The amino acid sensitive TOR pathway from yeast to mammals

The mitochondrial ribosomal protein of the large subunit, Afo1p, determines cellular longevity through mitochondrial back-signaling via TOR1

Yeast mother cell-specific aging constitutes a model of replicative aging as it occurs in stem cell populations of higher eukaryotes. Here, we present a new long-lived yeast deletion mutation,afo1 (for aging factor one), that confers a 60% increase in replicative lifespan. AFO1/MRPL25 codes for a protein that is contained in the large subunit of the mitochondrial ribosome. Double mutant experiments indicate that the longevity-increasing action of the afo1 mutation is independent of mitochondrial translation, yet involves the cytoplasmic Tor1p as well as the growth-controlling transcription factor Sfp1p. In their final cell cycle, the long-lived mutant cells do show the phenotypes of yeast apoptosis indicating that the longevity of the mutant is not caused by an inability to undergo programmed cell death. Furthermore, the afo1 mutation displays high resistance against oxidants. Despite the respiratory deficiency the mutant has paradoxical increase in growth rate compared to generic petite mutants. A comparison of the single and double mutant strains for afo1 and fob1 shows that the longevity phenotype of afo1 is independent of the formation of ERCs (ribosomal DNA minicircles). AFO1/MRPL25 function establishes a new connection between mitochondria, metabolism and aging.

The Drosophila deoxyhypusine hydroxylase homologue nero and its target eIF5A are required for cell growth and the regulation of autophagy

Hypusination is a unique posttranslational modification by which lysine is transformed into the atypical amino acid hypusine. eIF5A (eukaryotic initiation factor 5A) is the only known protein to contain hypusine. In this study, we describe the identification and characterization of nero, the Drosophila melanogaster deoxyhypusine hydroxylase (DOHH) homologue. nero mutations affect cell and organ size, bromodeoxyuridine incorporation, and autophagy. Knockdown of the hypusination target eIF5A via RNA interference causes phenotypes similar to nero mutations. However, loss of nero appears to cause milder phenotypes than loss of eIF5A. This is partially explained through a potential compensatory mechanism by which nero mutant cells up-regulate eIF5A levels. The failure of eIF5A up-regulation to rescue nero mutant phenotypes suggests that hypusination is required for eIF5A function. Furthermore, expression of enzymatically impaired forms of DOHH fails to rescue nero clones, indicating that hypusination activity is important for nero function. Our data also indicate that nero and eIF5A are required for cell growth and affect autophagy and protein synthesis.

A network of conserved damage survival pathways revealed by a genomic RNAi screen

Damage initiates a pleiotropic cellular response aimed at cellular survival when appropriate. To identify genes required for damage survival, we used a cell-based RNAi screen against the Drosophila genome and the alkylating agent methyl methanesulphonate (MMS). Similar studies performed in other model organisms report that damage response may involve pleiotropic cellular processes other than the central DNA repair components, yet an intuitive systems level view of the cellular components required for damage survival, their interrelationship, and contextual importance has been lacking. Further, by comparing data from different model organisms, identification of conserved and presumably core survival components should be forthcoming. We identified 307 genes, representing 13 signaling, metabolic, or enzymatic pathways, affecting cellular survival of MMS-induced damage. As expected, the majority of these pathways are involved in DNA repair; however, several pathways with more diverse biological functions were also identified, including the TOR pathway, transcription, translation, proteasome, glutathione synthesis, ATP synthesis, and Notch signaling, and these were equally important in damage survival. Comparison with genomic screen data from Saccharomyces cerevisiae revealed no overlap enrichment of individual genes between the species, but a conservation of the pathways. To demonstrate the functional conservation of pathways, five were tested in Drosophila and mouse cells, with each pathway responding to alkylation damage in both species. Using the protein interactome, a significant level of connectivity was observed between Drosophila MMS survival proteins, suggesting a higher order relationship. This connectivity was dramatically improved by incorporating the components of the 13 identified pathways within the network. Grouping proteins into "pathway nodes" qualitatively improved the interactome organization, revealing a highly organized "MMS survival network." We conclude that identification of pathways can facilitate comparative biology analysis when direct gene/orthologue comparisons fail. A biologically intuitive, highly interconnected MMS survival network was revealed after we incorporated pathway data in our interactome analysis.

Drosophila expresses a CD98 transporter with an evolutionarily conserved structure and amino acid-transport properties

Mammalian CD98 heterodimeric amino acid transporters consist of a promiscuous single-pass transmembrane glycoprotein, CD98hc (CD98 heavy chain), and one of six multipass transmembrane proteins or 'light chains'. The heterodimeric complexes of CD98hc and the light chains LAT1 (L-type amino acid transporter 1) or LAT2 specifically promote sodium-independent System L exchange of neutral amino acids, including leucine. CD98hc is also implicated in other processes, including cell fusion, cell adhesion and activation of TOR (target of rapamycin) signalling. Surprisingly, recent reports suggested that insects lack a membrane-bound CD98hc, but in the present study we show that Drosophila CG2791 encodes a functional CD98hc orthologue with conservation in intracellular, transmembrane and extracellular domains. We demonstrate by RNA-interference knockdown in Drosophila Schneider cells that CG2791 and two Drosophila homologues of the mammalian CD98 light chains, Mnd (Minidiscs) and JhI-21, are required for normal levels of System L transport. Furthermore, we show that System L activity is increased by methoprene, an analogue of the developmentally regulated endocrine hormone juvenile hormone, an effect that is potentially mediated by elevated Mnd expression. Co-expression of CG2791 and JhI-21, but not CG2791 and Mnd, in Xenopus oocytes mediates System L transport. Finally, mapping of conserved sequences on to the recently determined crystal structure of the human CD98hc extracellular domain highlights two conserved exposed hydrophobic patches at either end of the domain that are potential protein-protein-interaction surfaces. Therefore our results not only show that there is functional conservation of CD98hc System L transporters in flies, but also provide new insights into the structure, functions and regulation of heterodimeric amino acid transporters.

Amino acid transceptors: gate keepers of nutrient exchange and regulators of nutrient signaling

Amino acid transporters at the surface of cells are in an ideal location to relay nutritional information, as well as nutrients themselves, to the cell interior. These transporters are able to modulate signaling downstream of intracellular amino acid receptors by regulating intracellular amino acid concentrations through processes of coupled transport. The concept of dual-function amino acid transporter/receptor (or "transceptor") proteins is well established in primitive eukaryotes such as yeast, where detection of extracellular amino acid deficiency leads to upregulation of proteins involved in biosynthesis and transport of the deficient amino acid(s). The evolution of the "extracellular milieu" and nutrient-regulated endocrine controls in higher eukaryotes, alongside their frequent inability to synthesize all proteinaceous amino acids (and, hence, the requirement for indispensable amino acids in their diet), appears to have lessened the priority of extracellular amino acid sensing as a stimulus for metabolic signals. Nevertheless, recent studies of amino acid transporters in flies and mammalian cell lines have revealed perhaps unanticipated "echoes" of these transceptor functions, which are revealed by cellular stresses (notably starvation) or gene modification/silencing. APC-transporter superfamily members, including slimfast, path, and SNAT2 all appear capable of sensing and signaling amino acid availability to the target of rapamycin (TOR) pathway, possibly through PI 3-kinase-dependent mechanisms. We hypothesize (by extrapolation from knowledge of the yeast Ssy1 transceptor) that, at least for SNAT2, the transceptor discriminates between extracellular and intracellular amino acid stimuli when evoking a signal.

Bidirectional transport of amino acids regulates mTOR and autophagy

Amino acids are required for activation of the mammalian target of rapamycin (mTOR) kinase which regulates protein translation, cell growth, and autophagy. Cell surface transporters that allow amino acids to enter the cell and signal to mTOR are unknown. We show that cellular uptake of L-glutamine and its subsequent rapid efflux in the presence of essential amino acids (EAA) is the rate-limiting step that activates mTOR. L-glutamine uptake is regulated by SLC1A5 and loss of SLC1A5 function inhibits cell growth and activates autophagy. The molecular basis for L-glutamine sensitivity is due to SLC7A5/SLC3A2, a bidirectional transporter that regulates the simultaneous efflux of L-glutamine out of cells and transport of L-leucine/EAA into cells. Certain tumor cell lines with high basal cellular levels of L-glutamine bypass the need for L-glutamine uptake and are primed for mTOR activation. Thus, L-glutamine flux regulates mTOR, translation and autophagy to coordinate cell growth and proliferation.

Amino acid transporters: éminences grises of nutrient signalling mechanisms?

Nutrient signalling by the mTOR (mammalian target of rapamycin) pathway involves upstream sensing of free AA (amino acid) concentrations. Several AA-regulated kinases have recently been identified as putative intracellular AA sensors. Their activity will reflect the balance between AA flows through underlying mechanisms which together determine the size of the intracellular free AA pool. For indispensable AAs, these mechanisms are primarily (i) AA transport across the cell membrane, and (ii) protein synthesis/breakdown. The System L AA transporter is the primary conduit for cellular entry of indispensable neutral AAs (including leucine and phenylalanine) and potentially a key modulator of AA-sensitive mTOR signalling. Coupling of substrate flows through System L and other AA transporters (e.g. System A) may extend the scope for sensing nutrient abundance. Factors influencing AA transporter activity (e.g. hormones) may affect intracellular AA concentrations and hence indirectly mTOR pathway activity. Several AA transporters are themselves regulated by AA availability through 'adaptive regulation', which may help to adjust the gain of AA sensing. The substrate-binding sites of AA transporters are potentially direct sensors of AA availability at both faces of the cell surface, and there is growing evidence that AA transporters of the SNAT (sodium-coupled neutral AA transporter) and PAT (proton-assisted AA transporter) families may operate, at least under some circumstances, as transporter-like sensors (or 'transceptors') upstream of mTOR.

Potentiating effects of RAD001 (Everolimus) on vincristine therapy in childhood acute lymphoblastic leukemia

Despite advances in the treatment of acute lymphoblastic leukemia (ALL), the majority of children who relapse still die of ALL. Therefore, the development of more potent but less toxic drugs for the treatment of ALL is imperative. We investigated the effects of the mammalian target of rapamycin inhibitor, RAD001 (Everolimus), in a nonobese diabetic/severe combined immunodeficiency model of human childhood B-cell progenitor ALL. RAD001 treatment of established disease increased the median survival of mice from 21.3 days to 42.3 days (P < .02). RAD001 together with vincristine significantly increased survival compared with either treatment alone (P < .02). RAD001 induced a cell-cycle arrest in the G(0/1) phase with associated dephosphorylation of the retinoblastoma protein, and reduced levels of cyclin-dependent kinases 4 and 6. Ultrastructure analysis demonstrated the presence of autophagy and limited apoptosis in cells of RAD001-treated animals. In contrast, cleaved poly(ADP-ribose) polymerase suggested apoptosis in cells from animals treated with vincristine or the combination of RAD001 and vincristine, but not in those receiving RAD001 alone. In conclusion, we have demonstrated activity of RAD001 in an in vivo leukemia model supporting further clinical development of target of rapamycin inhibitors for the treatment of patients with ALL.

Function and regulation of macroautophagy in plants

The plant vacuole is a major site for the degradation of macromolecules, which are transferred from the cytoplasm by autophagy via double-membrane vesicles termed autophagosomes. Autophagy functions at a basal level under normal growth conditions and is induced during senescence and upon exposure to stress conditions to recycle nutrients or degrade damaged proteins and organelles. Autophagy is also required for the regulation of programmed cell death as a response to pathogen infection and possibly during certain developmental processes. Little is known about how autophagy is regulated under these different conditions in plants, but recent evidence suggests that plants contain a functional TOR pathway which may control autophagy induction in conjunction with hormonal and/or environmental signals.

Phosphoinositide (PI) 3-kinase assays

The regulation of phosphoinositide (PI) 3-kinase activities has been linked to many normal and disease-related processes, including cell survival, cell growth and proliferation, cell differentiation, cell motility, and intracellular vesicle trafficking. However, as the family of enzymes has now grown to include eight true members, in three functional classes, plus several related protein kinases that are also inhibited by the widely used PI 3-kinase selective inhibitors, wortmannin and LY294002, extended methodologies are required to identify which type of kinase is involved in a particular cellular process, or protein complex, under study. A robust in vitro PI 3-kinase assay, suitable for use with immunoprecipitates, or purified proteins, is described here together with a series of modifications of substrate and assay conditions that will aid researchers in the identification of the particular class and isoform of PI 3-kinase that is involved in a signaling process under investigation.

Nutrient transporters in cancer: relevance to Warburg hypothesis and beyond

Tumor cells have an increased demand for nutrients; this demand is met by increased availability of nutrients through vasculogenesis and by enhanced cellular entry of nutrients through upregulation of specific transporters. This review focuses on three groups of nutrient transporters relevant to cancer: glucose transporters, lactate transporters, and amino acid transporters. Tumor cells enhance glucose uptake via induction of GLUT1 and SGLT1, and coordinate the increased entry of glucose with increased glycolysis. Since enhanced glycolysis in cancer is associated with lactate production, tumor cells must find a way to eliminate lactic acid to prevent cellular acidification. This is achieved by the upregulation of MCT4, a H+-coupled lactate transporter. In addition, the Na+-coupled lactate transporter SMCT1 is silenced in cancer. SMCT1 also transports butyrate and pyruvate, which are inhibitors of histone deacetylases. The silencing of SMCT1 occurs in cancers of a variety of tissues. Re-expression of SMCT1 in cancer cell lines leads to growth arrest and apoptosis in the presence of butyrate or pyruvate, suggesting that the transporter may function as a tumor suppressor. Tumor cells meet their amino acid demands by inducing xCT/4F2hc, LAT1/4F2hc, ASCT2, and ATB0,+. xCT/4F2hc is related primarily to glutathione status, protection against oxidative stress, and cell cycle progression, whereas the other three transporters are related to amino acid nutrition. Pharmacologic blockade of LAT1/4F2hc, xCT/4F2hc, or ATB0,+ leads to inhibition of cancer cell growth. Since tumor cells selectively regulate these nutrient transporters to support their rapid growth, these transporters have potential as drug targets for cancer therapy.

RalA functions as an indispensable signal mediator for the nutrient-sensing system

Cells sense nutrients present in the extracellular environment and modulate the activities of intracellular signaling systems in response to nutrient availability. This study demonstrates that RalA and its activator RalGDS participate in nutrient sensing and are indispensable for activation of mammalian target of rapamycin complex 1 (mTORC1) induced by extracellular nutrients. Knockdown of RalA or RalGDS abolished amino acid- and glucose-induced mTORC1 activation, as judged by phosphorylation of S6 kinase and eukaryotic translation initiation factor 4E-binding protein 1. The amount of GTP-bound RalA increased in response to increased amino acid availability. In addition, RalA knockdown suppressed Rheb-induced S6 kinase phosphorylation, and the constitutively active form of RalA induced mTORC1 activation in the absence of Rheb. These results collectively suggest that RalGDS and RalA act downstream of Rheb and that RalA activation is a crucial step in nutrient-induced mTORC1 activation.

What controls TOR?

The target of rapamycin (TOR) is a protein kinase with numerous functions in cell growth control. Some of these functions can be potently inhibited by rapamycin, an immunosuppressive and potential anticancer drug. TOR exists as part of two functionally distinct protein complexes. The functions of TOR complex 1 (TORC1) are effectively inhibited by rapamycin, but the mechanism for this inhibition remains elusive. The identification of TORC2 and recent reports that rapamycin can inhibit TORC2 functions, in some cases, challenge current models of TOR regulation. This review discusses the latest findings in yeast and mammals on the possible mechanisms that control TOR activity leading to its many cellular functions

Amino acid regulation of autophagosome formation

Amino acids are not only substrates for various metabolic pathways, but can also serve as signaling molecules controlling signal transduction pathways. One of these signaling pathways is mTOR-dependent and is activated by amino acids (leucine in particular) in synergy with insulin. Activation of this pathway inhibits autophagy. Because activation of mTOR-mediated signaling also stimulates protein synthesis, it appears that protein synthesis and autophagic protein degradation are reciprocally controlled by the same signaling pathway. Recent developments indicate that amino acid-stimulated mTOR-dependent signaling is subject to complex regulation. The mechanism by which amino acids stimulate mTORdependent signaling (and other signaling pathways), and its molecular connection with the autophagic machinery, is still unknown.

Intracellular mechanisms of metabolism regulation: the role of signaling via the mammalian target of rapamycin pathway and other routes

The development of diabetes mellitus is the consequence of defects in the action of insulin in skeletal muscles and adipose tissue other than pancreas and liver defects. Insulin action defects are mostly the results of defects of the insulin intracellular signaling transduction pathway. This review summarizes the main mechanisms involved in insulin signaling and possible intracellular defects that lead to insulin resistance. We also report preliminary experimental data that suggest the existence of intracellular alternatives to insulin metabolic pathways stimulated by nutrients such as amino acids (AAs). Indeed, we found that oral supplements with AAs stimulated both glucose transporter-4 and protein synthesis through independent insulin signals in rat hearts. Evidence suggests that the mammalian target of rapamycin and/or other molecules could be involved in this insulin-independent metabolic pathway. This hypothesis suggests the presence of an ancestral metabolic pathway in eukaryotic cells that is not active when insulin intracellular signaling is efficient but can be activated by alternative stimuli, such as AAs, when insulin signaling is impaired. Our observations provide molecular evidence that supports the use of anabolic nutrients such as AAs, together with standard therapies, to overcome insulin resistance syndrome.

Loss of the tuberous sclerosis complex tumor suppressors triggers the unfolded protein response to regulate insulin signaling and apoptosis

Mammalian target of rapamycin, mTOR, is a major sensor of nutrient and energy availability in the cell and regulates a variety of cellular processes, including growth, proliferation, and metabolism. Loss of the tuberous sclerosis complex genes (TSC1 or TSC2) leads to constitutive activation of mTOR and downstream signaling elements, resulting in the development of tumors, neurological disorders, and at the cellular level, severe insulin/IGF-1 resistance. Here, we show that loss of TSC1 or TSC2 in cell lines and mouse or human tumors causes endoplasmic reticulum (ER) stress and activates the unfolded protein response (UPR). The resulting ER stress plays a significant role in the mTOR-mediated negative-feedback inhibition of insulin action and increases the vulnerability to apoptosis. These results demonstrate ER stress as a critical component of the pathologies associated with dysregulated mTOR activity and offer the possibility to exploit this mechanism for new therapeutic opportunities.

Mammalian target of rapamycin contributes to the acquired apoptotic resistance of human mesothelioma multicellular spheroids

When grown as three-dimensional structures, tumor cells can acquire an additional multicellular resistance to apoptosis that may mimic the chemoresistance found in solid tumors. We developed a multicellular spheroid model of malignant mesothelioma to investigate molecular mechanisms of acquired apoptotic resistance. We found that mesothelioma cell lines, when grown as multicellular spheroids, acquired resistance to a variety of apoptotic stimuli, including combinations of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), ribotoxic stressors, histone deacetylase, and proteasome inhibitors, that were highly effective against mesothelioma cells when grown as monolayers. Inhibitors of the phosphatidylinositol 3-kinase/Akt/mammalian target of rapamycin (mTOR) pathway, particularly rapamycin, blocked much of the acquired resistance of the spheroids, suggesting a key role for mTOR. Knockdown by small interference RNA of S6K, a major downstream target of mTOR, reproduced the effect of rapamycin, thereby confirming the role of mTOR and of S6K in the acquired resistance of three dimensional spheroids. Rapamycin or S6K knockdown increased TRAIL-induced caspase-8 cleavage in spheroids, suggesting initially that mTOR inhibited apoptosis by actions at the death receptor pathway; however, isolation of the apoptotic pathways by means of Bid knockdown ablated this effect showing that mTOR actually controls a step distal to Bid, probably at the level of the mitochondria. In sum, mTOR and S6K contribute to the apoptotic resistance of mesothelioma cells in three-dimensional, not in two-dimensional, cultures. The three-dimensional model may reflect a more clinically relevant in vitro setting in which mTOR exhibits anti-apoptotic properties.

Trading translation with RNA-binding proteins

RNA-binding proteins regulate every aspect of RNA metabolism, including pre-mRNA splicing, mRNA trafficking, stability, and translation. This review summarizes the available information on molecular mechanisms of translational repression by RNA-binding proteins. By using a specific set of well-defined examples, we also describe how regulation can be reversed.

Amino acid transporters and nutrient-sensing mechanisms: new targets for treating insulin-linked disorders?

The IIS (insulin/IGF (insulin-like growth factor) signalling) cascade has an important role in regulating normal development and physiology, as evidenced by its effects in a host of major human diseases including cancer, Type 2 diabetes and neurodegeneration. Recently, it has become clear that multiple types of local nutrient-sensing mechanisms have an impact on cellular insulin-sensitivity through the downstream kinase TOR (target of rapamycin). In vivo analysis in flies has surprisingly highlighted PATs (proton-assisted amino acid transporters) as having a uniquely potent role in regulating IIS/TOR activity and growth, potentially via a novel signalling mechanism. Other molecules such as the heterodimeric amino acid transporter, CD98, which provides the principal route for cellular uptake of leucine, an amino acid implicated in regulating TOR, also appear to have important effects. As our understanding of how nutrient sensing has an impact on IIS/TOR increases, novel targets to modulate aberrant IIS in disease are likely to emerge, which could complement current strategies designed to block kinases in this pathway.

Rapamycin inhibits polyglutamine aggregation independently of autophagy by reducing protein synthesis

Accumulation of misfolded proteins and protein assemblies is associated with neuronal dysfunction and death in several neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's disease (HD). It is therefore critical to understand the molecular mechanisms of drugs that act on pathways that modulate misfolding and/or aggregation. It is noteworthy that the mammalian target of rapamycin inhibitor rapamycin or its analogs have been proposed as promising therapeutic compounds clearing toxic protein assemblies in these diseases via activation of autophagy. However, using a cellular model of HD, we found that rapamycin significantly decreased aggregation-prone polyglutamine (polyQ) and expanded huntingtin and its inclusion bodies (IB) in both autophagy-proficient and autophagy-deficient cells (by genetic knockout of the atg5 gene in mouse embryonic fibroblasts). This result suggests that rapamycin modulates the levels of misfolded polyQ proteins via pathways other than autophagy. We show that rapamycin reduces the amount of soluble polyQ protein via a modest inhibition of protein synthesis that in turn significantly reduces the formation of insoluble polyQ protein and IB formation. Hence, a modest reduction in huntingtin synthesis by rapamycin may lead to a substantial decrease in the probability of reaching the critical concentration required for a nucleation event and subsequent toxic polyQ aggregation. Thus, in addition to its beneficial effect proposed previously of reducing polyQ aggregation/toxicity via autophagic pathways, rapamycin may alleviate polyQ disease pathology via its effect on global protein synthesis. This finding may have important therapeutic implications.

mTOR controls mitochondrial oxidative function through a YY1-PGC-1alpha transcriptional complex

Transcriptional complexes that contain peroxisome-proliferator-activated receptor coactivator (PGC)-1alpha control mitochondrial oxidative function to maintain energy homeostasis in response to nutrient and hormonal signals. An important component in the energy and nutrient pathways is mammalian target of rapamycin (mTOR), a kinase that regulates cell growth, size and survival. However, it is unknown whether and how mTOR controls mitochondrial oxidative activities. Here we show that mTOR is necessary for the maintenance of mitochondrial oxidative function. In skeletal muscle tissues and cells, the mTOR inhibitor rapamycin decreased the gene expression of the mitochondrial transcriptional regulators PGC-1alpha, oestrogen-related receptor alpha and nuclear respiratory factors, resulting in a decrease in mitochondrial gene expression and oxygen consumption. Using computational genomics, we identified the transcription factor yin-yang 1 (YY1) as a common target of mTOR and PGC-1alpha. Knockdown of YY1 caused a significant decrease in mitochondrial gene expression and in respiration, and YY1 was required for rapamycin-dependent repression of those genes. Moreover, mTOR and raptor interacted with YY1, and inhibition of mTOR resulted in a failure of YY1 to interact with and be coactivated by PGC-1alpha. We have therefore identified a mechanism by which a nutrient sensor (mTOR) balances energy metabolism by means of the transcriptional control of mitochondrial oxidative function. These results have important implications for our understanding of how these pathways might be altered in metabolic diseases and cancer.

Replicative aging in yeast: the means to the end

Progress in aging research is now rapid, and surprisingly, studies in a single-celled eukaryote are a driving force. The genetic modulators of replicative life span in yeast are being identified, the molecular events that accompany aging are being discovered, and the extent to which longevity pathways are conserved between yeast and multicellular eukaryotes is being tested. In this review, we provide a brief retrospective view on the development of yeast as a model for aging and then turn to recent discoveries that have pushed aging research into novel directions and also linked aging in yeast to well-developed hypotheses in mammals. Although the question of what causes aging still cannot be answered definitively, that day may be rapidly approaching.

Tumour suppressors in liver carcinogenesis

The circuitous cell signalling pathways of hepatocytes comprise several factors that operate to downgrade or even interrupt the transmission of a given signal. These down-regulating influences are essential to keep cell proliferation and cell survival in check and if impaired, can alter a delicate balance in favour of cell proliferation. Each signalling pathway that has been implicated in carcinogenesis is influenced by both oncogenic factors that promote tumour growth when activated as well as tumour suppressor proteins that have to be impaired to favour tumour growth. This summary of the Tumour Suppressors in Liver Carcinogenesis Symposium held at the 2007 EASL Annual Meeting discusses four pathways with pre-eminent tumour suppressor activity, each involved in hepatocarcinogenesis: p53, mTOR, beta-catenin and hedgehog.

Distinct signaling mechanisms activate the target of rapamycin in response to different B-cell stimuli

Phosphoinositide 3-kinase (PI3K) and the mammalian target of rapamycin (mTOR), a downstream kinase, are both required for proliferation of splenic B cells. However, the functions of PI3K and mTOR in response to different stimuli and among B cell subsets have not been fully elucidated. We used flow cytometry and magnetic cell sorting to examine the requirement for PI3K and mTOR in responses of splenic B cell subsets to BCR and LPS stimulation. BCR-mediated phosphorylation of Akt and Erk is sensitive to the PI3K catalytic inhibitor wortmannin in both marginal zone (MZ) and follicular (FO) cells. BCR-mediated mTOR activation in both subsets is inhibited by wortmannin, though less strongly in MZ cells. In contrast, LPS-induced mTOR signaling is strikingly resistant to wortmannin in both subsets. Similarly, functional responses to LPS are partially wortmannin resistant yet sensitive to mTOR inhibition by rapamycin. We also observed mitogen-independent mTOR activity that is regulated by nutrient availability, and is significantly elevated in MZ cells relative to FO cells. These data define both similarities and differences in PI3K/mTOR signaling mechanisms in MZ and FO cells, and suggest that mTOR signaling can occur in the absence of PI3K activation to promote B cell responses to LPS.

Prevention of atherosclerosis by the mTOR inhibitor everolimus in LDLR-/- mice despite severe hypercholesterolemia

Everolimus inhibits the mammalian target of rapamycin (mTOR) in proliferating cells. It is widely used in transplant patients and has also been exploited by drug-eluting stents for the treatment of cardiovascular disease. However, there is only limited data on the pathophysiological effects of mTOR-inhibitors on the vascular wall. We aimed to unravel the effects of everolimus on cholesterol-induced atherosclerosis and on circulating cell mediators in LDL-receptor-deficient (LDLR(-/-)) mice. Male hypercholesterolemic LDLR(-/-) mice received either solvent (group A; n=28) or everolimus at 0.05 mg/kg (group B, n=22) and 1.5 mg/kg (group C, n=29) per body weight per day by subcutaneously implanted osmotic minipumps for the study period of 12 weeks. Group B showed 44% reduction of atherosclerotic lesions at the brachiocephalic artery (BCA). In group C atherosclerotic lesions were reduced by 85% in the BCA and by 60% at the aortic root. This was associated with a significantly lower complexity of lesions in both treated groups (p<0.001) and despite a 40% increase of plasma cholesterol. Everolimus caused a significant reduction of circulating cell mediators such as interleukin-1alpha, interleukin-5, GM-CSF and interleukin-12p40. Everolimus increased the plasma levels of KC but had no effect on eighteen other circulating cell mediators studied. Everolimus strongly inhibits atherosclerosis development in LDL-receptor(-/-) mice despite severe hypercholesterolemia. Everolimus application had only small effects on circulating cell mediators. The significant reduction of atherosclerotic lesions was associated with a delayed transition from early macrophages enriched lesions to advanced atherosclerotic plaques.

Targeting phosphoinositide 3-kinase: moving towards therapy

Phosphoinositide 3-kinases (PI3K) orchestrate cell responses including mitogenic signaling, cell survival and growth, metabolic control, vesicular trafficking, degranulation, cytoskeletal rearrangement and migration. Deregulation of the PI3K pathway occurs by activating mutations in growth factor receptors or the PIK3CA locus coding for PI3Kalpha, by loss of function of the lipid phosphatase and tensin homolog deleted in chromosome ten (PTEN/MMAC/TEP1), by the up-regulation of protein kinase B (PKB/Akt), or the impairment of the tuberous sclerosis complex (TSC1/2). All these events are linked to growth and proliferation, and have thus prompted a significant interest in the pharmaceutical targeting of the PI3K pathway in cancer. Genetic targeting of PI3Kgamma (p110gamma) and PI3Kdelta (p110delta) in mice has underlined a central role of these PI3K isoforms in inflammation and allergy, as they modulate chemotaxis of leukocytes and degranulation in mast cells. Proof-of-concept molecules selective for PI3Kgamma have already successfully alleviated disease progress in murine models of rheumatoid arthritis and lupus erythematosus. As targeting PI3K moves forward to therapy of chronic, non-fatal disease, safety concerns for PI3K inhibitors increase. Many of the present inhibitor series interfere with target of rapamycin (TOR), DNA-dependent protein kinase (DNA-PK(cs)) and activity of the ataxia telangiectasia mutated gene product (ATM). Here we review the current disease-relevant knowledge for isoform-specific PI3K function in the above mentioned diseases, and review the progress of >400 recent patents covering pharmaceutical targeting of PI3K. Currently, several drugs targeting the PI3K pathway have entered clinical trials (phase I) for solid tumors and suppression of tissue damage after myocardial infarction (phases I,II).

Nutrient sensing in the mTOR/S6K1 signalling pathway

Nutrient overload induces constitutive S6K1 (S6 kinase 1) activation, which leads to insulin resistance by suppressing insulin-induced class I PI3K (phosphoinositide 3-kinase) signalling [Um, Frigerio, Watanabe, Picard, Joaquin, Sticker, Fumagalli, Allegrini, Kozma, Auwerx and Thomas (2004) Nature 431, 200-205]. This finding gave rise to the question of the mechanism by which nutrients, such as AAs (amino acids), enter the mTOR (mammalian target of rapamycin)/S6K1 signalling pathway. Counter to the prevailing view, our recent studies have shown that the AA input into the mTOR/S6K1 signalling pathway is not mediated by the tumour suppressor TSC1 (tuberous sclerosis complex 1)/TSC2 or its target, the proto-oncogene Rheb (Ras homologue enriched in brain). Instead, we found that the AA input was mediated by class 3 PI3K, or hVps34 (human vacuolar protein sorting 34). In brief, ectopic expression of hVps34 drives S6K1 activation, but only in the presence of AAs, and this effect is blocked by small interfering RNAs directed against hVps34. Moreover, stimulation of cells with AAs increases hVps34 activity, as indicated by the production of PI3P (phosphatidylinositol 3-phosphate). PI3P mediates the recruitment of proteins containing FYVE (Fab1p, YOTB, Vac1p and EEA1) or PX (Phox homology) domains to endosomal membranes, with PI3P-rich micro-domains acting as signalling platforms. Additional evidence indicating hVps34 as the mediator of AA input to S6K1 came from experiments in which S6K1 activation was attenuated by ectopic expression of a cDNA containing two FYVE domains, which bind to PI3P, preventing binding of proteins containing either FYVE or PX domains [Nobukuni, Joaquin, Roccio, Dann, Kim, Gulati, Byfield, Backer, Natt, Bos, Zwartkruis and Thomas (2005) Proc. Natl. Acad. Sci. U.S.A. 102, 14238-14243].

Regulation of macroautophagy by mTOR and Beclin 1 complexes

Macroautophagy or autophagy is a vacuolar degradative pathway terminating in the lysosomal compartment after forming a cytoplasmic vacuole or autophagosome that engulfs macromolecules and organelles. The original discovery that ATG (AuTophaGy related) genes in yeast are involved in the formation of autophagosomes has greatly increased our knowledge of the molecular basis of autophagy, and its role in cell function that extends far beyond non-selective degradation. The regulation of autophagy by signaling pathways overlaps the control of cell growth, proliferation, cell survival and death. The evolutionarily conserved TOR (Target of Rapamycin) kinase complex 1 plays an important role upstream of the Atg1 complex in the control of autophagy by growth factors, nutrients, calcium signaling and in response to stress situations, including hypoxia, oxidative stress and low energy. The Beclin 1 (Atg6) complex, which is involved in the initial step of autophagosome formation, is directly targeted by signaling pathways. Taken together, these data suggest that multiple signaling checkpoints are involved in regulating autophagosome formation.

mTOR is the rapamycin-sensitive kinase that confers mechanically-induced phosphorylation of the hydrophobic motif site Thr(389) in p70(S6k)

Mechanical stretch induces phosphorylation of the hydrophobic motif site Thr(389) in p70(S6k) through a rapamycin-sensitive (RS) pathway that involves a unique PI3K-independent mechanism. Rapamycin is considered to be a highly specific inhibitor of the protein kinase mTOR; however, mTOR is also considered to be a PI3K-dependent signaling molecule. Thus, questions remain as to whether mTOR is the RS element that confers mechanically-induced signaling to p70(S6k)(389). In this study, rapamycin-resistant mutants of mTOR along with mechanical stretch were used to address this question. The results indicate that mTOR is the RS element and reveal that mTOR signaling can be activated through a PI3K-independent mechanism.

Mechanisms through which sulfur amino acids control protein metabolism and oxidative status

Amino acids regulate protein synthesis and breakdown (i.e., protein turnover) and consequently protein deposition, which corresponds to the balance between the two processes. Elucidating the mechanisms involved in such regulation is important from fundamental and applied points of view since it can provide a basis to optimize amino acid requirements and to control protein mass, body composition and so forth. Amino acids, which have long been considered simply as precursors of protein synthesis, are now recognized to exert other significant influences; that is, they are precursors of essential molecules, act as mediators or signal molecules and affect numerous functions. For example, amino acids act as mediators of metabolic pathways in the same manner as certain hormones. Thus, they modulate the activity of intracellular protein kinases involved in the regulation of metabolic pathways such as mRNA translation. We provide here an overview of the roles of amino acids as regulators of protein metabolism, by focusing particularly on sulfur amino acids. The potential importance of methionine as a "nutrient signal" is discussed in the light of recent findings. Emphasis is also placed on mechanisms controlling oxidative status since sulfur amino acids are involved in the synthesis of intracellular antioxidants (glutathione, taurine etc.) and in the methionine sulfoxide reductase antioxidant system.

Regulation of muscle protein synthesis during sepsis and inflammation

Prolonged sepsis and exposure to an inflammatory milieu decreases muscle protein synthesis and reduces muscle mass. As a result of its ability to integrate diverse signals, including hormones and nutrients, the mammalian target of rapamycin (mTOR) is a dominant regulator in the translational control of protein synthesis. Under postabsorptive conditions, sepsis decreases mTOR kinase activity in muscle, as evidenced by reduced phosphorylation of both eukaryotic initiation factor (eIF)4E-binding protein (BP)-1 and ribosomal S6 kinase (S6K)1. These sepsis-induced changes, along with the redistribution of eIF4E from the active eIF4E.eIF4G complex to the inactive eIF4E.4E-BP1 complex, are preventable by neutralization of tumor necrosis factor (TNF)-alpha but not by antagonizing glucocorticoid action. Although the ability of mTOR to respond to insulin-like growth factor (IGF)-I is not disrupted by sepsis, the ability of leucine to increase 4E-BP1 and S6K1 phosphorylation is greatly attenuated. This "leucine resistance" results from a cooperative interaction between both TNF-alpha and glucocorticoids. Finally, although septic animals are not IGF-I resistant, the anabolic actions of IGF-I are nonetheless reduced because of the development of growth hormone resistance, which decreases both circulating and muscle IGF-I. Herein, we highlight recent advances in the mTOR signaling network and emphasize their connection to the atrophic response observed in skeletal muscle during sepsis. Although many unanswered questions remain, understanding the cellular basis of the sepsis-induced decrease in translational activity will contribute to the rational development of therapeutic interventions and thereby minimize the debilitating affects of the atrophic response that impairs patient recovery.

Coordinate regulation of ribosome biogenesis and function by the ribosomal protein S6 kinase, a key mediator of mTOR function

Current understanding of the mechanisms by which cell growth is regulated lags significantly behind our knowledge of the complex processes controlling cell cycle progression. Recent studies suggest that the mammalian target of rapamycin (mTOR) pathway is a key regulator of cell growth via the regulation of protein synthesis. The key mTOR effectors of cell growth are eukaryotic initiation factor 4E-binding protein 1 (4EBP-1) and the ribosomal protein S6 kinase (S6K). Here we will review the current models for mTOR dependent regulation of ribosome function and biogenesis as well as its role in coordinating growth factor and nutrient signaling to facilitate homeostasis of cell growth and proliferation. We will place particular emphasis on the role of S6K1 signaling and will highlight the points of cross talk with other key growth control pathways. Finally, we will discuss the impact of S6K signaling and the consequent feedback regulation of the PI3K/Akt pathway on disease processes including cancer.

Notch-induced T cell development requires phosphoinositide-dependent kinase 1

Phosphoinositide-dependent kinase l (PDK1) phosphorylates and activates multiple AGC serine kinases, including protein kinase B (PKB), p70Ribosomal S6 kinase (S6K) and p90Ribosomal S6 kinase (RSK). PDK1 is required for thymocyte differentiation and proliferation, and herein, we explore the molecular basis for these essential functions of PDK1 in T lymphocyte development. A key finding is that PDK1 is required for the expression of key nutrient receptors in T cell progenitors: CD71 the transferrin receptor and CD98 a subunit of L-amino acid transporters. PDK1 is also essential for Notch-mediated trophic and proliferative responses in thymocytes. A PDK1 mutant PDK1 L155E, which supports activation of PKB but no other AGC kinases, can restore CD71 and CD98 expression in pre-T cells and restore thymocyte differentiation. However, PDK1 L155E is insufficient for thymocyte proliferation. The role of PDK1 in thymus development thus extends beyond its ability to regulate PKB. In addition, PDK1 phosphorylation of AGC kinases such as S6K and RSK is also necessary for thymocyte development.

Regulation of various proteolytic pathways by insulin and amino acids in human fibroblasts

Intracellular protein degradation is a regulated process with several proteolytic pathways. Although regulation of macroautophagy has been investigated in some detail in hepatocytes and in few other cells, less is known on this regulation in other cells and proteolytic pathways. We show that in human fibroblasts insulin and amino acids reduce protein degradation by different signalling pathways and that this inhibition proceeds in part via the mammalian target of rapamycin, especially with amino acids, which probably increase lysosomal pH. Moreover, the regulatory amino acids (Phe, Arg, Met, Tyr, Trp and Cys) are partially different from other cells. Finally, and in addition to macroautophagy, insulin and amino acids modify, to different extents and sometimes in opposite directions, the activities of other proteolytic pathways.

Signaling pathways mediating chemotaxis in the social amoeba, Dictyostelium discoideum

Chemotaxis, or cell migration guided by chemical cues, is critical for a multitude of biological processes in a diverse array of organisms. Dictyostelium discoideum amoebae rely on chemotaxis to find food and to survive starvation conditions, and we have taken advantage of this system to study the molecular regulation of this vital cell behavior. Previous work has identified phosphoinositide signaling as one mechanism which may contribute to directional sensing and actin polymerization during chemotaxis; a mechanism which is conserved in mammalian neutrophils. In this review, we will discuss recent data on genes and pathways governing directional sensing and actin polymerization, with a particular emphasis on contributions from our laboratory.

Molecular determinants of caste differentiation in the highly eusocial honeybee Apis mellifera

Background

In honeybees, differential feeding of female larvae promotes the occurrence of two different phenotypes, a queen and a worker, from identical genotypes, through incremental alterations, which affect general growth, and character state alterations that result in the presence or absence of specific structures. Although previous studies revealed a link between incremental alterations and differential expression of physiometabolic genes, the molecular changes accompanying character state alterations remain unknown.

Results

By using cDNA microarray analyses of >6,000 Apis mellifera ESTs, we found 240 differentially expressed genes (DEGs) between developing queens and workers. Many genes recorded as up-regulated in prospective workers appear to be unique to A. mellifera, suggesting that the workers' developmental pathway involves the participation of novel genes. Workers up-regulate more developmental genes than queens, whereas queens up-regulate a greater proportion of physiometabolic genes, including genes coding for metabolic enzymes and genes whose products are known to regulate the rate of mass-transforming processes and the general growth of the organism (e.g., tor). Many DEGs are likely to be involved in processes favoring the development of caste-biased structures, like brain, legs and ovaries, as well as genes that code for cytoskeleton constituents. Treatment of developing worker larvae with juvenile hormone (JH) revealed 52 JH responsive genes, specifically during the critical period of caste development. Using Gibbs sampling and Expectation Maximization algorithms, we discovered eight overrepresented cis-elements from four gene groups. Graph theory and complex networks concepts were adopted to attain powerful graphical representations of the interrelation between cis-elements and genes and objectively quantify the degree of relationship between these entities.

Conclusion

We suggest that clusters of functionally related DEGs are co-regulated during caste development in honeybees. This network of interactions is activated by nutrition-driven stimuli in early larval stages. Our data are consistent with the hypothesis that JH is a key component of the developmental determination of queen-like characters. Finally, we propose a conceptual model of caste differentiation in A. mellifera based on gene-regulatory networks.

Insulin and amino acid availability regulate atrogin-1 in avian QT6 cells

New evidence has demonstrated that the expression of major genes, termed atrogenes, controls the ubiquitin-proteasome proteolytic pathway. The present work aimed to study the impact of insulin and amino acids on the expression of one of these atrogenes, the E3 ubiquitin ligase Muscle Atrophy F box (MAFbx, also called atrogin-1), in quail muscle (QT6) fibroblasts. First, we characterized atrogin-1 in QT6 cells and demonstrated the insulin sensitivity of these cells. Second, we showed that insulin reduced atrogin-1 mRNA via the phosphatidylinositol-3'kinase (PI3K)/protein kinase B (PKB or AKT)/target of rapamycin (TOR) pathway. Atrogin-1 expression also depended on the availability of an individual amino acid, i.e., methionine. Moreover, the amino acid-induced reduction of atrogin-1 was inhibited by rapamycin, indicating the involvement of the TOR pathway in such regulation. In conclusion, expression of the ubiquitin ligase atrogin-1 is regulated by both insulin and amino acids through the TOR pathway.

mTOR Complex1-S6K1 signaling: at the crossroads of obesity, diabetes and cancer

Recent studies demonstrate that the mammalian target of rapamycin (mTOR) and its effector, S6 kinase 1 (S6K1), lie at the crossroads of a nutrient-hormonal signaling network that is involved in specific pathological responses, including obesity, diabetes and cancer. mTOR exists in two complexes: mTOR Complex1, which is rapamycin-sensitive and phosphorylates S6K1 and initiation factor 4E binding proteins (4E-BPs), and mTOR Complex2, which is rapamycin-insensitive and phosphorylates protein kinase B (PKB, also known as Akt). Both mTOR complexes are stimulated by mitogens, but only mTOR Complex1 is under the control of nutrient and energy inputs. Thus, to orchestrate the control of homeostatic responses, mTOR Complex1 must integrate signals from distinct cues. Here, we review recent findings concerning the regulation and pathophysiology associated with mTOR Complex1 and S6K1.

Stress-responsive Gln3 localization in Saccharomyces cerevisiae is separable from and can overwhelm nitrogen source regulation

Intracellular localization of Saccharomyces cerevisiae GATA family transcription activator, Gln3, is used as a downstream readout of rapamycin-inhibited Tor1,2 control of Tap42 and Sit4 activities. Gln3 is cytoplasmic in cells provided with repressive nitrogen sources such as glutamine and is nuclear in cells growing with a derepressive nitrogen source such as proline or those treated with rapamycin or methionine sulfoximine (Msx). Although gross Gln3-Myc13 phosphorylation levels in wild type cells do not correlate with nitrogen source-determined intracellular Gln3-Myc13 localization, the phosphorylation levels are markedly influenced by several environmental perturbations. Msx treatment increases Snf1-independent Gln3-Myc13 phosphorylation, whereas carbon starvation increases both Snf1-dependent and -independent Gln3-Myc13 phosphorylation. Here we demonstrate that a broad spectrum of environmental stresses (temperature, osmotic, and oxidative) increase Gln3-Myc13 phosphorylation. In parallel, these stresses elicit rapid (<5 min for NaCl) Gln3-Myc13 relocalization from the nucleus to the cytoplasm. The response of Gln3-Myc13 localization to stressful conditions can completely overwhelm its response to nitrogen source quality or inhibitor-generated disruption of the Tor1,2 signal transduction pathway. Adding NaCl to cells cultured under conditions in which Gln3-Myc13 is normally nuclear, i.e. proline-grown, nitrogen-starved, Msx-, caffeine-, and rapamycin-treated wild type cells, or ure2Delta cells, results in its prompt relocalization to the cytoplasm. Together these data identify a major new level of regulation to which Gln3 responds, and adds a new dimension to mechanistic studies of the regulation of this transcription factor.

hvps34, an ancient player, enters a growing game: mTOR Complex1/S6K1 signaling

Recent studies have shown that the nutrient input to the mTOR Complex1/S6K1 signaling pathway is mediated by class 3 PI3K or hVps34, the oldest member of the PI3K family. Moreover, studies to date would suggest that during the evolution of multicellular organisms this ancient branch of the pathway was merged with the growth-factor-hormone-controlled class 1 PI3K pathway at the level of mTOR Complex1 to control the development and growth of the organism. However, hVps34 also plays a role in the regulation of macroautophagy - the mechanism by which cells generate nutrients, such as amino acids, through the degradation of intracellular complexes, including mitochondria and ribosomes. These functions of hVps34 initially appear contradictory, since increased mTOR Complex1 activation is triggered by increased amino acid levels, while autophagy is triggered when cells are faced with amino acid deprivation.

Amino acids and insulin act additively to regulate components of the ubiquitin-proteasome pathway in C2C12 myotubes

Background

The ubiquitin-proteasome system is the predominant pathway for myofibrillar proteolysis but a previous study in C2C12 myotubes only observed alterations in lysosome-dependent proteolysis in response to complete starvation of amino acids or leucine from the media. Here, we determined the interaction between insulin and amino acids in the regulation of myotube proteolysis

Results

Incubation of C2C12 myotubes with 0.2 × physiological amino acids concentration (0.2 × PC AA), relative to 1.0 × PC AA, significantly increased total proteolysis and the expression of 14-kDa E2 ubiquitin conjugating enzyme (p < 0.05). The proteasome inhibitor MG132 blocked the rise in proteolysis observed in the 0.2 × PC AA media. Addition of insulin to the medium inhibited proteolysis at both 0.2 and 1.0× PC AA and the expression of 14-kDa E2 proteins and C2 sub unit of 20 S proteasome (p < 0.05). Incubation of myotubes with increasing concentrations of leucine in the 0.2 × PC AA media inhibited proteolysis but only in the presence of insulin. Incubation of rapamycin (inhibitor of mTOR) inhibited amino acid or insulin-dependent p70 S6 kinase phosphorylation, blocked (P < 0.05) the inhibitory effects of 1.0 × PC AA on protein degradation, but did not alter the inhibitory effects of insulin or leucine

Conclusion

In a C2C12 myotube model of myofibrillar protein turnover, amino acid limitation increases proteolysis in a ubiquitin-proteasome-dependent manner. Increasing amino acids or leucine alone, act additively with insulin to down regulate proteolysis and expression of components of ubiquitin-proteasome pathway. The effects of amino acids on proteolysis but not insulin and leucine, are blocked by inhibition of the mTOR signalling pathway.

Point mutations in TOR confer Rheb-independent growth in fission yeast and nutrient-independent mammalian TOR signaling in mammalian cells

Rheb is a unique member of the Ras superfamily GTP-binding proteins. We as well as others previously have shown that Rheb is a critical component of the TSC/TOR signaling pathway. In fission yeast, Rheb is encoded by the rhb1 gene. Rhb1p is essential for growth and directly interacts with Tor2p. In this article, we report identification of 22 single amino acid changes in the Tor2 protein that enable growth in the absence of Rhb1p. These mutants also exhibit decreased mating efficiency. Interestingly, the mutations are located in the C-terminal half of the Tor2 protein, clustering mainly within the FAT and kinase domains. We noted some differences in the effect of a mutation in the FAT domain (L1310P) and in the kinase domain (E2221K) on growth and mating. Although the Tor2p mutations bypass Rhb1p's requirement for growth, they are incapable of suppressing Rhb1p's requirement for resistance to stress and toxic amino acids, pointing to multiple functions of Rhb1p. In mammalian systems, we find that mammalian target of rapamycin (mTOR) carrying analogous mutations (L1460P or E2419K), although sensitive to rapamycin, exhibits constitutive activation even when the cells are starved for nutrients. These mutations do not show significant difference in their ability to form complexes with Raptor, Rictor, or mLST8. Furthermore, we present evidence that mutant mTOR can complex with wild-type mTOR and that this heterodimer is active in nutrient-starved cells.