mTOR Ser-2481 autophosphorylation monitors mTORC-specific catalytic activity and clarifies rapamycin mechanism of action

TORC1 of fission yeast is rapamycin-sensitive

The target of rapamycin (TOR) protein kinase plays central roles in the regulation of cell growth in response to nutritional availability. TOR forms two distinct multiprotein complexes termed TOR complex 1 (TORC1) and TORC2. Typically, only the activity of TORC1 is inhibited by the immunosuppressant rapamycin. Although rapamycin strongly inhibits cell growth of the budding yeast Saccharomyces cerevisiae through inhibition of TORC1, growth of the fission yeast Schizosaccharomyces pombe appears to be resistant to rapamycin. Here, we demonstrate that rapamycin inhibits the kinase activity of S. pombe TORC1 in vitro in a similar manner to TORC1 of other organisms. We furthermore show that incomplete inhibition of TORC1 by rapamycin underlies the apparent rapamycin resistance of S. pombe. In the presence of caffeine, which potentially lowers TORC1 activity, the growth of wild-type S. pombe cells is sensitive to rapamycin in a TORC1-dependent manner. Moreover, treatment of S. pombe cells with rapamycin plus caffeine induces starvation-specific gene expression and autophagy, similarly to cells with reduced TORC1 activity. These results indicate that rapamycin does inhibit TORC1 in S. pombe, but the inhibition is not sufficient to cause a growth defect. These findings establish a universal action of rapamycin on TORC1 inhibition.

Wip1-dependent regulation of autophagy, obesity, and atherosclerosis

Obesity and atherosclerosis-related diseases account for over one-third of deaths in the western world. Controlling these conditions remains a major challenge due to an incomplete understanding of the molecular pathways involved. Here, we show that Wip1 phosphatase, a known negative regulator of Atm-dependent signaling, plays a major role in controlling fat accumulation and atherosclerosis in mice; specifically, Wip1 deficiency prevents both conditions. In the course of atherosclerosis, deletion of Wip1 results in suppression of macrophage conversion into foam cells, thus preventing the formation of atherosclerotic plaques. This process appears to be independent of p53 but rely on a noncanonical Atm-mTOR signaling pathway and on selective autophagy in regulation of cholesterol efflux. We propose that the Wip1-dependent control of autophagy and cholesterol efflux may provide avenues for treating obesity and atherosclerosis.

Chronic pharmacological mGlu5 inhibition corrects fragile X in adult mice

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability. Previous studies have implicated mGlu5 in the pathogenesis of the disease, but a crucial unanswered question is whether pharmacological mGlu5 inhibition is able to reverse an already established FXS phenotype in mammals. Here we have used the novel, potent, and selective mGlu5 inhibitor CTEP to address this issue in the Fmr1 knockout mouse. Acute CTEP treatment corrects elevated hippocampal long-term depression, protein synthesis, and audiogenic seizures. Chronic treatment that inhibits mGlu5 within a receptor occupancy range of 81% ± 4% rescues cognitive deficits, auditory hypersensitivity, aberrant dendritic spine density, overactive ERK and mTOR signaling, and partially corrects macroorchidism. This study shows that a comprehensive phenotype correction in FXS is possible with pharmacological intervention starting in young adulthood, after development of the phenotype. It is of great interest how these findings may translate into ongoing clinical research testing mGlu5 inhibitors in FXS patients.

PRR5L degradation promotes mTORC2-mediated PKC-δ phosphorylation and cell migration downstream of Gα12

Mammalian target of rapamycin complex (MTORC) 2 phosphorylates AGC protein kinases including PKC and regulates cellular functions including cell migration. However, its regulation remains poorly understood. Here we show that LPA induces two phases of PKCδ hydrophobic motif (HM) phosphorylation. The late phase is mediated by Gα12, which specifically activates ARAF, leading to upregulation of the expression of an E3 ubiquitin ligase RFFL and subsequent ubiquitination and degradation of PRR5L. Destabilization of PRR5L, a suppressor of mTORC2-mediated HM phosphorylation of PKCδ, but not AKT, results in PKCδ HM phosphorylation and activation. This Gα12-mediated pathway is critically important for fibroblast migration and pulmonary fibrosis development. Thus, our study unravels a signaling pathway for mTORC2 regulation and fibroblast migration.

InTERTesting association between telomerase, mTOR and phytochemicals

Telomeres are stretches of repeated DNA sequences located at the ends of chromosomes that are necessary to prevent loss of gene-coding DNA regions during replication. Telomerase – the enzyme responsible for immortalising cancer cells through the addition of telomeric repeats – is active in ~90% of human cancers. Telomerase activity is inhibited by various phytochemicals such as isoprenoids, genistein, curcumin, epigallocatechin-3-gallate, resveratrol and others. Human TERT (telomerase reverse transcriptase – the rate-limiting component of telomerase), heat shock protein 90, Akt, p70 S6 kinase (S6K) and mammalian target of rapamycin (mTOR) form a physical and functional complex with one another. The inclusion of Akt, mTOR and S6K in the TERT complex is compelling evidence to support mTOR-mediated control of telomerase activity. This review will define the role of mTOR, the master regulator of protein translation, in telomerase regulation and provide additional insights into the numerous ways in which telomerase activity is hindered by phytochemicals.

A dynamic network model of mTOR signaling reveals TSC-independent mTORC2 regulation

The kinase mammalian target of rapamycin (mTOR) exists in two multiprotein complexes (mTORC1 and mTORC2) and is a central regulator of growth and metabolism. Insulin activation of mTORC1, mediated by phosphoinositide 3-kinase (PI3K), Akt, and the inhibitory tuberous sclerosis complex 1/2 (TSC1-TSC2), initiates a negative feedback loop that ultimately inhibits PI3K. We present a data-driven dynamic insulin-mTOR network model that integrates the entire core network and used this model to investigate the less well understood mechanisms by which insulin regulates mTORC2. By analyzing the effects of perturbations targeting several levels within the network in silico and experimentally, we found that, in contrast to current hypotheses, the TSC1-TSC2 complex was not a direct or indirect (acting through the negative feedback loop) regulator of mTORC2. Although mTORC2 activation required active PI3K, this was not affected by the negative feedback loop. Therefore, we propose an mTORC2 activation pathway through a PI3K variant that is insensitive to the negative feedback loop that regulates mTORC1. This putative pathway predicts that mTORC2 would be refractory to Akt, which inhibits TSC1-TSC2, and, indeed, we found that mTORC2 was insensitive to constitutive Akt activation in several cell types. Our results suggest that a previously unknown network structure connects mTORC2 to its upstream cues and clarifies which molecular connectors contribute to mTORC2 activation.

Remarkable inhibition of mTOR signaling by the combination of rapamycin and 1,4-phenylenebis(methylene)selenocyanate in human prostate cancer cells

Preclinical studies and clinical analyses have implicated the mammalian target of rapamycin (mTOR) pathway in the progression of prostate cancer, suggesting mTOR as a potential target for new therapies. mTOR, a serine/threonine kinase, belongs to two distinct signaling complexes: mTORC1 and mTORC2. We previously showed that the synthetic organoselenium compound, p-XSC, effectively inhibits viability and critical signaling molecules (e.g., androgen receptor, Akt) in androgen responsive (AR) and androgen independent (AI) human prostate cancer cells. On the basis of its inhibition of Akt, we hypothesized that p-XSC modulates mTORC2, an upstream regulator of the kinase. We further hypothesized that combining p-XSC with rapamycin, an mTORC1 inhibitor, would be an effective combinatory strategy for the inhibition of prostate cancer. The effects of p-XSC and rapamycin, alone or in combination, on viability and mTOR signaling were examined in AR LNCaP prostate cancer cells and AI C4-2 and DU145 cells. Phosphorylation of downstream targets of mTORC1 and mTORC2 was analyzed by immunoblotting. The interaction of mTORC1- and mTORC2-specific proteins with mTOR was probed through immunoprecipitation and immunoblotting. p-XSC inhibited phosphorylation of mTORC2 downstream targets, Akt and PCKα, and decreased the levels of rictor, an mTORC2-specific protein, coimmunoprecipitated with mTOR in C4-2 cells. The combination of p-XSC and rapamycin more effectively inhibited viability and mTOR signaling in C4-2, LNCaP and DU145 cells than either agent individually.

Regulation and function of ribosomal protein S6 kinase (S6K) within mTOR signalling networks

The ribosomal protein S6K (S6 kinase) represents an extensively studied effector of the TORC1 [TOR (target of rapamycin) complex 1], which possesses important yet incompletely defined roles in cellular and organismal physiology. TORC1 functions as an environmental sensor by integrating signals derived from diverse environmental cues to promote anabolic and inhibit catabolic cellular functions. mTORC1 (mammalian TORC1) phosphorylates and activates S6K1 and S6K2, whose first identified substrate was rpS6 (ribosomal protein S6), a component of the 40S ribosome. Studies over the past decade have uncovered a number of additional S6K1 substrates, revealing multiple levels at which the mTORC1-S6K1 axis regulates cell physiology. The results thus far indicate that the mTORC1-S6K1 axis controls fundamental cellular processes, including transcription, translation, protein and lipid synthesis, cell growth/size and cell metabolism. In the present review we summarize the regulation of S6Ks, their cellular substrates and functions, and their integration within rapidly expanding mTOR (mammalian TOR) signalling networks. Although our understanding of the role of mTORC1-S6K1 signalling in physiology remains in its infancy, evidence indicates that this signalling axis controls, at least in part, glucose homoeostasis, insulin sensitivity, adipocyte metabolism, body mass and energy balance, tissue and organ size, learning, memory and aging. As dysregulation of this signalling axis contributes to diverse disease states, improved understanding of S6K regulation and function within mTOR signalling networks may enable the development of novel therapeutics.

Modulation of autophagy and ubiquitin-proteasome pathways during ultra-endurance running

In this study, the coordinated activation of ubiquitin-proteasome pathway (UPP), autophagy-lysosomal pathway (ALP), and mitochondrial remodeling including mitophagy was assessed by measuring protein markers during ultra-endurance running exercise in human skeletal muscle. Eleven male, experienced ultra-endurance athletes ran for 24 h on a treadmill. Muscle biopsy samples were taken from the vastus lateralis muscle 2 h before starting and immediately after finishing exercise. Athletes ran 149.8 ± 16.3 km with an effective running time of 18 h 42 min ( ± 41 min). The phosphorylation state of Akt (-74 ± 5%; P < 0.001), FOXO3a (-49 ± 9%; P < 0.001), mTOR Ser2448 (-32 ± 14%; P = 0.028), and 4E-BP1 (-34 ± 7%; P < 0.001) was decreased, whereas AMPK phosphorylation state increased by 247 ± 170% (P = 0.042). Proteasome β2 subunit activity increased by 95 ± 44% (P = 0.028), whereas the activities associated with the β1 and β5 subunits remained unchanged. MuRF1 protein level increased by 55 ± 26% (P = 0.034), whereas MAFbx protein and ubiquitin-conjugated protein levels did not change. LC3bII increased by 554 ± 256% (P = 0.005), and the form of ATG12 conjugated to ATG5 increased by 36 ± 17% (P = 0.042). The mitochondrial fission marker phospho-DRP1 increased by 110 ± 47% (P = 0.003), whereas the fusion marker Mfn1 and the mitophagy markers Parkin and PINK1 remained unchanged. These results fit well with a coordinated regulation of ALP and UPP triggered by FOXO3 and AMPK during ultra-endurance exercise.

The dual mTORC1 and mTORC2 inhibitor AZD8055 has anti-tumor activity in acute myeloid leukemia

The serine/threonine kinase mammalian target of rapamycin (mTOR) is crucial for cell growth and proliferation, and is constitutively activated in primary acute myeloid leukemia (AML) cells, therefore representing a major target for drug development in this disease. We show here that the specific mTOR kinase inhibitor AZD8055 blocked mTORC1 and mTORC2 signaling in AML. Particularly, AZD8055 fully inhibited multisite eIF4E-binding protein 1 phosphorylation, subsequently blocking protein translation, which was in contrast to the effects of rapamycin. In addition, the mTORC1-dependent PI3K/Akt feedback activation was fully abrogated in AZD8055-treated AML cells. Significantly, AZD8055 decreased AML blast cell proliferation and cell cycle progression, reduced the clonogenic growth of leukemic progenitors and induced caspase-dependent apoptosis in leukemic cells but not in normal immature CD34+ cells. Interestingly, AZD8055 strongly induced autophagy, which may be either protective or cell death inducing, depending on concentration. Finally, AZD8055 markedly increased the survival of AML transplanted mice through a significant reduction of tumor growth, without apparent toxicity. Our current results strongly suggest that AZD8055 should be tested in AML patients in clinical trials.

Toll-like receptor 4 is up-regulated by mTOR activation during THP-1 macrophage foam cells formation

Macrophage foam cells formation is the most important process in atherosclerotic plaque formation and development. Toll-like receptor 4 (TLR4) is one of the important innate immune sensors of endogenous damage signals and crucial for regulating inflammation. Growing evidence indicates that TLR4 plays a very important role in macrophage foam cells formation. However, the underlying mechanisms regulating TLR4 expression in macrophage are not fully understood. In this study, we induced THP-1 macrophage foam cells formation with oxidative modified low-density lipoprotein (ox-LDL). We observed that TLR4 mRNA and protein expression were markedly up-regulated, and the phosphorylation of mammalian target of rapamycin (mTOR) and its downstream target p70S6K were promoted during foam cells formation. The mTOR inhibitor rapamycin blocked mTOR phosphorylation and inhibited TLR4 expression induced by ox-LDL. Silencing mTOR, rictor or raptor protein expression by small interfering RNA, also inhibited the up-regulation of TLR4 expression, respectively. Inhibition of mTOR with rapamycin reversed the down-regulation of cellular lipid efflux mediator ABCA1, which resulted from the activation of TLR4 by ligands. These data suggested that TRL4 expression was up-regulated by a mechanism dependent on mTOR signal pathway activation during THP-1 macrophage foam cells formation. Inhibition of ox-LDL induced mTOR activation reduced TLR4 expression, and improved the impaired lipid efflux.

Mammalian TOR signaling to the AGC kinases

The mechanistic (or mammalian) target of rapamycin (mTOR), an evolutionarily conserved protein kinase, orchestrates cellular responses to growth, metabolic and stress signals. mTOR processes various extracellular and intracellular inputs as part of two mTOR protein complexes, mTORC1 or mTORC2. The mTORCs have numerous cellular targets but members of a family of protein kinases, the protein kinase (PK)A/PKG/PKC (AGC) family are the best characterized direct mTOR substrates. The AGC kinases control multiple cellular functions and deregulation of many members of this family underlies numerous pathological conditions. mTOR phosphorylates conserved motifs in these kinases to allosterically augment their activity, influence substrate specificity, and promote protein maturation and stability. Activation of AGC kinases in turn triggers the phosphorylation of diverse, often overlapping, targets that ultimately control cellular response to a wide spectrum of stimuli. This review will highlight recent findings on how mTOR regulates AGC kinases and how mTOR activity is feedback regulated by these kinases. We will discuss how this regulation can modulate downstream targets in the mTOR pathway that could account for the varied cellular functions of mTOR.

Proteomic analysis shows synthetic oleanane triterpenoid binds to mTOR

New multifunctional drugs that target multiple disease-relevant networks offer a novel approach to the prevention and treatment of many diseases. New synthetic oleanane triterpenoids (SO), such as CDDO (2-cyano-3,12-dioxooleana-1,9-dien-28-oic acid) and its derivatives, are multifunctional compounds originally developed for the prevention and treatment of inflammation and oxidative stress. However, the protein binding partners and mechanisms of action of these SO are not yet fully understood. Here we characterize the putative target profile of one SO, CDDO-Imidazolide (CDDO-Im), by combining affinity purification with mass spectroscopic proteomic analysis to identify 577 candidate binding proteins in whole cells. This SO pharmaco-interactome consists of a diverse but interconnected set of signaling networks; bioinformatic analysis of the protein interactome identified canonical signaling pathways targeted by the SO, including retinoic acid receptor (RAR), estrogen receptor (ER), insulin receptor (IR), janus kinase/signal transducers and activators of transcription (JAK/STAT), and phosphatase and tensin homolog (PTEN). Pull-down studies then further validated a subset of the putative targets. In addition, we now show for the first time that the mammalian target of rapamycin (mTOR) is a direct target of CDDO-Im. We also show that CDDO-Im blocks insulin-induced activation of this pathway by binding to mTOR and inhibiting its kinase activity. Our basic studies confirm that the SO, CDDO-Im, acts on a protein network to elicit its pharmacological activity.

mTOR complex 2 signaling and functions

The mechanistic target of rapamycin (mTOR) plays a central role in cellular growth and metabolism. mTOR forms two distinct protein complexes, mTORC1 and mTORC2. Much is known about the regulation and functions of mTORC1 due to availability of a natural compound, rapamycin, that inhibits this complex. Studies that define mTORC2 cellular functions and signaling have lagged behind. The development of pharmacological inhibitors that block mTOR kinase activity, and thereby inhibit both mTOR complexes, along with availability of mice with genetic knockouts in mTOR complex components have now provided new insights on mTORC2 function and regulation. Since prolonged effects of rapamycin can also disrupt mTORC2, it is worth re-evaluating the contribution of this less-studied mTOR complex in cancer, metabolic disorders and aging. In this review, we focus on recent developments on mammalian mTORC2 signaling mechanisms and its cellular and tissue-specific functions.

Phosphatidic acid activates mammalian target of rapamycin complex 1 (mTORC1) kinase by displacing FK506 binding protein 38 (FKBP38) and exerting an allosteric effect

Phosphatidic acid (PA) is a critical mediator of mitogenic activation of mammalian target of rapamycin complex 1 (mTORC1) signaling, a master regulator of mammalian cell growth and proliferation. The mechanism by which PA activates mTORC1 signaling has remained unknown. Here, we report that PA selectively stimulates mTORC1 but not mTORC2 kinase activity in cells and in vitro. Furthermore, we show that PA competes with the mTORC1 inhibitor, FK506 binding protein 38 (FKBP38), for mTOR binding at a site encompassing the rapamycin-FKBP12 binding domain. This leads to PA antagonizing FKBP38 inhibition of mTORC1 kinase activity in vitro and rescuing mTORC1 signaling from FKBP38 in cells. Phospholipase D 1, a PA-generating enzyme that is an established upstream regulator of mTORC1, is found to negatively affect mTOR-FKBP38 interaction, confirming the role of endogenous PA in this regulation. Interestingly, removal of FKBP38 alone is insufficient to activate mTORC1 kinase and signaling, which require PA even when the FKBP38 level is drastically reduced by RNAi. In conclusion, we propose a dual mechanism for PA activation of mTORC1: PA displaces FKBP38 from mTOR and allosterically stimulates the catalytic activity of mTORC1.

cAMP inhibits mammalian target of rapamycin complex-1 and -2 (mTORC1 and 2) by promoting complex dissociation and inhibiting mTOR kinase activity

cAMP and mTOR signalling pathways control a number of critical cellular processes including metabolism, protein synthesis, proliferation and cell survival and therefore understanding the signalling events which integrate these two signalling pathways is of particular interest. In this study, we show that the pharmacological elevation of [cAMP](i) in mouse embryonic fibroblasts (MEFs) and human embryonic kidney 293 (HEK293) cells inhibits mTORC1 activation via a PKA-dependent mechanism. Although the inhibitory effect of cAMP on mTOR could be mediated by impinging on signalling cascades (i.e. PKB, MAPK and AMPK) that inhibit TSC1/2, an upstream negative regulator of mTORC1, we show that cAMP inhibits mTORC1 in TSC2 knockout (TSC2(-/-)) MEFs. We also show that cAMP inhibits insulin and amino acid-stimulated mTORC1 activation independently of Rheb, Rag GTPases, TSC2, PKB, MAPK and AMPK, indicating that cAMP may act independently of known regulatory inputs into mTOR. Moreover, we show that the prolonged elevation in [cAMP](i) can also inhibit mTORC2. We provide evidence that this cAMP-dependent inhibition of mTORC1/2 is caused by the dissociation of mTORC1 and 2 and a reduction in mTOR catalytic activity, as determined by its auto-phosphorylation on Ser2481. Taken together, these results provide an important insight into how cAMP signals to mTOR and down-regulates its activity, which may lead to the identification of novel drug targets to inhibit mTOR that could be used for the treatment and prevention of human diseases such as cancer.

ULK1 inhibits mTORC1 signaling, promotes multisite Raptor phosphorylation and hinders substrate binding

Protein synthesis and autophagy work as two opposing processes to control cell growth in response to nutrient supply. The mammalian/mechanistic target of rapamycin complex 1 (mTORC1) pathway, which acts as a master regulator to control protein synthesis, has recently been shown to inhibit autophagy by phosphorylating and inactivating ULK1, an autophagy regulatory protein. ULK1 also inhibits phosphorylation of a mTORC1 substrate, S6K1, indicating that a complex signaling interplay exists between mTORC1 and ULK1. Here, we demonstrate that ULK1 induces multisite phosphorylation of Raptor in vivo and in vitro. Using phospho-specific antibodies we identify Ser855 and Ser859 as being strongly phosphorylated by ULK1, with moderate phosphorylation of Ser792 also observed. Interestingly, ULK1 overexpression also increases phosphorylation of Raptor Ser863 and the mTOR autophosphorylation site, Ser2481 in a mTORC1-dependent manner. Despite this evidence for heightened mTORC1 kinase activity following ULK1 overexpresssion, mTORC1-mediated phosphorylation of S6K1 and 4E-BP1 is significantly inhibited. ULK1 expression has no effect on protein-protein interactions between the components of mTORC1, but does reduce the ability of Raptor to bind to the substrate 4E-BP1. Furthermore, shRNA knockdown of ULK1 leads to increased phosphorylation of mTORC1 substrates and decreased phosphorylation of Raptor at Ser859 and Ser792. We propose a new mechanism whereby ULK1 contributes to mTORC1 inhibition through hindrance of substrate docking to Raptor. This is a novel negative feedback loop that occurs upon activation of autophagy to maintain mTORC1 inhibition when nutrient supplies are limiting.

Mammalian target of rapamycin: a central node of complex signaling cascades

The mammalian target of rapamycin (mTOR) is a serine/threonine kinase that regulates cell growth and metabolism in response to diverse external stimuli. In the presence of mitogenic stimuli, mTOR transduces signals that activate the translational machinery and promote cell growth. mTOR functions as a central node in a complex net of signaling pathways that are involved both in normal physiological, as well as pathogenic events. mTOR signaling occurs in concert with upstream Akt and tuberous sclerosis complex (TSC) and several downstream effectors. During the past few decades, the mTOR-mediated pathway has been shown to promote tumorigenesis through the coordinated phosphorylation of proteins that directly regulate cell-cycle progression and metabolism, as well as transcription factors that regulate the expression of genes involved in the oncogenic processes. The importance of mTOR signaling in oncology is now widely accepted, and agents that selectively target mTOR have been developed as anti-cancer drugs. In this review, we highlight the past research on mTOR, including clinical and pathological analyses, and describe its molecular mechanisms of signaling, and its roles in the physiology and pathology of human diseases, particularly, lung carcinomas. We also discuss strategies that might lead to more effective clinical treatments of several diseases by targeting mTOR.

Granulovacuolar degeneration (GVD) bodies of Alzheimer's disease (AD) resemble late-stage autophagic organelles

AIMS

Granulovacuolar degeneration involves the accumulation of large, double membrane-bound bodies within certain neurones during the course of Alzheimer's disease (AD) and other adult-onset dementias. Because of the two-layer membrane morphology, it has been proposed that the bodies are related to autophagic organelles. The aim of this study was to test this hypothesis, and determine the approximate stage at which the pathway stalls in AD.

METHODS

Spatial colocalization of autophagic and endocytic markers with casein kinase 1 delta, a marker for granulovacuolar degeneration (GVD) bodies, was evaluated in hippocampal sections prepared from post mortem Braak stage IV and V AD cases using double-label confocal fluorescence microscopy.

RESULTS

GVD bodies colocalized weakly with early-stage autophagy markers LC3 and p62, but strongly with late-stage marker lysosome-associated membrane protein 1 (LAMP1), which decorated their surrounding membranes. GVD bodies also colocalized strongly with charged multivesicular body protein 2B (CHMP2B), which colocalized with the core granule, but less strongly with lysosomal marker cathepsin D.

CONCLUSIONS

The resultant immunohistochemical signature suggests that granulovacuolar degeneration bodies (GVBs) do contain late-stage autophagic markers, and accumulate at the nexus of autophagic and endocytic pathways. The data further suggest that failure to complete autolysosome formation may be an important correlate of GVB accumulation.

mTOR kinase domain phosphorylation promotes mTORC1 signaling, cell growth, and cell cycle progression

The mammalian target of rapamycin complex 1 (mTORC1) functions as an environmental sensor to promote critical cellular processes such as protein synthesis, cell growth, and cell proliferation in response to growth factors and nutrients. While diverse stimuli regulate mTORC1 signaling, the direct molecular mechanisms by which mTORC1 senses and responds to these signals remain poorly defined. Here we investigated the role of mTOR phosphorylation in mTORC1 function. By employing mass spectrometry and phospho-specific antibodies, we demonstrated novel phosphorylation on S2159 and T2164 within the mTOR kinase domain. Mutational analysis of these phosphorylation sites indicates that dual S2159/T2164 phosphorylation cooperatively promotes mTORC1 signaling to S6K1 and 4EBP1. Mechanistically, S2159/T2164 phosphorylation modulates the mTOR-raptor and raptor-PRAS40 interactions and augments mTORC1-associated mTOR S2481 autophosphorylation. Moreover, mTOR S2159/T2164 phosphorylation promotes cell growth and cell cycle progression. We propose a model whereby mTOR kinase domain phosphorylation modulates the interaction of mTOR with regulatory partner proteins and augments intrinsic mTORC1 kinase activity to promote biochemical signaling, cell growth, and cell cycle progression.

Effects of rapamycin and TOR on aging and memory: implications for Alzheimer's disease

Rapamycin is a macrolide immunosuppressant drug, originally used as an anti-fungal agent, which is widely used in transplantation medicine to prevent organ rejection. Target of rapamycin (TOR) is an evolutionarily conserved serine/threonine kinase with pleiotropic cellular functions, regulating processes such as growth and metabolism, cell survival, transcription and autophagy. TOR intervenes in two distinct enzymatic complexes with different functions, a rapamycin-sensitive complex TORC1 and a rapamycin-insensitive complex TORC2. Rapamycin has an inhibitory effect on TORC1 activity and it has been suggested to increase life span, an effect correlated with decreased protein biosynthesis and autophagy activation. In the CNS, rapamycin shows beneficial effects in neuronal survival and plasticity, thus contributing to memory improvement. In this review, evidence implying rapamycin and TOR in aging/life span extension and memory improvement will be discussed. Recent findings about the effects of rapamycin on Alzheimer's disease-associated neuropathology will be also discussed.

Neutral sphingomyelinase-2 mediates growth arrest by retinoic acid through modulation of ribosomal S6 kinase

All-trans-retinoic acid (ATRA) induces growth arrest of many cell types. Previous studies have reported that ATRA can modulate cellular sphingolipids, but the role of sphingolipids in the ATRA response is not clear. Using MCF-7 cells as a model system, we show that ATRA stimulates an increase in ceramide levels followed by G(0)/G(1) growth arrest. Notably, induction of nSMase2 was the primary effect of ATRA on the sphingolipid network and was both time- and dose-dependent. Importantly, pretreatment with nSMase2 siRNA significantly inhibited ATRA effects on ceramide levels and growth arrest. In contrast, nSMase2 overexpression was sufficient to increase ceramide levels and induce G(0)/G(1) growth arrest of asynchronous MCF-7 cells. Surprisingly, neither ATRA stimulation nor nSMase2 overexpression had significant effects on classical cell cycle regulators such as p21/WAF1 or retinoblastoma. In contrast, ATRA suppressed phosphorylation of ribosomal S6 kinase (S6K) and its downstream targets S6 and eIF4B. Importantly, these effects were significantly inhibited by nSMase2 siRNA. Reciprocally, nSMase2 overexpression was sufficient to suppress S6K phosphorylation and signaling. Notably, neither ATRA effects nor nSMase2 effects on S6K phosphorylation required the ceramide-activated protein phosphatase PP2A, previously identified as important for S6K regulation. Finally, nSMase2 overexpression was sufficient to decrease translation as measured by methionine incorporation and analysis of polyribosome profiles. Taken together, these results implicate nSMase2 as a major component of ATRA-induced growth arrest of MCF-7 cells and identify S6K as a novel downstream target of nSMase2.

mTORC2 is required for proliferation and survival of TSC2-null cells

Mutational inactivation of the tumor suppressor tuberous sclerosis complex 2 (TSC2) constitutively activates mTORC1, increases cell proliferation, and induces the pathological manifestations observed in tuberous sclerosis (TS) and in pulmonary lymphangioleiomyomatosis (LAM). While the role of mTORC1 in TSC2-dependent growth has been extensively characterized, little is known about the role of mTORC2. Our data demonstrate that mTORC2 modulates TSC2-null cell proliferation and survival through RhoA GTPase and Bcl2 proteins. TSC2-null cell proliferation was inhibited not only by reexpression of TSC2 or small interfering RNA (siRNA)-induced downregulation of Rheb, mTOR, or raptor, but also by siRNA for rictor. Increased RhoA GTPase activity and P-Ser473 Akt were inhibited by siRNA for rictor. Importantly, constitutively active V14RhoA reversed growth inhibition induced by siRNA for rictor, siRNA TSC1, reexpression of TSC2, or simvastatin. While siRNA for RhoA had a modest effect on growth inhibition, downregulation of RhoA markedly increased TSC2-null cell apoptosis. Inhibition of RhoA activity downregulated antiapoptotic Bcl2 and upregulated proapoptotic Bim, Bok, and Puma. In vitro and in vivo, simvastatin alone or in combination with rapamycin inhibited cell growth and induced TSC2-null cell apoptosis, abrogated TSC2-null tumor growth, improved animal survival, and prevented tumor recurrence by inhibiting cell growth and promoting apoptosis. Our data demonstrate that mTORC2-dependent activation of RhoA is required for TSC2-null cell growth and survival and suggest that targeting both mTORC2 and mTORC1 by a combination of proapoptotic simvastatin and cytostatic rapamycin shows promise for combinational therapeutic intervention in diseases with TSC2 dysfunction.

A comprehensive map of the mTOR signaling network

The mammalian target of rapamycin (mTOR) is a central regulator of cell growth and proliferation. mTOR signaling is frequently dysregulated in oncogenic cells, and thus an attractive target for anticancer therapy. Using CellDesigner, a modeling support software for graphical notation, we present herein a comprehensive map of the mTOR signaling network, which includes 964 species connected by 777 reactions. The map complies with both the systems biology markup language (SBML) and graphical notation (SBGN) for computational analysis and graphical representation, respectively. As captured in the mTOR map, we review and discuss our current understanding of the mTOR signaling network and highlight the impact of mTOR feedback and crosstalk regulations on drug-based cancer therapy. This map is available on the Payao platform, a Web 2.0 based community-wide interactive process for creating more accurate and information-rich databases. Thus, this comprehensive map of the mTOR network will serve as a tool to facilitate systems-level study of up-to-date mTOR network components and signaling events toward the discovery of novel regulatory processes and therapeutic strategies for cancer.

mTOR signaling, function, novel inhibitors, and therapeutic targets

Mammalian target of rapamycin (mTOR) is an evolutionally conserved serine/threonine kinase that integrates signals from multiple pathways, including nutrients (e.g., amino acids and glucose), growth factors (e.g., insulin and insulinlike growth factor 1), hormones (e.g., leptin), and stresses (e.g., starvation, hypoxia, and DNA damage) to regulate a wide variety of eukaryotic cellular functions, such as translation, transcription, protein turnover, cell growth, differentiation, cell survival, metabolism, energy balance, and stress response. Dysregulation of the mTOR pathway is closely associated with cancers and other human diseases. Thus, mTOR is of considerable interest in view of its potential as a therapeutic drug target. However, only limited success has been achieved in clinical applications of mTOR inhibitors because of the inherent complexity in the regulation and function of mTOR. Emerging new developments in this area, such as novel readouts (potential biomarkers) for mTOR activity, dynamic assembly and translocation of the mTOR complex, cross-regulation between mTOR complex 1 and mTOR complex 2 via inter- and intracomplex loops, new mTOR regulators, and new inhibitors, are providing insights that may help overcome these challenges. The introduction of innovative imaging strategies is also expected to give rise to breakthroughs in understanding mTOR network complexity and mTOR inhibitor action by visualizing the regulation and function of mTOR.

mTOR is required for pulmonary arterial vascular smooth muscle cell proliferation under chronic hypoxia

Pulmonary arterial vascular smooth muscle (PAVSM) cell proliferation is a key pathophysiological component of vascular remodeling in pulmonary arterial hypertension (PAH) for which cellular and molecular mechanisms are poorly understood. The goal of our study was to determine the role of mammalian target of rapamycin (mTOR) in PAVSM cell proliferation, a major pathological manifestation of vascular remodeling in PAH. Our data demonstrate that chronic hypoxia promoted mTOR(Ser-2481) phosphorylation, an indicator of mTOR intrinsic catalytic activity, mTORC1-specific S6 and mTORC2-specific Akt (Ser-473) phosphorylation, and proliferation of human and rat PAVSM cells that was inhibited by siRNA mTOR. PAVSM cells derived from rats exposed to chronic hypoxia (VSM-H cells) retained increased mTOR(Ser-2481), S6, Akt (Ser-473) phosphorylation, and DNA synthesis compared to cells from normoxia-exposed rats. Suppression of mTORC2 signaling with siRNA rictor, or inhibition of mTORC1 signaling with rapamycin and metformin, while having little effect on other complex activities, inhibited VSM-H and chronic hypoxia-induced human and rat PAVSM cell proliferation. Collectively, our data demonstrate that up-regulation of mTOR activity and activation of both mTORC1 and mTORC2 are required for PAVSM cell proliferation induced by in vitro and in vivo chronic hypoxia and suggest that mTOR may serve as a potential therapeutic target to inhibit vascular remodeling in PAH.

Evidence for direct activation of mTORC2 kinase activity by phosphatidylinositol 3,4,5-trisphosphate

mTORC2 (mammalian target of rapamycin complex 2) plays important roles in signal transduction by regulating an array of downstream effectors, including protein kinase AKT. However, its regulation by upstream regulators remains poorly characterized. Although phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)) is known to regulate the phosphorylation of AKT Ser(473), the hydrophobic motif (HM) site, by mTORC2, it is not clear whether PtdIns(3,4,5)P(3) can directly regulate mTORC2 kinase activity. Here, we used two membrane-docked AKT mutant proteins, one with and the other without the pleckstrin homology (PH) domain, as substrates for mTORC2 to dissect the roles of PtdIns(3,4,5)P(3) in AKT HM phosphorylation in cultured cells and in vitro kinase assays. In HEK293T cells, insulin and constitutively active mutants of small GTPase H-Ras and PI3K could induce HM phosphorylation of both AKT mutants, which was blocked by the PI3K inhibitor LY294002. Importantly, PtdIns(3,4,5)P(3) was able to stimulate the phosphorylation of both AKT mutants by immunoprecipitated mTOR2 complexes in an in vitro kinase assay. In both in vivo and in vitro assays, the AKT mutant containing the PH domain appeared to be a better substrate than the one without the PH domain. Therefore, these results suggest that PtdIns(3,4,5)P(3) can regulate HM phosphorylation by mTORC2 via multiple mechanisms. One of the mechanisms is to directly stimulate the kinase activity of mTORC2.

mTORC1 inhibition via rapamycin promotes triacylglycerol lipolysis and release of free fatty acids in 3T3-L1 adipocytes

Signaling by mTOR complex 1 (mTORC1) promotes anabolic cellular processes in response to growth factors, nutrients, and hormonal cues. Numerous clinical trials employing the mTORC1 inhibitor rapamycin (aka sirolimus) to immuno-suppress patients following organ transplantation have documented the development of hypertriglyceridemia and elevated serum free fatty acids (FFA). We therefore investigated the cellular role of mTORC1 in control of triacylglycerol (TAG) metabolism using cultured murine 3T3-L1 adipocytes. We found that treatment of adipocytes with rapamycin reduced insulin-stimulated TAG storage ~50%. To determine whether rapamycin reduces TAG storage by upregulating lipolytic rate, we treated adipocytes in the absence and presence of rapamycin and isoproterenol, a β2-adrenergic agonist that activates the cAMP/protein kinase A (PKA) pathway to promote lipolysis. We found that rapamycin augmented isoproterenol-induced lipolysis without altering cAMP levels. Rapamycin enhanced the isoproterenol-stimulated phosphorylation of hormone sensitive lipase (HSL) on Ser-563 (a PKA site), but had no effect on the phosphorylation of HSL S565 (an AMPK site). Additionally, rapamycin did not affect the isoproterenol-mediated phosphorylation of perilipin, a protein that coats the lipid droplet to initiate lipolysis upon phosphorylation by PKA. These data demonstrate that inhibition of mTORC1 signaling synergizes with the β-adrenergic-cAMP/PKA pathway to augment phosphorylation of HSL to promote hormone-induced lipolysis. Moreover, they reveal a novel metabolic function for mTORC1; mTORC1 signaling suppresses lipolysis, thus augmenting TAG storage.

mTORC2 can associate with ribosomes to promote cotranslational phosphorylation and stability of nascent Akt polypeptide

The mechanisms that couple translation and protein processing are poorly understood in higher eukaryotes. Although mammalian target of rapamycin (mTOR) complex 1 (mTORC1) controls translation initiation, the function of mTORC2 in protein synthesis remains to be defined. In this study, we find that mTORC2 can colocalize with actively translating ribosomes and can stably interact with rpL23a, a large ribosomal subunit protein present at the tunnel exit. Exclusively during translation of Akt, mTORC2 mediates phosphorylation of the nascent polypeptide at the turn motif (TM) site, Thr450, to avoid cotranslational Akt ubiquitination. Constitutive TM phosphorylation occurs because the TM site is accessible, whereas the hydrophobic motif (Ser473) site is concealed in the ribosomal tunnel. Thus, mTORC2 can function cotranslationally by phosphorylating residues in nascent chains that are critical to attain proper conformation. Our findings reveal that mTOR links protein production with quality control.

Mammalian target of rapamycin: hitting the bull's-eye for neurological disorders

The mammalian target of rapamycin (mTOR) and its associated cell signaling pathways have garnered significant attention for their roles in cell biology and oncology. Interestingly,the explosion of information in this field has linked mTOR to neurological diseases with promising initial studies. mTOR, a 289 kDa serine/threonine protein kinase, plays an important role in cell growth and proliferation and is activated through phosphorylation in response to growth factors, mitogens and hormones. Growth factors, amino acids, cellular nutrients and oxygen deficiency can downregulate mTOR activity. The function of mTOR signaling is mediated primarily through two mTOR complexes: mTORC1 and mTORC2. mTORC1 initiates cap-dependent protein translation, a rate-limiting step of protein synthesis, through the phosphorylation of the targets eukaryotic initiation factor 4E-binding protein 1 (4EBP1) and p70 ribosomal S6 kinase (p70S6K). In contrast, mTORC2 regulates development of the cytoskeleton and also controls cell survival. Although closely tied to tumorigenesis, mTOR and the downstream signaling pathways are significantly involved in the central nervous system (CNS) with synaptic plasticity, memory retention, neuroendocrine regulation associated with food intake and puberty and modulation of neuronal repair following injury. The signaling pathways of mTOR also are believed to be a significant component in a number of neurological diseases, such as Alzheimer disease, Parkinson disease and Huntington disease, tuberous sclerosis, neurofibromatosis, fragile X syndrome, epilepsy, traumatic brain injury and ischemic stroke. Here we describe the role of mTOR in the CNS and illustrate the potential for new strategies directed against neurological disorders.

Dual inhibition of PI3K and mTORC1/2 signaling by NVP-BEZ235 as a new therapeutic strategy for acute myeloid leukemia

PURPOSE

The growth and survival of acute myeloid leukemia (AML) cells are enhanced by the deregulation of signaling pathways such as phosphoinositide 3-kinase (PI3K)/Akt and mammalian target of rapamycin (mTOR). Major efforts have thus been made to develop molecules targeting these activated pathways. The mTOR serine/threonine kinase belongs to two separate complexes: mTORC1 and mTORC2. The mTORC1 pathway is rapamycin sensitive and controls protein translation through the phosphorylation of 4E-BP1 in most models. In AML, however, the translation process is deregulated and rapamycin resistant. Furthermore, the activity of PI3K/Akt and mTOR is closely related, as mTORC2 activates the oncogenic kinase Akt. We therefore tested, in this study, the antileukemic activity of the dual PI3K/mTOR ATP-competitive inhibitor NVP-BEZ235 compound (Novartis).

EXPERIMENTAL DESIGN

The activity of NVP-BEZ235 was tested in primary AML samples (n = 21) and human leukemic cell lines. The different signaling pathways were analyzed by Western blotting. The cap-dependent mRNA translation was studied by 7-methyl-GTP pull-down experiments, polysomal analysis, and [(3)H]leucine incorporation assays. The antileukemic activity of NVP-BEZ235 was tested by analyzing its effects on leukemic progenitor clonogenicity, blast cell proliferation, and survival.

RESULTS

The NVP-BEZ235 compound was found to inhibit PI3K and mTORC1 signaling and also mTORC2 activity. Furthermore, NVP-BEZ235 fully inhibits the rapamycin-resistant phosphorylation of 4E-BP1, resulting in a marked inhibition of protein translation in AML cells. Hence, NVP-BEZ235 reduces the proliferation rate and induces an important apoptotic response in AML cells without affecting normal CD34(+) survival.

CONCLUSIONS

Our results clearly show the antileukemic efficiency of the NVP-BEZ235 compound, which therefore represents a promising option for future AML therapies.

Steady-state kinetic and inhibition studies of the mammalian target of rapamycin (mTOR) kinase domain and mTOR complexes

The mammalian target of rapamycin (mTOR) is a Ser/Thr protein kinase and a major controller of cell growth. In cells, mTOR forms two distinct multiprotein complexes, mTORC1 and mTORC2. The mTORC1 complex can phosphorylate 4EBP1 and S6K1, two key regulators of translation initiation, whereas mTORC2 phosphorylates AKT1, an event required for AKT1 activation. Here, we expressed and purified human mTORC1 and mTORC2 from HEK-293 cells using FLAG-M2 affinity chromatography. Western blotting analysis using phospho-specific antibodies indicated that recombinant mTORC1 and mTORC2 exhibit distinct substrate preferences in vitro, consistent with their roles in cells. To improve our understanding of the enzymatic properties of mTOR alone and mTOR in its complex form, steady-state kinetic profiles of truncated mTOR containing the kinase domain (residues 1360-2549) and mTORC1 were determined. The results revealed that mTORC1 is catalytically less active than truncated mTOR, as evidenced by 4.7- and 3.1-fold decreases in catalytic efficiency, k(cat)/K(m), for ATP and 4EBP1, respectively. We also found that truncated mTOR undergoes autophosphorylation through an intramolecular mechanism. Mass spectrometric analysis identified two novel mTOR autophosphorylation sites, Ser2454 and either Thr2473 or Thr2474, in addition to the previously reported Ser2481 site. Truncated mTOR and mTORC1 were completely inhibited by ATP competitive inhibitors PI103 and BEZ235 and partially inhibited by rapamycin/FKBP12 in a noncompetitive fashion toward ATP. All inhibitors tested exhibited similar inhibitory potencies between mTORC1 and truncated mTOR containing the kinase domain. Our studies presented here provide the first detailed kinetic studies of a recombinant mTOR complex.

Mammalian target of rapamycin (mTOR): conducting the cellular signaling symphony

The mammalian target of rapamycin (mTOR) protein kinase responds to diverse environmental cues to control a plethora of cellular processes. mTOR forms the catalytic core of at least two distinct signaling complexes known as mTOR complexes 1 and 2. Differing sensitivities to the mTOR inhibitor rapamycin, unique partner proteins, distinct substrates, and unique cellular functions distinguish the complexes. Here, we review recent progress in our understanding of the regulation and function of mTOR signaling networks in cellular physiology.