. Excessive or insufficient autophagy may lead to cell death. Therefore, how to regulate autophagic balance during myocardial ischemia/reperfusion is critical to the treatment of myocardial I/R injury. Raptor is an mTOR regulatory related protein and closely related to the induction of autophagy. ZNF143 is widely expressed in various cells and acts as a transcription factor, which is involved in the regulation of autophagy, cell growth and development. In this study, we aimed to explore the mechanism by which ZNF143 regulated autophagy in myocardial I/R injury and the relationship between ZNF143 and Raptor. In our results, we found that ZNF143 expression was down-regulated in myocardial I/R. Inhibition of ZNF143 expression further enhanced autophagy and restored the deficiency of autophagic flux caused by myocardial I/R, subsequently alleviating myocardial I/R injury. On the other hand, overexpression of ZNF143 up-regulated Raptor expression and reduced autophagic activity, consequently exacerbating myocardial I/R injury. Taken together, our study revealed that ZNF143 might be a key target of the regulation of autophagy and a novel therapeutic target of myocardial I/R injury.

Effects of dietary L-leucine supplementation on testicular development and semen quality in boars

Sperm and seminal plasma are rich in leucine, and leucine can promote the protein synthesis. This property makes it an interesting amino acid to increase sperm quality of human and livestock spermatogenesis. The goal of this study was to explore the effects of dietary leucine supplementation on testicular development and semen quality in boars from weaning to 10 months of age. 30 pure-bred, weaned Duroc boars (8.0 ± 1.0 kg) were randomly divided into two groups: control group (CON; fed the basal diet) and leucine group (LEU; fed the basal diet supplemented with 1.2% leucine); then, their body weight and testicular volume were recorded every 4 weeks. Testes were collected for histological and genes expression analysis from 150-day-old boars. Semen was collected and analyzed. Amino acids contents of blood plasma, seminal plasma, sperm, and testes were determined. Dietary supplementation with leucine increased the testicular volume and weight of boars, compared with CON. Sperm viability, sperm count per ejaculation, and average curve speed of sperm in leucine-supplemented boars were increased. Furthermore, leucine supplementation increased the blood plasma and seminal plasma leucine concentrations, and enhanced the gene expressions of branch chain amino acid transaminase, protein kinase B, mammalian target of rapamycin (mTOR), and cyclinb1 in the testes. Interestingly, the expressions of the p-mTOR and mTOR proteins in the testes were also upregulated. Thus, dietary leucine supplementation increased leucine absorption and utilization in the testes, promoted testicular development, and improved semen quality of boars, partly through the mTOR signaling pathway.

CORP: Gene delivery into murine skeletal muscle using in vivo electroporation

The strategy of gene delivery into skeletal muscles has provided exciting avenues in identifying new potential therapeutics towards muscular disorders and addressing basic research questions in muscle physiology through overexpression and knockdown studies. In vivo electroporation methodology offers a simple, rapidly effective technique for the delivery of plasmid DNA into post-mitotic skeletal muscle fibers and the ability to easily explore the molecular mechanisms of skeletal muscle plasticity. The purpose of this review is to describe how to robustly electroporate plasmid DNA into different hindlimb muscles of rodent models. Further, key parameters (e.g., voltage, hyaluronidase, plasmid concentration) which contribute to the successful introduction of plasmid DNA into skeletal muscle fibers will be discussed. In addition, details on processing tissue for immunohistochemistry and fiber cross-sectional area (CSA) analysis will be outlined. The overall goal of this review is to provide the basic and necessary information needed for successful implementation of in vivo electroporation of plasmid DNA and thus open new avenues of discovery research in skeletal muscle physiology.

Targeting mTOR Signaling in Type 2 Diabetes Mellitus and Diabetes Complications

The mechanistic target of rapamycin (mTOR) is a pivotal regulator of cell metabolism and growth. In the form of two different multi-protein complexes, mTORC1 and mTORC2, mTOR integrates cellular energy, nutrient and hormonal signals to regulate cellular metabolic homeostasis. In type 2 diabetes mellitus (T2DM) aberrant mTOR signaling underlies its pathological conditions and end-organ complications. Substantial evidence suggests that two mTOR-mediated signaling schemes, mTORC1-p70S6 kinase 1 (S6K1) and mTORC2-protein kinase B (AKT), play a critical role in insulin sensitivity and that their dysfunction contributes to development of T2DM. This review summaries our current understanding of the role of mTOR signaling in T2DM and its associated complications, as well as the potential use of mTOR inhibitors in treatment of T2DM.

Targeting PRAS40: a novel therapeutic strategy for human diseases

Proline-rich Akt substrate of 40 kD (PRAS40) is not only the substrate of protein kinase B (PKB/Akt), but also the binding protein of 14-3-3 protein. PRAS40 is expressed in a variety of tissues in vivo and has multiple phosphorylation sites, which its activity is closely related to phosphorylation. Studies have shown that PRAS40 is involved in regulating cell growth, cell apoptosis, oxidative stress, autophagy and angiogenesis, as well as various of signalling pathways such as mammalian target of mammalian target rapamycin (mTOR), protein kinase B (PKB/Akt), nuclear factor kappa-B(NF-κB), proto-oncogene serine/threonine-protein kinase PIM-1(PIM1) and pyruvate kinase M2 (PKM2). The interactive roles between PRAS40 and these signal proteins were analysed by bioinformatics in this paper. Moreover, it is of great necessity for analyse the important roles of PRAS40 in some human diseases including cardiovascular disease, ischaemia-reperfusion injury, neurodegenerative disease, cancer, diabetes and other metabolic diseases. Finally, the effects of miRNA on the regulation of PRAS40 function and the occurrence and development of PRAS40-related diseases are also discussed. Overall, PRAS40 is expected to be a drug target and provide a new treatment strategy for human diseases.

Repeated bouts of resistance exercise in rats alter mechanistic target of rapamycin complex 1 activity and ribosomal capacity but not muscle protein synthesis

NEW FINDINGS

What is the central question of this study? Is muscle protein synthesis (MPS) additionally activated following exercise when ribosomal capacity is increased after repeated bouts of resistance exercise (RE)? What is the main finding and its importance? Skeletal muscles with increased ribosome content through repeated RE bouts showed sufficient activation of MPS with lower mechanistic target of rapamycin complex 1 signalling. Thus, repeated bouts of RE possibly change the translational capacity and efficiency to optimize translation activation following RE.

ABSTRACT

Resistance exercise (RE) activates ribosome biogenesis and increases ribosome content in skeletal muscles. However, it is unclear whether the increase in ribosome content subsequently causes an increase in RE-induced activation of muscle protein synthesis (MPS). Thus, this study aimed to investigate the relationship between ribosome content and MPS after exercise using a rat RE model. Male Sprague-Dawley rats were categorized into three groups (n = 6 for each group): sedentary (SED) and RE trained with one bout (1B) or three bouts (3B). The RE stimulus was applied to the right gastrocnemius muscle by transcutaneous electrical stimulation under isoflurane anaesthesia. The 3B group underwent stimulation every other day. Our results revealed that 6 h after the last bout of RE, muscles in the 3B group showed an increase in total RNA and 18S+28S rRNA content per muscle weight compared with the SED and 1B groups. In both the 1B and 3B groups, MPS, estimated by puromycin incorporation in proteins, was higher than that in the SED group 6 h after exercise; however, no significant difference was observed between the 1B and 3B groups. In the 1B and 3B groups, phosphorylated p70S6K at Thr-389 increased, indicating mechanistic target of rapamycin complex 1 (mTORC1) activity. p70S6K phosphorylation level was lower in the 3B group than in the 1B group. Finally, protein synthesis per ribosome (indicator of translation efficiency) was lower in the 3B group than in the 1B group. Thus, three bouts of RE changed the ribosome content and mTORC1 activation, but not the degree of RE-induced global MPS activation.

Muscle transcriptional networks linked to resistance exercise training hypertrophic response heterogeneity

The skeletal muscle hypertrophic response to resistance exercise training (RT) is highly variable across individuals. The molecular underpinnings of this heterogeneity are unclear. This study investigated transcriptional networks linked to RT-induced muscle hypertrophy, classified as 1) predictive of hypertrophy, 2) responsive to RT independent of muscle hypertrophy, or 3) plastic with hypertrophy. Older adults (n = 31, 18 F/13 M, 70 ± 4 yr) underwent 14-wk RT (3 days/wk, alternating high-low-high intensity). Muscle hypertrophy was assessed by pre- to post-RT change in mid-thigh muscle cross-sectional area (CSA) [computed tomography (CT), primary outcome] and thigh lean mass [dual-energy X-ray absorptiometry (DXA), secondary outcome]. Transcriptome-wide poly-A RNA-seq was performed on vastus lateralis tissue collected pre- (n = 31) and post-RT (n = 22). Prediction networks (using only baseline RNA-seq) were identified by weighted gene correlation network analysis (WGCNA). To identify Plasticity networks, WGCNA change indices for paired samples were calculated and correlated to changes in muscle size outcomes. Pathway-level information extractor (PLIER) was applied to identify Response networks and link genes to biological annotation. Prediction networks (n = 6) confirmed transcripts previously connected to resistance/aerobic training adaptations in the MetaMEx database while revealing novel member genes that should fuel future research to understand the influence of baseline muscle gene expression on hypertrophy. Response networks (n = 6) indicated RT-induced increase in aerobic metabolism and reduced expression of genes associated with spliceosome biology and type-I myofibers. A single exploratory Plasticity network was identified. Findings support that interindividual differences in baseline gene expression may contribute more than RT-induced changes in gene networks to muscle hypertrophic response heterogeneity. Code/Data: https://github.com/kallavin/MASTERS_manuscript/tree/master.

Regulation of mTORC1 by Upstream Stimuli

The mammalian target of rapamycin (mTOR) is an evolutionary conserved Ser/Thr protein kinase that senses multiple upstream stimuli to control cell growth, metabolism, and autophagy. mTOR is the catalytic subunit of mTOR complex 1 (mTORC1). A significant amount of research has uncovered the signaling pathways regulated by mTORC1, and the involvement of these signaling cascades in human diseases like cancer, diabetes, and ageing. Here, we review advances in mTORC1 regulation by upstream stimuli. We specifically focus on how growth factors, amino acids, G-protein coupled receptors (GPCRs), phosphorylation, and small GTPases regulate mTORC1 activity and signaling.

Whey for Sarcopenia; Can Whey Peptides, Hydrolysates or Proteins Play a Beneficial Role?

As the human body ages, skeletal muscle loses its mass and strength. It is estimated that in 10% of individuals over the age of 60, this muscle frailty has progressed to sarcopenia. Biomarkers of sarcopenia include increases in inflammatory markers and oxidative stress markers and decreases in muscle anabolic markers. Whey is a high-quality, easily digested dairy protein which is widely used in the sports industry. This review explores the evidence that whey protein, hydrolysates or peptides may have beneficial effects on sarcopenic biomarkers in myoblast cell lines, in aged rodents and in human dietary intervention trials with the older consumer. A daily dietary supplementation of 35 g of whey is likely to improve sarcopenic biomarkers in frail or sarcopenia individuals. Whey supplementation, consumed by an older, healthy adult certainly improves muscle mTOR signaling, but exercise appears to have the greatest benefit to older muscle. In vitro cellular assays are central for bioactive and bioavailable peptide identification and to determine their mechanism of action on ageing muscle.

mTOR at the nexus of nutrition, growth, ageing and disease

The mTOR pathway integrates a diverse set of environmental cues, such as growth factor signals and nutritional status, to direct eukaryotic cell growth. Over the past two and a half decades, mapping of the mTOR signalling landscape has revealed that mTOR controls biomass accumulation and metabolism by modulating key cellular processes, including protein synthesis and autophagy. Given the pathway's central role in maintaining cellular and physiological homeostasis, dysregulation of mTOR signalling has been implicated in metabolic disorders, neurodegeneration, cancer and ageing. In this Review, we highlight recent advances in our understanding of the complex regulation of the mTOR pathway and discuss its function in the context of physiology, human disease and pharmacological intervention.

Raptor/mTORC1 Acts as a Modulatory Center to Regulate Anti-bacterial Immune Response in Rockfish

The mammalian target of rapamycin (mTOR) is an evolutionarily highly conserved atypical serine/threonine protein kinase, which regulates cell growth, proliferation, apoptosis, autophagy, and metabolism. As a regulatory protein, Raptor is awfully important for the stability and function of mTOR complex 1 (mTORC1). However, the studies about how Raptor/mTORC1 participates in and regulates immune response in lower vertebrates are still limited. In this study, we investigated the regulation of immune response by the Raptor/mTORC1 signaling pathway in rockfish Sebastes schlegelii. Sebastes schlegelii Raptor (Ss-Raptor) is a highly conserved protein during the evolution, in both primary and tertiary structure. Ss-Raptor mRNA was widely distributed in various tissues of rockfish and has a relative higher expression in spleen and blood. After infected by Micrococcus luteus or Listonella anguillarum, mRNA expression of Ss-Raptor rapidly increased within 48 h. Once Raptor/mTORC1 signaling was blocked by rapamycin, expression of the pro-inflammatory cytokines IL-1β and IL-8 was severely impaired, suggesting potential regulatory role of Raptor/mTORC1 signaling in the innate immune response of rockfish. In addition, Raptor/mTORC1 pathway participated in lymphocyte activation of rockfish through promoting 4EBP1 and S6 phosphorylation. Inhibition of Raptor/mTORC1 signaling crippled the lymphocyte expansion during primary adaptive immune response, manifesting by the decrease of lymphoid organ weight and lymphocyte numbers. More importantly, inhibition of Raptor/mTORC1 signaling impaired the lymphocyte mediated cytotoxic response, and made the fish more vulnerable to the bacterial infection. Together, our results suggested that Raptor and its tightly regulated mTORC1 signaling acts as modulatory center to regulate both innate and lymphocyte-mediated adaptive immune response during bacterial infection. This research has shed new light on regulatory mechanism of teleost immune response, and provide helpful evidences to understand the evolution of immune system.

Cross-talk between ribosome biogenesis, translation, and mTOR in CD133+ 4/CD44+ prostate cancer stem cells

OBJECTIVE

To investigate the gene expression profile of CSCs and to explore the key pathways and specific molecular signatures involved in the characteristic of CSCs.

MATERIALS AND METHODS

CD133+ /CD44+ CSCs and bulk population (non-CSCs) were isolated from DU-145 cells using fluorescence-activated cell sorting (FACS). We used Illumina HumanHT-12 v4 Expression to investigate gene expression profiling of CSCs and non-CSCs. Protein-protein interaction (PPI) network analysis was performed using the STRING database. Biomarkers selected based on gene expression profiling were visually analyzed using immunofluorescence staining method. An image analysis program, ImageJ®, was used for the analysis of fluorescence intensity.

RESULTS

In microarray analysis, we found that many ribosomal proteins and translation initiation factors that constitute the mTOR complex were highly expressed. PPI analysis using the 33 genes demonstrated that there was a close interaction between ribosome biogenesis, translation, and mTOR signaling. The fluorescence amount of mTOR and MLST8 were higher in CSCs compared to non-CSCs.

CONCLUSIONS

The increase in a number of genes associated with ribosome biogenesis, translation, and mTOR signaling may be important to evaluate prognosis and determine treatment approach for prostate cancer (PCa). A better understanding of the molecular pathways associated with CSCs may be promising to develop targeted therapies to prolong survival in PCa.

A novel rapamycin analog is highly selective for mTORC1 in vivo

Rapamycin, an inhibitor of mechanistic Target Of Rapamycin Complex 1 (mTORC1), extends lifespan and shows strong potential for the treatment of age-related diseases. However, rapamycin exerts metabolic and immunological side effects mediated by off-target inhibition of a second mTOR-containing complex, mTOR complex 2. Here, we report the identification of DL001, a FKBP12-dependent rapamycin analog 40x more selective for mTORC1 than rapamycin. DL001 inhibits mTORC1 in cell culture lines and in vivo in C57BL/6J mice, in which DL001 inhibits mTORC1 signaling without impairing glucose homeostasis and with substantially reduced or no side effects on lipid metabolism and the immune system. In cells, DL001 efficiently represses elevated mTORC1 activity and restores normal gene expression to cells lacking a functional tuberous sclerosis complex. Our results demonstrate that highly selective pharmacological inhibition of mTORC1 can be achieved in vivo, and that selective inhibition of mTORC1 significantly reduces the side effects associated with conventional rapalogs.

mTOR as a central regulator of lifespan and aging

The mammalian/mechanistic target of rapamycin (mTOR) is a key component of cellular metabolism that integrates nutrient sensing with cellular processes that fuel cell growth and proliferation. Although the involvement of the mTOR pathway in regulating life span and aging has been studied extensively in the last decade, the underpinning mechanisms remain elusive. In this review, we highlight the emerging insights that link mTOR to various processes related to aging, such as nutrient sensing, maintenance of proteostasis, autophagy, mitochondrial dysfunction, cellular senescence, and decline in stem cell function.

mTOR Signaling Pathway Regulates Sperm Quality in Older Men

Understanding how age affects fertility becomes increasingly relevant as couples delay childbearing toward later stages of their lives. While the influence of maternal age on fertility is well established, the impact of paternal age is poorly characterized. Thus, this study aimed to understand the molecular mechanisms responsible for age-dependent decline in spermatozoa quality. To attain it, we evaluated the impact of male age on the activity of signaling proteins in two distinct spermatozoa populations: total spermatozoa fraction and highly motile/viable fraction. In older men, we observed an inhibition of the mechanistic target of rapamycin complex 1 (mTORC1) in the highly viable spermatozoa population. On the contrary, when considering the entire spermatozoa population (including defective/immotile/apoptotic cells) our findings support an active mTORC1 signaling pathway in older men. Additionally, total spermatozoa fractions of older men presented increased levels of apoptotic/stress markers [e.g., cellular tumor antigen p53 (TP53)] and mitogen-activated protein kinases (MAPKs) activity. Moreover, we established that the levels of most signaling proteins analyzed were consistently and significantly altered in men older than 27 years of age. This study was the first to associate the mTOR signaling pathway with the age impact on spermatozoa quality. Additionally, we constructed a network of the sperm proteins associated with male aging, identifying TP53 as a central player in spermatozoa aging.

RNA processing in skeletal muscle biology and disease

RNA processing encompasses the capping, cleavage, polyadenylation and alternative splicing of pre-mRNA. Proper muscle development relies on precise RNA processing, driven by the coordination between RNA-binding proteins. Recently, skeletal muscle biology has been intensely investigated in terms of RNA processing. High throughput studies paired with deletion of RNA-binding proteins have provided a high-level understanding of the molecular mechanisms controlling the regulation of RNA-processing in skeletal muscle. Furthermore, misregulation of RNA processing is implicated in muscle diseases. In this review, we comprehensively summarize recent studies in skeletal muscle that demonstrated: (i) the importance of RNA processing, (ii) the RNA-binding proteins that are involved, and (iii) diseases associated with defects in RNA processing.

Silent Information Regulator 1 Negatively Regulates Atherosclerotic Angiogenesis via Mammalian Target of Rapamycin Complex 1 Signaling Pathway

BACKGROUND

This study aimed to investigate the interactions between silent information regulator 1 (SIRT1) and mammalian target of rapamycin (mTOR) in intraplaque angiogenesis and their potential mechanisms through in vivo and in vitro studies.

METHODS

An atherosclerosis model was established in 12 rabbits on a high-cholesterol diet. The rabbits were equally divided into 3 groups: a control group (high-lipid diet), RAP group (high-lipid diet supplemented with rapamycin) and RAP + NAM group (high-lipid diet supplemented with rapamycin and nicotinamide). At the end of 4 weeks, the area of plaques in the aorta was determined and the protein expression of CD31 and vascular endothelial growth factor (VEGF) was detected through hematoxylin and eosin staining and immunohistochemical staining, respectively. For in vitro study, a hypoxia model was established in human umbilical vein endothelial cells (HUVECs) by using the chemical method (CoCl₂). The MTT assay, scratch assay and tube formation assay were performed to evaluate the proliferation and angiogenesis abilities of HUVECs. Reverse transcription polymerase chain reaction was used to examine the mRNA levels of SIRT1, hypoxia-inducible factor-1α (HIF-1α), mTOR and p70 ribosomal S6 kinase (p70S6K). Western blotting was used to examine the protein levels of SIRT1, HIF-1α, mTOR, p-mTOR, p-raptor and p-p70S6K.

RESULTS

The results of the in vivo study indicated a significant inhibitory effect of rapamycin on plaque size and intraplaque angiogenesis (0.05 ± 0.02mm² versus 5.44 ± 0.50mm², P < 0.05). This effect was attenuated by nicotinamide (0.76 ± 0.15mm² versus 0.05 ± 0.02mm², P < 0.05). Compared with the RAP group, CD31- and VEGF-positive vessels were abundant in the RAP + NAM group. The RAP group showed lower expression of p-mTOR, p-p70S6K and HIF-1α than did the control group (P < 0.05), whereas the RAP + NAM group showed slightly higher expression of these factors than did the RAP group (P < 0.05). Furthermore, in vitro studies revealed that the inhibitory effect of rapamycin on the angiogenic ability of HUVECs and its significant inhibitory effects on the protein level of HIF-1α and the phosphorylation of proteins involved in the mTORC1 pathway, including mTOR, raptor and p70S6K (P < 0.05), were enhanced by cotreatment with SRT1720 and rapamycin (P < 0.05). In contrast to mTOR and SIRT1, the mRNA levels of p70S6K and HIF-1α were reduced by rapamycin (P < 0.05) and further reduced by cotreatment with SRT1720 and rapamycin.

CONCLUSIONS

The study results indicate that SIRT1 might negatively regulate atherosclerotic angiogenesis via mTORC1 and HIF-1α signaling pathway and cointervention of SIRT1 and mTOR may serve as a crucial therapeutic strategy in cardiovascular medicine.

mTOR signalling: jack-of-all-trades 1

The mechanistic target of rapamycin (mTOR) is an evolutionarily conserved serine/threonine kinase that senses and integrates environmental information into cellular regulation and homeostasis. Accumulating evidence has suggested a master role of mTOR signalling in many fundamental aspects of cell biology and organismal development. mTOR deregulation is implicated in a broad range of pathological conditions, including diabetes, cancer, neurodegenerative diseases, myopathies, inflammatory, infectious, and autoimmune conditions. Here, we review recent advances in our knowledge of mTOR signalling in mammalian physiology. We also discuss the impact of mTOR alteration in human diseases and how targeting mTOR function can treat human diseases.

Akt1 deficiency diminishes skeletal muscle hypertrophy by reducing satellite cell proliferation

Skeletal muscle mass is determined by the net dynamic balance between protein synthesis and degradation. Although the Akt/mechanistic target of rapamycin (mTOR)-dependent pathway plays an important role in promoting protein synthesis and subsequent skeletal muscle hypertrophy, the precise molecular regulation of mTOR activity by the upstream protein kinase Akt is largely unknown. In addition, the activation of satellite cells has been indicated as a key regulator of muscle mass. However, the requirement of satellite cells for load-induced skeletal muscle hypertrophy is still under intense debate. In this study, female germline Akt1 knockout (KO) mice were used to examine whether Akt1 deficiency attenuates load-induced skeletal muscle hypertrophy through suppressing mTOR-dependent signaling and satellite cell proliferation. Akt1 KO mice showed a blunted hypertrophic response of skeletal muscle, with a diminished rate of satellite cell proliferation following mechanical overload. In contrast, Akt1 deficiency did not affect the load-induced activation of mTOR signaling and the subsequent enhanced rate of protein synthesis in skeletal muscle. These observations suggest that the load-induced activation of mTOR signaling occurs independently of Akt1 regulation and that Akt1 plays a critical role in regulating satellite cell proliferation during load-induced muscle hypertrophy.

Autophagy induced by cardamonin is associated with mTORC1 inhibition in SKOV3 cells

BACKGROUND

The mammalian target of rapamycin (mTOR) integrates energy level to modulate cell proliferation and autophagy. Cardamonin exhibits anti-proliferative activity through inhibiting mTOR. In this study, the effect of cardamonin on autophagy and its mechanism on mTOR inhibition were investigated.

METHODS

Cell viability and proliferation were measured by MTT assay and BrdU incorporation, respectively. Cell apoptosis was assayed by flow cytometry and cell autophagy was detected by electron microscopy and GFP-LC3 fluorescence. The mechanism of cardamonin on mTORC1 inhibition was investigated by Raptor siRNA and Raptor over-expression.

RESULTS

The cell viability and proliferation were inhibited by cardamonin. The autophagosomes and the protein level of LC3-II were increased by cardamonin. Cell apoptosis and the levels of cleaved PARP and Caspase-3 were increased by cardamonin. Cardamonin inhibited the phosphorylation of mTOR and ribosome S6 protein kinase 1 (S6K1) as well as the protein level of regulatory associated protein of mTOR (Raptor). However, cardamonin had no effect on the component of mTORC2 and its downstream substrate Akt. The inhibitory effect of cardamonin on the phosphorylation of mTOR and S6K1 was eliminated by Raptor knockdown with siRNA, whereas this effect of cardamonin was stronger than that of rapamycin and AZD8055 in Raptor over-expression cells. Cell viability was inhibited by cardamonin in both Raptor knockdown and Raptor over-expression cells, which was consistent with the inhibitory effect of cardamonin on mTOR.

CONCLUSION

These findings demonstrated that the autophagy induced by cardamonin was associated with mTORC1 inhibition through decreasing the protein level of Raptor in SKOV3 cells.

Raptor mediates the antiproliferation of cardamonin by mTORC1 inhibition in SKOV3 cells

Purpose

Cardamonin inhibits the proliferation of SKOV3 cells by suppressing the mammalian target of rapamycin complex 1 (mTORC1). However, the mechanism of cardamonin on mTORC1 inhibition has not been well demonstrated. The regulatory-associated protein of TOR (Raptor) is an essential component of mTORC1. Here, we investigated the role of Raptor in the mTORC1 inhibition effect of cardamonin in SKOV3 cells.

Methods

The expression of Raptor was knockdown by small interfering RNA (siRNA). The expressions of specific binding proteins of mTORC1 were analyzed by Western blotting, and the cell proliferation was detected by methyl thiazolyl tetrazolium (MTT) assay.

Results

Rapamycin, AZD8055, and cardamonin inhibited the activity of mammalian target of rapamycin (mTOR). Different from rapamycin and AZD8055, cardamonin suppressed the phosphorylation and protein expression of Raptor. Transfected with Raptor siRNA, the mTOR activation and proliferation of SKOV3 cells were decreased, and these effects were strengthened by cardamonin in Raptor siRNA SKOV3 cells. Cardamonin interfered with the lysosomal colocalization of mTOR with lysosomal associated membrane protein 2 (LAMP2), which was also hindered by Raptor siRNA. Furthermore, cardamonin strengthened the inhibitory effect on the lysosomal localization of mTOR in Raptor siRNA cells.

Conclusion

Our results suggested that Raptor mainly mediated the inhibition of cardamonin on mTORC1 in SKOV3 cells.

Contraction mode itself does not determine the level of mTORC1 activity in rat skeletal muscle

Resistance training with eccentric contraction has been shown to augment muscle hypertrophy more than other contraction modes do (i.e., concentric and isometric contraction). However, the molecular mechanisms involved remain unclear. The purpose of this study was to investigate the effect of muscle contraction mode on mammalian target of rapamycin complex 1 (mTORC1) signaling using a standardized force‐time integral (load (weight) × contraction time). Male Sprague–Dawley rats were randomly assigned to three groups: eccentric contraction, concentric contraction, and isometric contraction. The right gastrocnemius muscle was exercised via percutaneous electrical stimulation‐induced maximal contraction. In experiment 1, different modes of muscle contraction were exerted using the same number of reps in all groups, while in experiment 2, muscle contractions were exerted using a standardized force‐time integral. Muscle samples were obtained immediately and 3 h after exercise. Phosphorylation of molecules associated with mTORC1 activity was assessed using western blot analysis. In experiment 1, the force‐time integral was significantly different among contraction modes with a higher force‐time integral for eccentric contraction compared to that for other contraction modes (P < 0.05). In addition, the force‐time integral was higher for concentric contraction compared to that for isometric contraction (P < 0.05). Similarly, p70S6K phosphorylation level was higher for eccentric contraction than for other modes of contraction (P < 0.05), and concentric contraction was higher than isometric contraction (P < 0.05) 3 h after exercise. In experiment 2, under the same force‐time integral, p70S6K (Thr389) and 4E‐BP1 phosphorylation levels were similar among contraction modes 3 h after exercise. Our results suggest that mTORC1 activity is not determined by differences in muscle contraction mode itself. Instead, mTORC1 activity is determined by differences in the force‐time integral during muscle contraction.

A map of the phosphoproteomic alterations that occur after a bout of maximal-intensity contractions

KEY POINTS

Mechanical signals play a critical role in the regulation of muscle mass, but the molecules that sense mechanical signals and convert this stimulus into the biochemical events that regulate muscle mass remain ill-defined. Here we report a mass spectrometry-based workflow to study the changes in protein phosphorylation that occur in mouse skeletal muscle 1 h after a bout of electrically evoked maximal-intensity contractions (MICs). Our dataset provides the first comprehensive map of the MIC-regulated phosphoproteome. Using unbiased bioinformatics approaches, we demonstrate that our dataset leads to the identification of many well-known MIC-regulated signalling pathways, as well as to a plethora of novel MIC-regulated events. We expect that our dataset will serve as a fundamentally important resource for muscle biologists, and help to lay the foundation for entirely new hypotheses in the field.

ABSTRACT

The maintenance of skeletal muscle mass is essential for health and quality of life. It is well recognized that maximal-intensity contractions, such as those which occur during resistance exercise, promote an increase in muscle mass. Yet, the molecules that sense the mechanical information and convert it into the signalling events (e.g. phosphorylation) that drive the increase in muscle mass remain undefined. Here we describe a phosphoproteomics workflow to examine the effects of electrically evoked maximal-intensity contractions (MICs) on protein phosphorylation in mouse skeletal muscle. While a preliminary phosphoproteomics experiment successfully identified a number of MIC-regulated phosphorylation events, a large proportion of these identifications were present on highly abundant myofibrillar proteins. We subsequently incorporated a centrifugation-based fractionation step to deplete the highly abundant myofibrillar proteins and performed a second phosphoproteomics experiment. In total, we identified 5983 unique phosphorylation sites of which 663 were found to be regulated by MIC. GO term enrichment, phosphorylation motif analyses, and kinase-substrate predictions indicated that the MIC-regulated phosphorylation sites were chiefly modified by mTOR, as well as multiple isoforms of the MAPKs and CAMKs. Moreover, a high proportion of the regulated phosphorylation sites were found on proteins that are associated with the Z-disc, with over 74% of the Z-disc proteins experiencing robust changes in phosphorylation. Finally, our analyses revealed that the phosphorylation state of two Z-disc kinases (striated muscle-specific serine/threonine protein kinase and obscurin) was dramatically altered by MIC, and we propose ways these kinases could play a fundamental role in skeletal muscle mechanotransduction.

Proteome-wide Adaptations of Mouse Skeletal Muscles during a Full Month in Space

The safety of space flight is challenged by a severe loss of skeletal muscle mass, strength, and endurance that may compromise the health and performance of astronauts. The molecular mechanisms underpinning muscle atrophy and decreased performance have been studied mostly after short duration flights and are still not fully elucidated. By deciphering the muscle proteome changes elicited in mice after a full month aboard the BION-M1 biosatellite, we observed that the antigravity soleus incurred the greatest changes compared with locomotor muscles. Proteomics data notably suggested mitochondrial dysfunction, metabolic and fiber type switching toward glycolytic type II fibers, structural alterations, and calcium signaling-related defects to be the main causes for decreased muscle performance in flown mice. Alterations of the protein balance, mTOR pathway, myogenesis, and apoptosis were expected to contribute to muscle atrophy. Moreover, several signs reflecting alteration of telomere maintenance, oxidative stress, and insulin resistance were found as possible additional deleterious effects. Finally, 8 days of recovery post flight were not sufficient to restore completely flight-induced changes. Thus in-depth proteomics analysis unraveled the complex and multifactorial remodeling of skeletal muscle structure and function during long-term space flight, which should help define combined sets of countermeasures before, during, and after the flight.

mTOR Signaling in Growth, Metabolism, and Disease

The mechanistic target of rapamycin (mTOR) coordinates eukaryotic cell growth and metabolism with environmental inputs, including nutrients and growth factors. Extensive research over the past two decades has established a central role for mTOR in regulating many fundamental cell processes, from protein synthesis to autophagy, and deregulated mTOR signaling is implicated in the progression of cancer and diabetes, as well as the aging process. Here, we review recent advances in our understanding of mTOR function, regulation, and importance in mammalian physiology. We also highlight how the mTOR signaling network contributes to human disease and discuss the current and future prospects for therapeutically targeting mTOR in the clinic.

Gene polymorphisms in the PI3K/AKT/mTOR signaling pathway contribute to prostate cancer susceptibility in Chinese men

In this hospital-based case-control study of 413 prostate cancer (PCa) cases and 807 cancer-free controls, we investigated the role of functional single nucleotide polymorphisms (SNPs) of pivotal genes in the PI3K/AKT/mTOR pathway. We genotyped 17 SNPs in mTOR, Raptor, AKT1, AKT2, PTEN, and K-ras and found that 4 were associated with PCa susceptibility. Among the variants, the homozygote variant CC genotype of mTOR rs17036508 C>T were associated with higher PCa risk than the wild TT genotypes (adjusted OR = 3.73 (95% CI = 1.75-7.94), P = 0.001). The GT genotype of mTOR rs2295080 G>T was more protective than the TT genotypes (adjusted OR=0.54 (95% CI=0.32-0.91), P=0.020). The distributions of Raptor rs1468033 A>G genotypes differed between cases and controls, especially in subgroups defined by age, BMI, smoking status, and ethnicity. The CT/CC genotypes of AKT2 rs7250897 C>T were associated with an increased risk of PCa, particularly in subgroups of age >71 and BMI >24 kg/m². These findings suggest that SNPs in the PI3K/AKT/mTOR pathway may contribute to the risk of PCa in Chinese men.

The Glial Cell-Derived Neurotrophic Factor (GDNF)-responsive Phosphoprotein Landscape Identifies Raptor Phosphorylation Required for Spermatogonial Progenitor Cell Proliferation

Cytokine-dependent renewal of stem cells is a fundamental requisite for tissue homeostasis and regeneration. Spermatogonial progenitor cells (SPCs) including stem cells support life-long spermatogenesis and male fertility, but pivotal phosphorylation events that regulate fate decisions in SPCs remain unresolved. Here, we described a quantitative mass-spectrometry-based proteomic and phosphoproteomic analyses of SPCs following sustained stimulation with glial cell-derived neurotrophic factor (GDNF), an extrinsic factor supporting SPC proliferation. Stimulated SPCs contained 3382 identified phosphorylated proteins and 12141 phosphorylation sites. Of them, 325 differentially phosphorylated proteins and 570 phosphorylation sites triggered by GDNF were highly enriched for ERK1/2, GSK3, CDK1, and CDK5 phosphorylating motifs. We validated that inhibition of GDNF/ERK1/2-signaling impaired SPC proliferation and increased G2/M cell cycle arrest. Significantly, we found that proliferation of SPCs requires phosphorylation of the mTORC1 component Raptor at Ser⁸⁶³ Tissue-specific deletion of Raptor in mouse germline cells results in impaired spermatogenesis and progressive loss of spermatogonia, but in vitro increased phosphorylation of Raptor by raptor over-expression in SPCs induced a more rapidly growth of SPCs in culture. These findings implicate previously undescribed signaling networks in governing fate decision of SPCs, which is essential for the understanding of spermatogenesis and of potential consequences of pathogenic insult for male infertility.

Identification of mechanically regulated phosphorylation sites on tuberin (TSC2) that control mechanistic target of rapamycin (mTOR) signaling

Mechanistic target of rapamycin (mTOR) signaling is necessary to generate a mechanically induced increase in skeletal muscle mass, but the mechanism(s) through which mechanical stimuli regulate mTOR signaling remain poorly defined. Recent studies have suggested that Ras homologue enriched in brain (Rheb), a direct activator of mTOR, and its inhibitor, the GTPase-activating protein tuberin (TSC2), may play a role in this pathway. To address this possibility, we generated inducible and skeletal muscle-specific knock-out mice for Rheb (iRhebKO) and TSC2 (iTSC2KO) and mechanically stimulated muscles from these mice with eccentric contractions (EC). As expected, the knock-out of TSC2 led to an elevation in the basal level of mTOR signaling. Moreover, we found that the magnitude of the EC-induced activation of mTOR signaling was significantly blunted in muscles from both inducible and skeletal muscle-specific knock-out mice for Rheb and iTSC2KO mice. Using mass spectrometry, we identified six sites on TSC2 whose phosphorylation was significantly altered by the EC treatment. Employing a transient transfection-based approach to rescue TSC2 function in muscles of the iTSC2KO mice, we demonstrated that these phosphorylation sites are required for the role that TSC2 plays in the EC-induced activation of mTOR signaling. Importantly, however, these phosphorylation sites were not required for an insulin-induced activation of mTOR signaling. As such, our results not only establish a critical role for Rheb and TSC2 in the mechanical activation of mTOR signaling, but they also expose the existence of a previously unknown branch of signaling events that can regulate the TSC2/mTOR pathway.

Disrupted autophagy undermines skeletal muscle adaptation and integrity

This review assesses the importance of proteostasis in skeletal muscle maintenance with a specific emphasis on autophagy. Skeletal muscle appears to be particularly vulnerable to genetic defects in basal and induced autophagy, indicating that autophagy is co-substantial to skeletal muscle maintenance and adaptation. We discuss emerging evidence that tension-induced protein unfolding may act as a direct link between mechanical stress and autophagic pathways. Mechanistic links between protein damage, autophagy and muscle hypertrophy, which is also induced by mechanical stress, are still poorly understood. However, some mouse models of muscle disease show ameliorated symptoms upon effective targeting of basal autophagy. These findings highlight the importance of autophagy as therapeutic target and suggest that elucidating connections between protein unfolding and mTOR-dependent or mTOR-independent hypertrophic responses is likely to reveal specific therapeutic windows for the treatment of muscle wasting disorders.

The Mechanistic Target of Rapamycin: The Grand ConducTOR of Metabolism and Aging

Since the discovery that rapamycin, a small molecule inhibitor of the protein kinase mTOR (mechanistic target of rapamycin), can extend the lifespan of model organisms including mice, interest in understanding the physiological role and molecular targets of this pathway has surged. While mTOR was already well known as a regulator of growth and protein translation, it is now clear that mTOR functions as a central coordinator of organismal metabolism in response to both environmental and hormonal signals. This review discusses recent developments in our understanding of how mTOR signaling is regulated by nutrients and the role of the mTOR signaling pathway in key metabolic tissues. Finally, we discuss the molecular basis for the negative metabolic side effects associated with rapamycin treatment, which may serve as barriers to the adoption of rapamycin or similar compounds for the treatment of diseases of aging and metabolism.

Aging Reduces the Activation of the mTORC1 Pathway after Resistance Exercise and Protein Intake in Human Skeletal Muscle: Potential Role of REDD1 and Impaired Anabolic Sensitivity

This study was designed to better understand the molecular mechanisms involved in the anabolic resistance observed in elderly people. Nine young (22 ± 0.1 years) and 10 older (69 ± 1.7 years) volunteers performed a one-leg extension exercise consisting of 10 × 10 repetitions at 70% of their 3-RM, immediately after which they ingested 30 g of whey protein. Muscle biopsies were taken from the vastus lateralis at rest in the fasted state and 30 min after protein ingestion in the non-exercised (Pro) and exercised (Pro+ex) legs. Plasma insulin levels were determined at the same time points. No age difference was measured in fasting insulin levels but the older subjects had a 50% higher concentration than the young subjects in the fed state (p < 0.05). While no difference was observed in the fasted state, in response to exercise and protein ingestion, the phosphorylation state of PKB (p < 0.05 in Pro and Pro+ex) and S6K1 (p = 0.059 in Pro; p = 0.066 in Pro+ex) was lower in the older subjects compared with the young subjects. After Pro+ex, REDD1 expression tended to be higher (p = 0.087) in the older group while AMPK phosphorylation was not modified by any condition. In conclusion, we show that the activation of the mTORC1 pathway is reduced in skeletal muscle of older subjects after resistance exercise and protein ingestion compared with young subjects, which could be partially due to an increased expression of REDD1 and an impaired anabolic sensitivity.

Lewis lung carcinoma regulation of mechanical stretch-induced protein synthesis in cultured myotubes

Mechanical stretch can activate muscle and myotube protein synthesis through mammalian target of rapamycin complex 1 (mTORC1) signaling. While it has been established that tumor-derived cachectic factors can induce myotube wasting, the effect of this catabolic environment on myotube mechanical signaling has not been determined. We investigated whether media containing cachectic factors derived from Lewis lung carcinoma (LLC) can regulate the stretch induction of myotube protein synthesis. C2C12 myotubes preincubated in control or LLC-derived media were chronically stretched. Protein synthesis regulation by anabolic and catabolic signaling was then examined. In the control condition, stretch increased mTORC1 activity and protein synthesis. The LLC treatment decreased basal mTORC1 activity and protein synthesis and attenuated the stretch induction of protein synthesis. LLC media increased STAT3 and AMP-activated protein kinase phosphorylation in myotubes, independent of stretch. Both stretch and LLC independently increased ERK1/2, p38, and NF-κB phosphorylation. In LLC-treated myotubes, the inhibition of ERK1/2 and p38 rescued the stretch induction of protein synthesis. Interestingly, either leukemia inhibitory factor or glycoprotein 130 antibody administration caused further inhibition of mTORC1 signaling and protein synthesis in stretched myotubes. AMP-activated protein kinase inhibition increased basal mTORC1 signaling activity and protein synthesis in LLC-treated myotubes, but did not restore the stretch induction of protein synthesis. These results demonstrate that LLC-derived cachectic factors can dissociate stretch-induced signaling from protein synthesis through ERK1/2 and p38 signaling, and that glycoprotein 130 signaling is associated with the basal stretch response in myotubes.

Defining the Domain Arrangement of the Mammalian Target of Rapamycin Complex Component Rictor Protein

Mammalian target of rapamycin (mTOR) complexes play a pivotal role in the cell. Raptor and Rictor proteins interact with mTOR to form two distinct complexes, mTORC1 and mTORC2, respectively. While the domain structure of Raptor is known, current bioinformatics tools failed to classify the domains in Rictor. Here we focus on identifying specific domains in Rictor by searching for conserved regions. We scanned the pdb structural database and constructed three protein domain datasets. Next we carried out multiple pairwise sequence alignments of the proteins in the domain dataset. By analyzing the z-scores of Rictor sequence similarity to protein sequences in the dataset, we assigned the structural and functional domains of Rictor. We found that, like Raptor, Rictor also has HEAT and WD40 domains, which could be the common motif binding to mTORC. Rictor may also have pleckstrin homology domains, which mediate cellular localization and transmit signals to downstream targets, as well as a domain that is homologous to 50S protein L17 and human 39S protein L17. This putative ribosome binding domain could mediate mTORC2-ribosome interaction.

GSK3-mediated raptor phosphorylation supports amino-acid-dependent mTORC1-directed signalling

Glycogen synthase kinase-3 (GSK3) mediates phosphorylation of raptor on Ser⁸⁵⁹, which crucially supports activation of mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) signalling in response to amino acid availability. GSK3 inhibition is associated with reduced mTORC1 signalling that impacts negatively on cell growth, protein synthesis and promotes cellular autophagy.

The mammalian or mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) is a ubiquitously expressed multimeric protein kinase complex that integrates nutrient and growth factor signals for the co-ordinated regulation of cellular metabolism and cell growth. Herein, we demonstrate that suppressing the cellular activity of glycogen synthase kinase-3 (GSK3), by use of pharmacological inhibitors or shRNA-mediated gene silencing, results in substantial reduction in amino acid (AA)-regulated mTORC1-directed signalling, as assessed by phosphorylation of multiple downstream mTORC1 targets. We show that GSK3 regulates mTORC1 activity through its ability to phosphorylate the mTOR-associated scaffold protein raptor (regulatory-associated protein of mTOR) on Ser⁸⁵⁹. We further demonstrate that either GSK3 inhibition or expression of a S859A mutated raptor leads to reduced interaction between mTOR and raptor and under these circumstances, irrespective of AA availability, there is a consequential loss in phosphorylation of mTOR substrates, such as p70S6K1 (ribosomal S6 kinase 1) and uncoordinated-51-like kinase (ULK1), which results in increased autophagic flux and reduced cellular proliferation.

mLST8 Promotes mTOR-Mediated Tumor Progression

The activity of the mechanistic target of rapamycin (mTOR) is elevated in various types of human cancers, implicating a role in tumor progression. However, the molecular mechanisms underlying mTOR upregulation remain unclear. In this study, we found that the expression of mLST8, a required subunit of both mTOR complex 1 (mTORC1) and complex 2 (mTORC2), was upregulated in several human colon and prostate cancer cell lines and tissues. Knockdown of mLST8 significantly suppressed mTORC1 and mTORC2 complex formation, and it also inhibited tumor growth and invasiveness in human colon carcinoma (HCT116) and prostate cancer (LNCaP) cells. Overexpression of mLST8 induced anchorage-independent cell growth in normal epithelial cells (HaCaT), although mLST8 knockdown had no effect on normal cell growth. mLST8 knockdown reduced mTORC2-mediated phosphorylation of AKT in both cancer and normal cells, whereas it potently inhibited mTORC1-mediated phosphorylation of 4E-BP1 specifically in cancer cells. These results suggest that mLST8 plays distinct roles in normal and cancer cells, depending upon its expression level, and that mLST8 upregulation may contribute to tumor progression by constitutively activating both the mTORC1 and mTORC2 pathways.

Uterine histotroph and conceptus development. I. cooperative effects of arginine and secreted phosphoprotein 1 on proliferation of ovine trophectoderm cells via activation of the PDK1-Akt/PKB-TSC2-MTORC1 signaling cascade

The greatest limitation to reproductive performance in most mammals, including humans, is embryonic mortality, which, in general, claims 20%-40% of the embryos during the peri-implantation period of pregnancy. Both arginine and secreted phosphoprotein 1 (SPP1) are multifunctional molecules that increase significantly in ovine uterine histotroph during early pregnancy. However, little is known about the relationship and underlying mechanisms for synergistic effects of arginine and SPP1, if any, on conceptus (embryo/fetus and associated extraembryonic membranes) development. Therefore, we conducted in vitro experiments using our established ovine trophectoderm cell line (oTr1) isolated from Day 15 ovine conceptuses to determine their proliferative response to individual and synergistic effects of arginine and recombinant SPP1 (rSPP1) that contains an RGD binding sequence. At physiological concentrations, arginine (0.2 mM) stimulated oTr1 cell proliferation 1.7-fold (P < 0.05) at 48 h, whereas rSPP1 (10 ng/ml) had no such effect. However, an additive effect on oTr1 cell proliferation was induced by combination of arginine and SPP1 as compared to the control (2.1-fold increase; P < 0.01), arginine alone (1.3-fold increase; P < 0.05), and rSPP1 alone (1.5-fold increase; P < 0.01). This additive effect was mediated through cooperative activation of the PDK1-Akt/PKB-TSC2-MTORC1 cell signaling cascade. Collectively, results suggest that arginine and SPP1 in histotroph act cooperatively to enhance survival, growth, and development of ovine conceptuses.

Molecular brakes regulating mTORC1 activation in skeletal muscle following synergist ablation

The goal of the current work was to profile positive (mTORC1 activation, autocrine/paracrine growth factors) and negative [AMPK, unfolded protein response (UPR)] pathways that might regulate overload-induced mTORC1 (mTOR complex 1) activation with the hypothesis that a number of negative regulators of mTORC1 will be engaged during a supraphysiological model of hypertrophy. To achieve this, mTORC1-IRS-1/2 signaling, BiP/CHOP/IRE1α, and AMPK activation were determined in rat plantaris muscle following synergist ablation (SA). SA resulted in significant increases in muscle mass of ~4% per day throughout the 21 days of the experiment. The expression of the insulin-like growth factors (IGF) were high throughout the 21st day of overload. However, IGF signaling was limited, since IRS-1 and -2 were undetectable in the overloaded muscle from day 3 to day 9. The decreases in IRS-1/2 protein were paralleled by increases in GRB10 Ser(501/503) and S6K1 Thr(389) phosphorylation, two mTORC1 targets that can destabilize IRS proteins. PKB Ser(473) phosphorylation was higher from 3-6 days, and this was associated with increased TSC2 Thr(939) phosphorylation. The phosphorylation of TSC2 (Thr1345) (an AMPK site) was also elevated, whereas phosphorylation at the other PKB site, Thr(1462), was unchanged at 6 days. In agreement with the phosphorylation of Thr(1345), SA led to activation of AMPKα1 during the initial growth phase, lasting the first 9 days before returning to baseline by day 12. The UPR markers CHOP and BiP were elevated over the first 12 days following ablation, whereas IRE1α levels decreased. These data suggest that during supraphysiological muscle loading at least three potential molecular brakes engage to downregulate mTORC1.

Quantitative phosphoproteomics reveals novel phosphorylation events in insulin signaling regulated by protein phosphatase 1 regulatory subunit 12A

Serine/threonine protein phosphatase 1 regulatory subunit 12A (PPP1R12A) modulates the activity and specificity of the catalytic subunit of protein phosphatase 1, regulating various cellular processes via dephosphorylation. Nonetheless, little is known about phosphorylation events controlled by PPP1R12A in skeletal muscle insulin signaling. Here, we used quantitative phosphoproteomics to generate a global picture of phosphorylation events regulated by PPP1R12A in a L6 skeletal muscle cell line, which were engineered for inducible PPP1R12A knockdown. Phosphoproteomics revealed 3876 phosphorylation sites (620 were novel) in these cells. Furthermore, PPP1R12A knockdown resulted in increased overall phosphorylation in L6 cells at the basal condition, and changed phosphorylation levels for 698 sites (assigned to 295 phosphoproteins) at the basal and/or insulin-stimulated conditions. Pathway analysis on the 295 phosphoproteins revealed multiple significantly enriched pathways related to insulin signaling, such as mTOR signaling and RhoA signaling. Moreover, phosphorylation levels for numerous regulatory sites in these pathways were significantly changed due to PPP1R12A knockdown. These results indicate that PPP1R12A indeed plays a role in skeletal muscle insulin signaling, providing novel insights into the biology of insulin action. This new information may facilitate the design of experiments to better understand mechanisms underlying skeletal muscle insulin resistance and type 2 diabetes.

BIOLOGICAL SIGNIFICANCE

These results identify a large number of potential new substrates of serine/threonine protein phosphatase 1 and suggest that serine/threonine protein phosphatase 1 regulatory subunit 12A indeed plays a regulatory role in multiple pathways related to insulin action, providing novel insights into the biology of skeletal muscle insulin signaling. This information may facilitate the design of experiments to better understand the molecular mechanism responsible for skeletal muscle insulin resistance and associated diseases, such as type 2 diabetes and cardiovascular diseases.

The order of concurrent endurance and resistance exercise modifies mTOR signaling and protein synthesis in rat skeletal muscle

Concurrent training, a combination of endurance (EE) and resistance exercise (RE) performed in succession, may compromise the muscle hypertrophic adaptations induced by RE alone. However, little is known about the molecular signaling interactions underlying the changes in skeletal muscle adaptation during concurrent training. Here, we used an animal model to investigate whether EE before or after RE affects the molecular signaling associated with muscle protein synthesis, specifically the interaction between RE-induced mammalian target of rapamycin complex 1 (mTORC1) signaling and EE-induced AMP-activated protein kinase (AMPK) signaling. Male Sprague-Dawley rats were divided into five groups: an EE group (treadmill, 25 m/min, 60 min), an RE group (maximum isometric contraction via percutaneous electrical stimulation for 3 × 10 s, 5 sets), an EE before RE group, an EE after RE group, and a nonexercise control group. Phosphorylation of p70S6K, a marker of mTORC1 activity, was significantly increased 3 h after RE in both the EE before RE and EE after RE groups, but the increase was smaller in latter. Furthermore, protein synthesis was greatly increased 6 h after RE in the EE before RE group. Increases in the phosphorylation of AMPK and Raptor were observed only in the EE after RE group. Akt and mTOR phosphorylation were increased in both groups, with no between-group differences. Our results suggest that the last bout of exercise dictates the molecular responses and that mTORC1 signaling induced by any prior bout of RE may be downregulated by a subsequent bout of EE.