Metabolic stress controls mTORC1 lysosomal localization and dimerization by regulating the TTT-RUVBL1/2 complex

Metabolism. Differential regulation of mTORC1 by leucine and glutamine

The mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) integrates environmental and intracellular signals to regulate cell growth. Amino acids stimulate mTORC1 activation at the lysosome in a manner thought to be dependent on the Rag small guanosine triphosphatases (GTPases), the Ragulator complex, and the vacuolar H⁺-adenosine triphosphatase (v-ATPase). We report that leucine and glutamine stimulate mTORC1 by Rag GTPase-dependent and -independent mechanisms, respectively. Glutamine promoted mTORC1 translocation to the lysosome in RagA and RagB knockout cells and required the v-ATPase but not the Ragulator. Furthermore, we identified the adenosine diphosphate ribosylation factor-1 GTPase to be required for mTORC1 activation and lysosomal localization by glutamine. Our results uncover a signaling cascade to mTORC1 activation independent of the Rag GTPases and suggest that mTORC1 is differentially regulated by specific amino acids.

A nexus for cellular homeostasis: the interplay between metabolic and signal transduction pathways

In multicellular organisms, individual cells have evolved to sense external and internal cues in order to maintain cellular homeostasis and survive under different environmental conditions. Cells efficiently adjust their metabolism to reflect the abundance of nutrients, energy and growth factors. The ability to rewire cellular metabolism between anabolic and catabolic processes is crucial for cells to thrive. Thus, cells have developed, through evolution, metabolic networks that are highly plastic and tightly regulated to meet the requirements necessary to maintain cellular homeostasis. The plasticity of these cellular systems is tightly regulated by complex signaling networks that integrate the intracellular and extracellular information. The coordination of signal transduction and metabolic pathways is essential in maintaining a healthy and rapidly responsive cellular state.

Npr2 inhibits TORC1 to prevent inappropriate utilization of glutamine for biosynthesis of nitrogen-containing metabolites

Cells must be capable of switching between growth and autophagy in unpredictable nutrient environments. The conserved Npr2 protein complex (comprising Iml1, Npr2, and Npr3; also called SEACIT) inhibits target of rapamycin complex 1 (TORC1) kinase signaling, which inhibits autophagy in nutrient-rich conditions. In yeast cultured in media with nutrient limitations that promote autophagy and inhibit growth, loss of Npr2 enables cells to bypass autophagy and proliferate. We determined that Npr2-deficient yeast had a metabolic state distinct from that of wild-type yeast when grown in minimal media containing ammonium as a nitrogen source and a nonfermentable carbon source (lactate). Unlike wild-type yeast, which accumulated glutamine, Npr2-deficient yeast metabolized glutamine into nitrogen-containing metabolites and maintained a high concentration of S-adenosyl methionine (SAM). Moreover, in wild-type yeast grown in these nutrient-limited conditions, supplementation with methionine stimulated glutamine consumption for synthesis of nitrogenous metabolites, demonstrating integration of a sulfur-containing amino acid cue and nitrogen utilization. These data revealed the metabolic basis by which the Npr2 complex regulates cellular homeostasis and demonstrated a key function for TORC1 in regulating the synthesis and utilization of glutamine as a nitrogen source.

Stoichiometry and assembly of mTOR complexes revealed by single-molecule pulldown

The mammalian target of rapamycin (mTOR) kinase is a master regulator of cellular, developmental, and metabolic processes. Deregulation of mTOR signaling is implicated in numerous human diseases including cancer and diabetes. mTOR functions as part of either of the two multisubunit complexes, mTORC1 and mTORC2, but molecular details about the assembly and oligomerization of mTORCs are currently lacking. We use the single-molecule pulldown (SiMPull) assay that combines principles of conventional pulldown assays with single-molecule fluorescence microscopy to investigate the stoichiometry and assembly of mTORCs. After validating our approach with mTORC1, confirming a dimeric assembly as previously reported, we show that all major components of mTORC2 exist in two copies per complex, indicating that mTORC2 assembles as a homodimer. Interestingly, each mTORC component, when free from the complexes, is present as a monomer and no single subunit serves as the dimerizing component. Instead, our data suggest that dimerization of mTORCs is the result of multiple subunits forming a composite surface. SiMPull also allowed us to distinguish complex disassembly from stoichiometry changes. Physiological conditions that abrogate mTOR signaling such as nutrient deprivation or energy stress did not alter the stoichiometry of mTORCs. On the other hand, rapamycin treatment leads to transient appearance of monomeric mTORC1 before complete disruption of the mTOR-raptor interaction, whereas mTORC2 stoichiometry is unaffected. These insights into assembly of mTORCs may guide future mechanistic studies and exploration of therapeutic potential.

Regulation of rDNA transcription in response to growth factors, nutrients and energy

Exquisite control of ribosome biogenesis is fundamental for the maintenance of cellular growth and proliferation. Importantly, synthesis of ribosomal RNA by RNA polymerase I is a key regulatory step in ribosome biogenesis and a major biosynthetic and energy consuming process. Consequently, ribosomal RNA gene transcription is tightly coupled to the availability of growth factors, nutrients and energy. Thus cells have developed an intricate sensing network to monitor the cellular environment and modulate ribosomal DNA transcription accordingly. Critical controllers in these sensing networks, which mediate growth factor activation of ribosomal DNA transcription, include the PI3K/AKT/mTORC1, RAS/RAF/ERK pathways and MYC transcription factor. mTORC1 also responds to amino acids and energy status, making it a key hub linking all three stimuli to the regulation of ribosomal DNA transcription, although this is achieved via overlapping and distinct mechanisms. This review outlines the current knowledge of how cells respond to environmental cues to control ribosomal RNA synthesis. We also highlight the critical points within this network that are providing new therapeutic opportunities for treating cancers through modulation of RNA polymerase I activity and potential novel imaging strategies.

Hexokinase II integrates energy metabolism and cellular protection: Akting on mitochondria and TORCing to autophagy

Accumulating evidence reveals that metabolic and cell survival pathways are closely related, sharing common signaling molecules. Hexokinase catalyzes the phosphorylation of glucose, the rate-limiting first step of glycolysis. Hexokinase II (HK-II) is a predominant isoform in insulin-sensitive tissues such as heart, skeletal muscle, and adipose tissues. It is also upregulated in many types of tumors associated with enhanced aerobic glycolysis in tumor cells, the Warburg effect. In addition to the fundamental role in glycolysis, HK-II is increasingly recognized as a component of a survival signaling nexus. This review summarizes recent advances in understanding the protective role of HK-II, controlling cellular growth, preventing mitochondrial death pathway and enhancing autophagy, with a particular focus on the interaction between HK-II and Akt/mTOR pathway to integrate metabolic status with the control of cell survival.

Therapeutic targeting of cellular metabolism in cells with hyperactive mTORC1: a paradigm shift

mTORC1 is an established master regulator of cellular metabolic homeostasis, via multiple mechanisms that include altered glucose and glutamine metabolism, and decreased autophagy. mTORC1 is hyperactive in the human disease tuberous sclerosis complex (TSC), an autosomal dominant disorder caused by germline mutations in the TSC1 or TSC2 gene. In TSC-deficient cells, metabolic wiring is extensively disrupted and rerouted as a consequence of mTORC1 hyperactivation, leading to multiple vulnerabilities, including "addiction" to glutamine, glucose, and autophagy. There is synergy between two rapidly evolving trajectories: elucidating the metabolic vulnerabilities of TSC-associated tumor cells, and the development of therapeutic agents that selectively target cancer-associated metabolic defects. The current review focuses on recent work supporting the targeting of cellular metabolic dysregulation for the treatment of tumors in TSC, with relevance to the many other human neoplasms with mTORC1 hyperactivation. These data expose a fundamental paradox in the therapeutic targeting of tumor cells with hyperactive mTORC1: inhibition of mTORC1 may not represent the optimal therapeutic strategy. Inhibiting mTORC1 "fixes" the metabolic vulnerabilities, results in a cytostatic response, and closes the door to metabolic targeting. In contrast, leaving mTORC1 active allows the metabolic vulnerabilities to be targeted with the potential for a cytocidal cellular response. The insights provided here suggest that therapeutic strategies for TSC and other tumors with activation of mTORC1 are at the verge of a major paradigm shift, in which optimal clinical responses will be accomplished by targeting mTORC1-associated metabolic vulnerabilities without inhibiting mTORC1 itself.

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

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

Hepatic mTORC1 controls locomotor activity, body temperature, and lipid metabolism through FGF21

The liver is a key metabolic organ that controls whole-body physiology in response to nutrient availability. Mammalian target of rapamycin (mTOR) is a nutrient-activated kinase and central controller of growth and metabolism that is negatively regulated by the tumor suppressor tuberous sclerosis complex 1 (TSC1). To investigate the role of hepatic mTOR complex 1 (mTORC1) in whole-body physiology, we generated liver-specific Tsc1 (L-Tsc1 KO) knockout mice. L-Tsc1 KO mice displayed reduced locomotor activity, body temperature, and hepatic triglyceride content in a rapamycin-sensitive manner. Ectopic activation of mTORC1 also caused depletion of hepatic and plasma glutamine, leading to peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α)-dependent fibroblast growth factor 21 (FGF21) expression in the liver. Injection of glutamine or knockdown of PGC-1α or FGF21 in the liver suppressed the behavioral and metabolic defects due to mTORC1 activation. Thus, mTORC1 in the liver controls whole-body physiology through PGC-1α and FGF21. Finally, mTORC1 signaling correlated with FGF21 expression in human liver tumors, suggesting that treatment of glutamine-addicted cancers with mTOR inhibitors might have beneficial effects at both the tumor and whole-body level.

Parkin ubiquitinates mTOR to regulate mTORC1 activity under mitochondrial stress

mTORC1, a kinase complex that is considered a master regulator of cellular growth and proliferation, is regulated by many extra- and intracellular signals. Among these signals, mitochondrial status is known to have an impact on the effects of mTORC1 on cell growth and survival. However, how mitochondrial status affects mTORC1 activity, notably the molecular link, is not fully elucidated. Here, we found that Parkin can interact with and ubiquitinate mTOR. We also identified K2066 and K2306 as Parkin-dependent and mitochondrial stress-induced mTOR ubiquitination residues. This ubiquitination by Parkin is required for maintenance of mTORC1 activity under mitochondrial stress. With regard to the physiological meaning of mTORC1 activity under mitochondrial stress, we suggest that mTORC1 plays a pro-survival role.

Making new contacts: the mTOR network in metabolism and signalling crosstalk

More than 20 years after its discovery, our understanding of target of rapamycin (TOR) signalling continues to grow. Recent global 'omics' studies have revealed physiological roles of mammalian TOR (mTOR) in protein, nucleotide and lipid synthesis. Furthermore, emerging evidence provides new insight into the control of mTOR by other pathways such as Hippo, WNT and Notch signalling. Together, this progress has expanded the list of downstream effectors and upstream regulators of mTOR signalling.

Phosphorylation-dependent PIH1D1 interactions define substrate specificity of the R2TP cochaperone complex

The R2TP cochaperone complex plays a critical role in the assembly of multisubunit machines, including small nucleolar ribonucleoproteins (snoRNPs), RNA polymerase II, and the mTORC1 and SMG1 kinase complexes, but the molecular basis of substrate recognition remains unclear. Here, we describe a phosphopeptide binding domain (PIH-N) in the PIH1D1 subunit of the R2TP complex that preferentially binds to highly acidic phosphorylated proteins. A cocrystal structure of a PIH-N domain/TEL2 phosphopeptide complex reveals a highly specific phosphopeptide recognition mechanism in which Lys57 and 64 in PIH1D1, along with a conserved DpSDD phosphopeptide motif within TEL2, are essential and sufficient for binding. Proteomic analysis of PIH1D1 interactors identified R2TP complex substrates that are recruited by the PIH-N domain in a sequence-specific and phosphorylation-dependent manner suggestive of a common mechanism of substrate recognition. We propose that protein complexes assembled by the R2TP complex are defined by phosphorylation of a specific motif and recognition by the PIH1D1 subunit.

Dennd3 functions as a guanine nucleotide exchange factor for small GTPase Rab12 in mouse embryonic fibroblasts

Small GTPase Rab12 regulates mTORC1 (mammalian target of rapamycin complex 1) activity and autophagy through controlling PAT4 (proton/amino acid transporter 4) trafficking from recycling endosomes to lysosomes, where PAT4 is degraded. However, the precise regulatory mechanism of the Rab12-mediated membrane trafficking pathway remained to be determined because a physiological Rab12-GEF (guanine nucleotide exchange factor) had yet to be identified. In this study we performed functional analyses of Dennd3, which has recently been shown to possess a GEF activity toward Rab12 in vitro. The results showed that knockdown of Dennd3 in mouse embryonic fibroblast cells caused an increase in the amount of PAT4 protein, the same as Rab12 knockdown did, and knockdown of Dennd3 and overexpression of Dennd3 were found to result in an increase and a decrease, respectively, in the intracellular amino acid concentration. Dennd3 overexpression was also found to reduce mTORC1 activity and promoted autophagy in a Rab12-dependent manner. Unexpectedly, however, Dennd3 knockdown had no effect on mTORC1 activity or autophagy despite increasing the intracellular amino acid concentration. Further study showed that Dennd3 knockdown reduced Akt activity, and the reduction in Akt activity is likely to have canceled out amino acid-induced mTORC1 activation through PAT4. These findings indicated that Dennd3 not only functions as a Rab12-GEF but also modulates Akt signaling in mouse embryonic fibroblast cells.

Where is mTOR and what is it doing there?

Target of rapamycin (TOR) forms two conserved, structurally distinct kinase complexes termed TOR complex 1 (TORC1) and TORC2. Each complex phosphorylates a different set of substrates to regulate cell growth. In mammals, mTOR is stimulated by nutrients and growth factors and inhibited by stress to ensure that cells grow only during favorable conditions. Studies in different organisms have reported localization of TOR to several distinct subcellular compartments. Notably, the finding that mTORC1 is localized to the lysosome has significantly enhanced our understanding of mTORC1 regulation. Subcellular localization may be a general principle used by TOR to enact precise spatial and temporal control of cell growth.

Hexokinase-II positively regulates glucose starvation-induced autophagy through TORC1 inhibition

Hexokinase-II (HK-II) catalyzes the first step of glycolysis and also functions as a protective molecule; however, its role in protective autophagy has not been determined. Results showed that inhibition of HK-II diminished, while overexpression of HK-II potentiated, autophagy induced by glucose deprivation in cardiomyocyte and noncardiomyocyte cells. Immunoprecipitation studies revealed that HK-II binds to and inhibits the autophagy suppressor, mTOR complex 1 (TORC1), and that this binding was increased by glucose deprivation. The TOS motif, a scaffold sequence responsible for binding TORC1 substrates, is present in HK-II, and mutating it blocked its ability to bind to TORC1 and regulate protective autophagy. The transition from glycolysis to autophagy appears to be regulated by a decrease in glucose-6 phosphate. We suggest that HK-II binds TORC1 as a decoy substrate and provides a previously unrecognized mechanism for switching cells from a metabolic economy, based on plentiful energy, to one of conservation, under starvation.

Cell cycle regulation by the nutrient-sensing mammalian target of rapamycin (mTOR) pathway

Cell division involves a series of ordered and controlled events that lead to cell proliferation. Cell cycle progression implies not only demanding amounts of cell mass, protein, lipid, and nucleic acid content but also a favorable energy state. The mammalian target of rapamycin (mTOR), in response to the energy state, nutrient status, and growth factor stimulation of cells, plays a pivotal role in the coordination of cell growth and the cell cycle. Here, we review how the nutrient-sensing mTOR-signaling cascade molecularly integrates nutritional and mitogenic/anti-apoptotic cues to accurately coordinate cell growth and cell cycle. First, we briefly outline the structure, functions, and regulation of the mTOR complexes (mTORC1 and mTORC2). Second, we concisely evaluate the best known ability of mTOR to control G1-phase progression. Third, we discuss in detail the recent evidence that indicates a new genome stability caretaker function of mTOR based on the specific ability of phosphorylated forms of several mTOR-signaling components (AMPK, raptor, TSC, mTOR, and S6K1), which spatially and temporally associate with essential mitotic regulators at the mitotic spindle and at the cytokinetic cleavage furrow.

Signal integration by mTORC1 coordinates nutrient input with biosynthetic output

Flux through metabolic pathways is inherently sensitive to the levels of specific substrates and products, but cellular metabolism is also managed by integrated control mechanisms that sense the nutrient and energy status of a cell or organism. The mechanistic target of rapamycin complex 1 (mTORC1), a protein kinase complex ubiquitous to eukaryotic cells, has emerged as a critical signalling node that links nutrient sensing to the coordinated regulation of cellular metabolism. Here, we discuss the role of mTORC1 as a conduit between cellular growth conditions and the anabolic processes that promote cell growth. The emerging network of signalling pathways through which mTORC1 integrates systemic signals (secreted growth factors) with local signals (cellular nutrients - amino acids, glucose and oxygen - and energy, ATP) is detailed. Our expanding understanding of the regulatory network upstream of mTORC1 provides molecular insights into the integrated sensing mechanisms by which diverse cellular signals converge to control cell physiology.

The mTORC1 pathway stimulates glutamine metabolism and cell proliferation by repressing SIRT4

Proliferating mammalian cells use glutamine as a source of nitrogen and as a key anaplerotic source to provide metabolites to the tricarboxylic acid cycle (TCA) for biosynthesis. Recently, mammalian target of rapamycin complex 1 (mTORC1) activation has been correlated with increased nutrient uptake and metabolism, but no molecular connection to glutaminolysis has been reported. Here, we show that mTORC1 promotes glutamine anaplerosis by activating glutamate dehydrogenase (GDH). This regulation requires transcriptional repression of SIRT4, the mitochondrial-localized sirtuin that inhibits GDH. Mechanistically, mTORC1 represses SIRT4 by promoting the proteasome-mediated destabilization of cAMP-responsive element binding 2 (CREB2). Thus, a relationship between mTORC1, SIRT4, and cancer is suggested by our findings. Indeed, SIRT4 expression is reduced in human cancer, and its overexpression reduces cell proliferation, transformation, and tumor development. Finally, our data indicate that targeting nutrient metabolism in energy-addicted cancers with high mTORC1 signaling may be an effective therapeutic approach.

The FoxO-BNIP3 axis exerts a unique regulation of mTORC1 and cell survival under energy stress

Normal cells possess adaptive mechanisms to couple energy availability with cell growth (cell size increase) and survival, and imbalances are associated with major diseases such as cancer. Inactivation of critical regulators involved in energy stress response, including adenosine monophosphate-activated protein kinase (AMPK), liver kinase B1 (LKB1), tuberous sclerosis complex 1 (TSC1) and tuberous sclerosis complex 2 (TSC2), leads to uncontrolled cell growth yet increased apoptosis under energy stress. These energy stress regulators are also important in tumor suppression and metabolism. Here, we show that forkhead box O (FoxO) transcription factor, a central regulator of tumor suppression and metabolism, plays a unique role in energy stress response. FoxOs inhibit the mammalian target of rapamycin complex 1 (mTORC1), a key regulator of cell growth, under energy stress, and inactivation of FoxOs alleviates energy stress-mediated mTORC1 repression. Surprisingly, unlike AMPK-, Lkb1- or Tsc1/2-deficient cells, FoxO-deficient cells exhibit decreased apoptosis under energy stress. FoxOs operate to inhibit mTORC1 signaling and cell survival independent of AMPK and TSC. Integrated transcriptomic and functional analyses identified BCL2/adenovirus E1B 19 kDa protein-interacting protein 3 (BNIP3)-a negative regulator of both Rheb and Bcl2 prosurvival family members-as a key downstream target of FoxOs to inhibit mTORC1 function and promote apoptosis in response to energy stress. We show that p38β, but not AMPK, is likely to function upstream of FoxO-BNIP3 to mediate energy stress response. Finally, we reveal that low expression of FoxO or BNIP3 correlates with poor clinical outcomes in renal cancer patients. Together, our study uncovers a novel signaling circuit functioning to mediate cellular energy responses to control cell growth and survival. These findings also have important implications to human cancers.

The role of mechanistic target of rapamycin (mTOR) complexes signaling in the immune responses

The mechanistic Target of Rapamycin (mTOR) is an evolutionarily conserved serine/threonine kinase which is a member of the PI3K related kinase (PIKK) family. mTOR emerged as a central node in cellular metabolism, cell growth, and differentiation, as well as cancer metabolism. mTOR senses the nutrients, energy, insulin, growth factors, and environmental cues and transmits signals to downstream targets to effectuate the cellular and metabolic response. Recently, mTOR was also implicated in the regulation of both the innate and adaptive immune responses. This paper will summarize the current knowledge of mTOR, as related to the immune microenvironment and immune responses.

Nutrient regulation of the mTOR complex 1 signaling pathway

The mammalian target of rapamycin (mTOR) is an evolutionally conserved kinase which exists in two distinct structural and functional complexes, mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2). Of the two complexes, mTORC1 couples nutrient abundance to cell growth and proliferation by sensing and integrating a variety of inputs arising from amino acids, cellular stresses, energy status, and growth factors. Defects in mTORC1 regulation are implicated in the development of many metabolic diseases, including cancer and diabetes. Over the past decade, significant advances have been made in deciphering the complexity of the signaling processes contributing to mTORC1 regulation and function, but the mechanistic details are still not fully understood. In particular, how amino acid availability is sensed by cells and signals to mTORC1 remains unclear. In this review, we discuss the current understanding of nutrient-dependent control of mTORC1 signaling and will focus on the key components involved in amino acid signaling to mTORC1.

Loss of TSC2 confers resistance to ceramide and nutrient deprivation

Nutrient stress that produces quiescence and catabolism in normal cells is lethal to cancer cells, because oncogenic mutations constitutively drive anabolism. One driver of biosynthesis in cancer cells is the mammalian target of rapamycin complex 1 (mTORC1) signaling complex. Activating mTORC1 by deleting its negative regulator tuberous sclerosis complex 2 (TSC2) leads to hypersensitivity to glucose deprivation. We have previously shown that ceramide kills cells in part by triggering nutrient transporter loss and restricting access to extracellular amino acids and glucose, suggesting that TSC2-deficient cells would be hypersensitive to ceramide. However, murine embryonic fibroblasts (MEFs) lacking TSC2 were highly resistant to ceramide-induced death. Consistent with the observation that ceramide limits access to both amino acids and glucose, TSC2(-/-) MEFs also had a survival advantage when extracellular amino acids and glucose were both reduced. As TSC2(-/-) MEFs were resistant to nutrient stress despite sustained mTORC1 activity, we assessed whether mTORC1 signaling might be beneficial under these conditions. In low amino acid and glucose medium, and following ceramide-induced nutrient transporter loss, elevated mTORC1 activity significantly enhanced the adaptive upregulation of new transporter proteins for amino acids and glucose. Strikingly, the introduction of oncogenic Ras abrogated the survival advantage of TSC2(-/-) MEFs upon ceramide treatment most likely by increasing nutrient demand. These results suggest that, in the absence of oncogene-driven biosynthetic demand, mTORC1-dependent translation facilitates the adaptive cellular response to nutrient stress.

Nutrient sensing, metabolism, and cell growth control

Cell growth is regulated by coordination of both extracellular nutrients and intracellular metabolite concentrations. AMP-activated kinase and mammalian target of rapamycin complex 1 serve as key molecules that sense cellular energy and nutrients levels, respectively. In addition, the members of the dioxygenase family, including prolylhydroxylase, lysine demethylase, and DNA demethylase, have emerged as possible sensors of intracellular metabolic status. The interplay among nutrients, metabolites, gene expression, and protein modification are involved in the coordination of cell growth with extracellular and intracellular conditions.

The emergence of the conserved AAA+ ATPases Pontin and Reptin on the signaling landscape

Pontin (also known as RUVBL1 and RVB1) and Reptin (also called RUVBL2 and RVB2) are related members of the large AAA+ (adenosine triphosphatase associated with diverse cellular activities) superfamily of conserved proteins. Various cellular functions depend on Pontin and Reptin, mostly because of their functions in the assembly of protein complexes that play a role in the regulation of cellular energetic metabolism, transcription, chromatin remodeling, and the DNA damage response. Little is known, though, about the interconnections between these multiple functions, how the relevant signaling pathways are regulated, whether the interconnections are affected in human disease, and whether components of these pathways are suitable targets for therapeutic intervention. The First International Workshop on Pontin (RUVBL1) and Reptin (RUVBL2), held between 16 and 19 October 2012, discussed the nature of the oligomeric organization of these proteins, their structures, their roles as partners in various protein complexes, and their involvement in cellular regulation, signaling, and pathophysiology, as well as their potential for therapeutic targeting. A major outcome of the meeting was a general consensus that most functions of Pontin and Reptin are related to their roles as chaperones or adaptor proteins that are important for the assembly and function of large signaling protein complexes.

Nutrient signaling to mTOR and cell growth

The mammalian target of rapamycin (mTOR) is a conserved protein kinase involved in a multitude of cellular processes including cell growth. Increased mTOR activation is observed in multiple human cancers and inhibition of mTOR has proven efficacious in numerous clinical trials. mTOR comprises two complexes, termed mTORC1 and mTORC2. Both complexes respond to growth factors, whereas only mTORC1 is controlled by nutrients, such as glucose and amino acids. Since the discovery of mTOR, extensive studies have intricately detailed the molecular mechanisms by which mTORC1 is regulated. Somewhat paradoxically, amino acid (AA)-induced mTORC1 activation -arguably the most essential stimulus leading to mTORC1 activation - is the least understood. Here we review the current knowledge of nutrient-dependent regulation of mTORC1.

Hippo-YAP and mTOR pathways collaborate to regulate organ size

Regulation of organ size is achieved through the action of the mTOR and Hippo signalling pathways, which control cell proliferation and cell growth in response to extracellular cues. A link between these pathways is revealed by the finding that YAP downregulates PTEN to promote cell growth and tissue hyperplasia.