Cytoplasmic-nuclear shuttling of FKBP12-rapamycin-associated protein is involved in rapamycin-sensitive signaling and translation initiation

The Spectrum of Renal "TFEopathies": Flipping the mTOR Switch in Renal Tumorigenesis

The mammalian target of Rapamycin complex 1 (mTORC1) is a serine/threonine kinase that couples nutrient and growth factor signaling to the cellular control of metabolism and plays a fundamental role in aberrant proliferation in cancer. mTORC1 has previously been considered an "on/off" switch, capable of phosphorylating the entire pool of its substrates when activated. However, recent studies have indicated that mTORC1 may be active toward its canonical substrates, eukaryotic translation initiation factor 4E-binding protein 1 (4EBP1) and S6 kinase (S6K), involved in mRNA translation and protein synthesis, and inactive toward TFEB and TFE3, transcription factors involved in the regulation of lysosome biogenesis, in several pathological contexts. Among these conditions are Birt-Hogg-Dubé syndrome (BHD) and, recently, tuberous sclerosis complex (TSC). Furthermore, increased TFEB and TFE3 nuclear localization in these syndromes, and in translocation renal cell carcinomas (tRCC), drives mTORC1 activity toward the canonical substrates, through the transcriptional activation of the Rag GTPases, thereby positioning TFEB and TFE3 upstream of mTORC1 activity toward 4EBP1 and S6K. The expanding importance of TFEB and TFE3 in the pathogenesis of these renal diseases warrants a novel clinical grouping that we term "TFEopathies." Currently, there are no therapeutic options directly targeting TFEB and TFE3, which represents a challenging and critically required avenue for cancer research.

Nuclear mTOR Signaling Orchestrates Transcriptional Programs Underlying Cellular Growth and Metabolism

mTOR is a central regulator of cell growth and metabolism in response to mitogenic and nutrient signals. Notably, mTOR is not only found in the cytoplasm but also in the nucleus. This review highlights direct involvement of nuclear mTOR in regulating transcription factors, orchestrating epigenetic modifications, and facilitating chromatin remodeling. These effects intricately modulate gene expression programs associated with growth and metabolic processes. Furthermore, the review underscores the importance of nuclear mTOR in mediating the interplay between metabolism and epigenetic modifications. By integrating its functions in nutrient signaling and gene expression related to growth and metabolism, nuclear mTOR emerges as a central hub governing cellular homeostasis, malignant transformation, and cancer progression. Better understanding of nuclear mTOR signaling has the potential to lead to novel therapies against cancer and other growth-related diseases.

p85S6K sustains synaptic GluA1 to ameliorate cognitive deficits in Alzheimer's disease

BACKGROUND

Ribosomal protein S6 kinase 1 (S6K1) is a serine-threonine kinase that has two main isoforms: p70S6K (70-kDa isoform) and p85S6K (85-kDa isoform). p70S6K, with its upstream mammalian target of rapamycin (mTOR), has been shown to be involved in learning and memory and participate in the pathophysiology of Alzheimer's disease (AD). However, the function of p85S6K has long been neglected due to its high similarity to p70S6k. The role of p85S6K in learning and memory is still largely unknown.

METHODS

We fractionated the postsynaptic densities to illustrate the differential distribution of p85S6K and p70S6K. Coimmunoprecipitation was performed to unveil interactions between p85S6K and the GluA1 subunit of AMPA receptor. The roles of p85S6K in synaptic targeting of GluA1 and learning and memory were evaluated by specific knockdown or overexpression of p85S6K followed by a broad range of methodologies including immunofluorescence, Western blot, in situ proximity ligation assay, morphological staining and behavioral examination. Further, the expression level of p85S6K was measured in brains from AD patients and AD model mice.

RESULTS

p85S6K, but not p70S6K, was enriched in the postsynaptic densities. Moreover, knockdown of p85S6K resulted in defective spatial and recognition memory. In addition, p85S6K could interact with the GluA1 subunit of AMPA receptor through synapse-associated protein 97 and A-kinase anchoring protein 79/150. Mechanistic studies demonstrated that p85S6K could directly phosphorylate GluA1 at Ser845 and increase the amount of GluA1 in synapses, thus sustaining synaptic function and spine densities. Moreover, p85S6K was found to be specifically decreased in the synaptosomal compartment in the brains of AD patients and AD mice. Overexpression of p85S6K ameliorated the synaptic deficits and cognitive impairment in transgenic AD model mice.

CONCLUSIONS

These results strongly imply a significant role for p85S6K in maintaining synaptic and cognitive function by interacting with GluA1. The findings provide an insight into the rational targeting of p85S6K as a therapeutic potential for AD.

P2RY2-AKT activation is a therapeutically actionable consequence of XPO1 inhibition in acute myeloid leukemia

Selinexor is a first-in-class inhibitor of the nuclear exportin XPO1 that was recently approved by the US Food and Drug Administration for the treatment of multiple myeloma and diffuse large B-cell lymphoma. In relapsed/refractory acute myeloid leukemia (AML), selinexor has shown promising activity, suggesting that selinexor-based combination therapies may have clinical potential. Here, motivated by the hypothesis that selinexor's nuclear sequestration of diverse substrates imposes pleiotropic fitness effects on AML cells, we systematically catalog the pro- and anti-fitness consequences of selinexor treatment. We discover that selinexor activates PI3Kγ-dependent AKT signaling in AML by upregulating the purinergic receptor P2RY2. Inhibiting this axis potentiates the anti-leukemic effects of selinexor in AML cell lines, patient-derived primary cultures and multiple mouse models of AML. In a syngeneic, MLL-AF9-driven mouse model of AML, treatment with selinexor and ipatasertib outperforms both standard-of-care chemotherapy and chemotherapy with selinexor. Together, these findings establish drug-induced P2RY2-AKT signaling as an actionable consequence of XPO1 inhibition in AML.

The non-canonical nuclear functions of key players of the PI3K-AKT-MTOR pathway

The PI3K-AKT-MTOR signal transduction pathway is one of the essential signalling cascades within the cell due to its involvement in many vital functions. The pathway initiates with the recruitment of phosphatidylinositol-3 kinases (PI3Ks) onto the plasma membrane, generating phosphatidylinositol-3,4,5-triphosphate [PtdIns(3,4,5)P₃ ] and subsequently activating AKT. Being the central node of the PI3K network, AKT activates the mechanistic target of rapamycin kinase complex 1 (MTORC1) via Tuberous sclerosis complex 2 inhibition in the cytoplasm. Although the cytoplasmic role of the pathway has been widely explored for decades, we now know that most of the effector molecules of the PI3K axis diverge from the canonical route and translocate to other cell organelles including the nucleus. The presence of phosphoinositides (PtdIns) inside the nucleus itself indicates the existence of a nuclear PI3K signalling. The nuclear localization of these signaling components is evident in regulating many nuclear processes like DNA replication, transcription, DNA repair, maintenance of genomic integrity, chromatin architecture, and cell cycle control. Here, our review intends to present a comprehensive overview of the nuclear functions of the PI3K-AKT-MTOR signaling biomolecules.

Rheb regulates nuclear mTORC1 activity independent of farnesylation

The small GTPase Ras homolog enriched in brain (Rheb) plays a critical role in activating the mechanistic target of rapamycin complex 1 (mTORC1), a signaling hub that regulates various cellular functions. We recently observed nuclear mTORC1 activity, raising an intriguing question as to how Rheb, which is known to be farnesylated and localized to intracellular membranes, regulates nuclear mTORC1. In this study, we found that active Rheb is present in the nucleus and required for nuclear mTORC1 activity. We showed that inhibition of farnesyltransferase reduced cytosolic, but not nuclear, mTORC1 activity. Furthermore, a farnesylation-deficient Rheb mutant, with preferential nuclear localization and specific lysosome tethering, enables nuclear and cytosolic mTORC1 activities, respectively. These data suggest that non-farnesylated Rheb is capable of interacting with and activating mTORC1, providing mechanistic insights into the molecular functioning of Rheb as well as regulation of the recently observed, active pool of nuclear mTORC1.

Unraveling the multifaceted nature of the nuclear function of mTOR

Positioned at the axis between the cell and its environment, mTOR directs a wide range of cellular activity in response to nutrients, growth factors, and stress. Our understanding of the role of mTOR is evolving beyond the spatial confines of the cytosol, and its role in the nucleus becoming ever more apparent. In this review, we will address various studies that explore the role of nuclear mTOR (nmTOR) in specific cellular programs and how these pathways influence one another. To understand the emerging roles of nuclear mTOR, we discuss data and propose plausible mechanisms to offer novel ideas, hypotheses, and future research directions.

Schwannoma development is mediated by Hippo pathway dysregulation and modified by RAS/MAPK signaling

Schwannomas are tumors of the Schwann cells that cause chronic pain, numbness, and potentially life-threatening impairment of vital organs. Despite the identification of causative genes, including NF2 (Merlin), INI1/SMARCB1, and LZTR1, the exact molecular mechanism of schwannoma development is still poorly understood. Several studies have identified Merlin as a key regulator of the Hippo, MAPK, and PI3K signaling pathways; however, definitive evidence demonstrating the importance of these pathways in schwannoma pathogenesis is absent. Here, we provide direct genetic evidence that dysregulation of the Hippo pathway in the Schwann cell lineage causes development of multiple schwannomas in mice. We found that canonical Hippo signaling through the effectors YAP/TAZ is required for schwannomagenesis and that MAPK signaling modifies schwannoma formation. Furthermore, cotargeting YAP/TAZ transcriptional activity and MAPK signaling demonstrated a synergistic therapeutic effect on schwannomas. Our new model provides a tractable platform to dissect the molecular mechanisms underpinning schwannoma formation and the role of combinatorial targeted therapy in schwannoma treatment.

Canonical Hippo signaling through the effectors YAP/TAZ is required for the development of peripheral nervous system tumors of Schwann cells, and MAPK signaling modifies schwannoma formation.

Nuclear Functions of TOR: Impact on Transcription and the Epigenome

The target of rapamycin (TOR) protein kinase is at the core of growth factor- and nutrient-dependent signaling pathways that are well-known for their regulation of metabolism, growth, and proliferation. However, TOR is also involved in the regulation of gene expression, genomic and epigenomic stability. TOR affects nuclear functions indirectly through its activity in the cytoplasm, but also directly through active nuclear TOR pools. The mechanisms by which TOR regulates its nuclear functions are less well-understood compared with its cytoplasmic activities. TOR is an important pharmacological target for several diseases, including cancer, metabolic and neurological disorders. Thus, studies of the nuclear functions of TOR are important for our understanding of basic biological processes, as well as for clinical implications.

CONSTANS-FKBP12 interaction contributes to modulation of photoperiodic flowering in Arabidopsis

Flowering time is a key process in plant development. Photoperiodic signals play a crucial role in the floral transition in Arabidopsis thaliana, and the protein CONSTANS (CO) has a central regulatory function that is tightly regulated at the transcriptional and post-translational levels. The stability of CO protein depends on a light-driven proteasome process that optimizes its accumulation in the evening to promote the production of the florigen FLOWERING LOCUS T (FT) and induce seasonal flowering. To further investigate the post-translational regulation of CO protein we have dissected its interactome network employing in vivo and in vitro assays and molecular genetics approaches. The immunophilin FKBP12 has been identified in Arabidopsis as a CO interactor that regulates its accumulation and activity. FKBP12 and CO interact through the CCT domain, affecting the stability and function of CO. fkbp12 insertion mutants show a delay in flowering time, while FKBP12 overexpression accelerates flowering, and these phenotypes can be directly related to a change in accumulation of FT protein. The interaction is conserved between the Chlamydomonas algal orthologs CrCO-CrFKBP12, revealing an ancient regulatory step in photoperiod regulation of plant development.

Spatial regulation of mTORC1 signalling: Beyond the Rag GTPases

The mechanistic (or mammalian) Target of Rapamycin Complex 1 (mTORC1) is a central regulator of cell growth and metabolism. By integrating mitogenic signals, mTORC1-dependent phosphorylation of substrates dictates the balance between anabolic, pro-growth and catabolic, recycling processes in the cell. The discovery that amino acids activate mTORC1 by promoting its translocation to the lysosome was a fundamental advance in the understanding of mTORC1 signalling. It has since become clear that the lysosome-cytoplasm shuttling of mTORC1 represents just one layer of spatial control of this signalling pathway. This review will focus on exploring the subcellular localisation of mTORC1 and its regulators to multiple sites within the cell. We will discuss how these spatially distinct regions such as endoplasmic reticulum, plasma membrane and the endosomal pathway co-operate to transduce nutrient availability to mTORC1, allowing for tight control of cell growth.

Quantum Blue Reduces the Severity of Woody Breast Myopathy via Modulation of Oxygen Homeostasis-Related Genes in Broiler Chickens

The incidence of woody breast (WB) is increasing on a global scale representing a significant welfare problem and economic burden to the poultry industry and for which there is no effective treatment due to its unknown etiology. In this study, using diffuse reflectance spectroscopy (DRS) coupled with iSTAT portable clinical analyzer, we provide evidence that the circulatory- and breast muscle-oxygen homeostasis is dysregulated [low oxygen and hemoglobin (HB) levels] in chickens with WB myopathy compared to healthy counterparts. Molecular analysis showed that blood HB subunit Mu (HBM), Zeta (HBZ), and hephaestin (HEPH) expression were significantly down regulated; however, the expression of the subunit rho of HB beta (HBBR) was upregulated in chicken with WB compared to healthy counterparts. The breast muscle HBBR, HBE, HBZ, and hypoxia-inducible factor prolyl hydroxylase 2 (PHD2) mRNA abundances were significantly down regulated in WB-affected compared to normal birds. The expression of HIF-1α at mRNA and protein levels was significantly induced in breasts of WB-affected compared to unaffected birds confirming a local hypoxic status. The phosphorylated levels of the upstream mediators AKT at Ser473 site, mTOR at Ser2481 site, and PI3K P85 at Tyr458 site, as well as their mRNA levels were significantly increased in breasts of WB-affected birds. In attempt to identify a nutritional strategy to reduce WB incidence, male broiler chicks (Cobb 500, n = 576) were randomly distributed into 48 floor pens and subjected to six treatments (12 birds/pen; 8 pens/treatment): a nutrient adequate control group (PC), the PC supplemented with 0.3% myo-inositol (PC + MI), a negative control (NC) deficient in available P and Ca by 0.15 and 0.16%, respectively, the NC fed with quantum blue (QB) at 500 (NC + 500 FTU), 1,000 (NC + 1,000 FTU), or 2,000 FTU/kg of feed (NC + 2,000 FTU). Although QB-enriched diets did not affect growth performances (FCR and FE), it did reduce the severity of WB by 5% compared to the PC diet. This effect is mediated by reversing the expression profile of oxygen homeostasis-related genes; i.e., significant down regulation of HBBR and upregulation of HBM, HBZ, and HEPH in blood, as well as a significant upregulation of HBA1, HBBR, HBE, HBZ, and PHD2 in breast muscle compared to the positive control.

XPO1 inhibitor KPT-330 synergizes with Bcl-xL inhibitor to induce cancer cell apoptosis by perturbing rRNA processing and Mcl-1 protein synthesis

XPO1 (exportin1) mediates nuclear export of proteins and RNAs and is frequently overexpressed in cancers. In this study, we show that the orally bioavailable XPO1 inhibitor KPT-330 reduced Mcl-1 protein level, by which it synergized with Bcl-xL inhibitor A-1331852 to induce apoptosis in cancer cells. KPT-330/A-1331852 combination disrupted bindings of Mcl-1 and Bcl-xL to Bax, Bak, and/or Bim, elicited mitochondrial outer membrane permeabilization, and triggered apoptosis. KPT-330 generally mitigated mRNA expression and protein synthesis rather than mRNA nuclear export or protein stability of Mcl-1. KPT-330 inhibited mTORC1/4E-BP1 and Mnk1/eIF4E axes, which disrupted the eIF4F translation initiation complex but was dispensable for Mcl-1 reduction and KPT-330/A-1331852 combination-induced apoptosis. Mature rRNAs are integral components of the ribosome that determines protein synthesis ability. KPT-330 impeded nucleolar rRNA processing and reduced total levels of multiple mature rRNAs. Reconstitution of XPO1 by expressing degradation-resistant C528S mutant retained rRNA amount, Mcl-1 expression, and Bcl-xL inhibitor resistance upon KPT-330 treatment. KPT-330/A-1331852 combination suppressed growth and enhanced apoptosis of non-small cell lung cancer xenografts. Therefore, we clarify the reason of apoptosis resistance of cancer cells to XPO1 inhibition and develop a potential strategy for treating solid tumors.

Direct imaging of the recruitment and phosphorylation of S6K1 in the mTORC1 pathway in living cells

Knowledge of protein signalling pathways in the working cell is seen as a primary route to identifying and developing targeted medicines. In recent years there has been a growing awareness of the importance of the mTOR pathway, making it an attractive target for therapeutic intervention in several diseases. Within this pathway we have focused on S6 kinase 1 (S6K1), the downstream phosphorylation substrate of mTORC1, and specifically identify its juxtaposition with mTORC1. When S6K1 is co-expressed with raptor we show that S6K1 is translocated from the nucleus to the cytoplasm. By developing a novel biosensor we demonstrate in real-time, that phosphorylation and de-phosphorylation of S6K1 occurs mainly in the cytoplasm of living cells. Furthermore, we show that the scaffold protein raptor, that typically recruits mTOR substrates, is not always involved in S6K1 phosphorylation. Overall, we demonstrate how FRET-FLIM imaging technology can be used to show localisation of S6K1 phosphorylation in living cells and hence a key site of action of inhibitors targeting mTOR phosphorylation.

Deptor transcriptionally regulates endoplasmic reticulum homeostasis in multiple myeloma cells

Multiple myeloma (MM) is a malignant disorder of plasma cells characterized by active production and secretion of monoclonal immunoglobulins (IgG), thus rendering cells prone to endoplasmic reticulum (ER) stress. For this reason, MM cell survival requires to maintain ER homeostasis at basal levels. Deptor is an mTOR binding protein, belonging to the mTORC1 and mTORC2 complexes. It was reported that Deptor is overexpressed in MM cells where it inhibits mTOR kinase activity and promotes cell survival by activating Akt signaling. Here we identify Deptor as a nuclear protein, able to bind DNA and regulate transcription in MM cells. In particular, we found that Deptor plays an important role in the maintenance of the ER network, sustaining the expression of several genes involved in this pathway. In agreement with this, Deptor depletion induces ER stress and synergizes the effect of the proteasome inhibitor bortezomib (Bz) in MM cells. These findings provide important new insights in the ER stress control in MM cells.

Canonical signaling and nuclear activity of mTOR-a teamwork effort to regulate metabolism and cell growth

Mechanistic (or mammalian) target of rapamycin (mTOR) is a kinase that regulates almost all functions related to cell growth and metabolism in response to extra- and intracellular stimuli, such as availability of nutrients, the presence of growth factors, or the energy status of the cell. As part of two distinct protein complexes, mTORC1 and mTORC2, the kinase has been shown to influence cell growth and proliferation by controlling ribosome biogenesis, mRNA translation, carbohydrate and lipid metabolism, protein degradation, autophagy as well as microtubule and actin dynamics. In addition to these well-characterized functions, mTOR can also influence gene transcription. While most studies focused on investigating how canonical mTOR signaling regulates the activity of transcription factors outside the nucleus, recent findings point to a more direct role for mTOR as a transcription factor operating on chromatin in the nucleus. In particular, recent genome-wide identification of mTOR targets on chromatin reveals that its activities in the nucleus and cytoplasm are functionally and biologically linked, thus uncovering a novel paradigm in mTOR function.

A Unique Homeostatic Signaling Pathway Links Synaptic Inactivity to Postsynaptic mTORC1

mTORC1-dependent translational control plays a key role in several enduring forms of synaptic plasticity such as long term potentiation (LTP) and mGluR-dependent long term depression. Recent evidence demonstrates an additional role in regulating synaptic homeostasis in response to inactivity, where dendritic mTORC1 serves to modulate presynaptic function via retrograde signaling. Presently, it is unclear whether LTP and homeostatic plasticity use a common route to mTORC1-dependent signaling or whether each engage mTORC1 through distinct pathways. Here, we report a unique signaling pathway that specifically couples homeostatic signaling to postsynaptic mTORC1 after loss of excitatory synaptic input. We find that AMPAR blockade, but not LTP-inducing stimulation, induces phospholipase D (PLD)-dependent synthesis of the lipid second messenger phosphatidic acid (PA) in rat cultured hippocampal neurons of either sex. Pharmacological blockade of PLD1/2 or pharmacogenetic disruption of PA interactions with mTOR eliminates mTORC1 signaling and presynaptic compensation driven by AMPAR blockade, but does not alter mTORC1 activation or functional changes during chemical LTP (cLTP). Overexpression of PLD1, but not PLD2, recapitulates both functional synaptic changes as well as signature cellular adaptations associated with homeostatic plasticity. Finally, transient application of exogenous PA is sufficient to drive rapid presynaptic compensation requiring mTORC1-dependent translation of BDNF in the postsynaptic compartment. These results thus define a unique homeostatic signaling pathway coupling mTORC1 activation to changes in excitatory synaptic drive. Our results further imply that more than one canonical mTORC1 activation pathway may be relevant for the design of novel therapeutic approaches against neurodevelopmental disorders associated with mTORC1 dysregulation.SIGNIFICANCE STATEMENT Homeostatic and Hebbian forms of synaptic plasticity are thought to play complementary roles in regulating neural circuit function, but we know little about how these forms of plasticity are distinguished at the single neuron level. Here, we define a signaling pathway that uniquely links mTORC1 with homeostatic signaling in neurons.

Blocking Stemness and Metastatic Properties of Ovarian Cancer Cells by Targeting p70S6K with Dendrimer Nanovector-Based siRNA Delivery

Metastasis is the cause of most (>90%) cancer deaths and currently lacks effective treatments. Approaches to understanding the biological process, unraveling the most effective molecular target(s), and implementing nanotechnology to increase the therapeutic index are expected to facilitate cancer therapy against metastasis. Here, we demonstrate the potential advantages of bringing these three approaches together through the rational design of a small interfering RNA (siRNA) that targets p70^S6K in cancer stem cells (CSCs) in combination with dendrimer nanotechnology-based siRNA delivery. Our results demonstrated that the generation 6 (G₆) poly(amidoamine) dendrimer can be used as a nanovector to effectively deliver p70^S6K siRNA by forming uniform dendriplex nanoparticles that protect the siRNA from degradation. These nanoparticles were able to significantly knock down p70^S6K in ovarian CSCs, leading to a marked reduction in CSC proliferation and expansion without obvious toxicity toward normal ovarian surface epithelial cells. Furthermore, treatment with the p70^S6K siRNA/G₆ dendriplexes substantially decreased mesothelial interaction, migration and invasion of CSCs in vitro, as well as tumor growth and metastasis in vivo. Collectively, these results suggest that p70^S6K constitutes a promising therapeutic target, and the use of siRNA in combination with nanotechnology-based delivery may constitute a new approach for molecularly targeted cancer therapy to treat metastasis.

PPAR Gamma Coactivator 1 Beta (PGC-1β) Reduces Mammalian Target of Rapamycin (mTOR) Expression via a SIRT1-Dependent Mechanism in Neurons

Mammalian target of rapamycin (mTOR) is a key regulator of metabolism, cell growth, and protein synthesis. Since decreased mTOR activity has been found to slow aging in many species, the aim of this study was to examine the activity of mTOR and its phosphorylated form in in vitro and in vivo models mimicking Alzheimer's disease (AD), and investigate the potential pathway of PGC-1β in regulating mTOR expression. Primary neurons and N2a cells were treated with Aβ25-35, while untreated cells served as controls. The expression of mTOR, p-mTOR (Ser2448), and PGC-1β was determined with Western blotting and RT-PCR assay, and the translocation of mTOR was detected using confocal microscopy. Aβ25-35 treatment stimulated the translocation of mTOR from cytoplasm to nucleus, and resulted in elevated expression of mTOR and p-mTOR (Ser2448) and reduced PGC-1β expression. In addition, overexpression of PGC-1β was found to decrease mTOR expression. The results of this study demonstrate that Aβ increases the expression of mTOR and p-mTOR at the site of Ser2448, and the stimulation of Aβ is likely to depend on sirtuin 1, PPARγ, and PGC-1β pathway in regulating mTOR expression.

An Ancient, Unified Mechanism for Metformin Growth Inhibition in C. elegans and Cancer

Metformin has utility in cancer prevention and treatment, though the mechanisms for these effects remain elusive. Through genetic screening in C. elegans, we uncover two metformin response elements: the nuclear pore complex (NPC) and acyl-CoA dehydrogenase family member-10 (ACAD10). We demonstrate that biguanides inhibit growth by inhibiting mitochondrial respiratory capacity, which restrains transit of the RagA-RagC GTPase heterodimer through the NPC. Nuclear exclusion renders RagC incapable of gaining the GDP-bound state necessary to stimulate mTORC1. Biguanide-induced inactivation of mTORC1 subsequently inhibits growth through transcriptional induction of ACAD10. This ancient metformin response pathway is conserved from worms to humans. Both restricted nuclear pore transit and upregulation of ACAD10 are required for biguanides to reduce viability in melanoma and pancreatic cancer cells, and to extend C. elegans lifespan. This pathway provides a unified mechanism by which metformin kills cancer cells and extends lifespan, and illuminates potential cancer targets. PAPERCLIP.

Phenethyl isothiocyanate suppresses the metastasis of ovarian cancer associated with the inhibition of CRM1-mediated nuclear export and mTOR-STAT3 pathway

Epithelial ovarian cancer is prone to metastasizing at an early stage, but their mechanisms remain unclear. CRM1 is an important nuclear exportin and inhibitors targeting CRM1 has been explored as an anti-cancer strategy. In previous study, we observed that PEITC could combine with the hydrophobic pocket of CRM1. In this study, we focused on the effects of PEITC on EOC and its mechanisms. Results showed that IC50 values of PEITC on SKOV3 and HO8910 cell line were 42.14 μM and 37.29 μM, respectively. PEITC inhibits the migration and invasion of SKOV3 and HO8910 cells in vitro. Oral administration of 10 μmol PEITC suppressed the metastasis of EOC in a xenograft mouse model in vivo. PEITC treatment decreased the expressions of CRM1 and mTOR (cargo protein of CRM1) in EOC cell lines and in xenograft mouse tissues. Moreover, CRM1-mediated nuclear export was attenuated by PEITC, mTOR accumulated in nucleus, expressions of mTORS2448 and downstream effectors STAT3S727, MMP2 and MMP9 were decreased in a dose- and time-dependent manner. Furthermore, immunohistochemical analysis showed that CRM1 and mTOR were increased in EOC tissues compared with benign ovarian tumors, and related with advanced stage, type II EOC, positive peritoneal cytology and decreased overall survival. In addition, CRM1 was positively correlated with mTOR levels. In conclusion, our data demonstrated that PEITC suppresses the metastasis of EOC through inhibiting CRM1-mediated nuclear export, subsequently suppressing the mTOR-STAT3 pathway. Both CRM1 and mTOR were increased in EOC patients, providing a rationale for further clinical investigation of PEITC in EOC treatment.

Vascular endothelial growth factor-D modulates oxidant-antioxidant balance of human vascular endothelial cells

Vascular endothelial growth factor‐D (VEGF‐D) is an angiogenic and lymphangiogenic glycoprotein that facilitates tumour growth and distant organ metastasis. Our previous studies showed that VEGF‐D stimulates the expression of proteins involved in cell–matrix interactions and promoting the migration of endothelial cells. In this study, we focused on the redox homoeostasis of endothelial cells, which is significantly altered in the process of tumour angiogenesis. Our analysis revealed up‐regulated expression of proteins that form the antioxidant barrier of the cell in VEGF‐D‐treated human umbilical endothelial cells and increased production of reactive oxygen and nitrogen species in addition to a transient elevation in the total thiol group content. Despite a lack of changes in the total antioxidant capacity, modification of the antioxidant barrier induced by VEGF‐D was sufficient to protect cells against the oxidative stress caused by hypochlorite and paraquat. These results suggest that exogenous stimulation of endothelial cells with VEGF‐D induces an antioxidant response of cells that maintains the redox balance. Additionally, VEGF‐D‐induced changes in serine/threonine kinase mTOR shuttling between the cytosol and nucleus and its increased phosphorylation at Ser‐2448, lead us to the conclusion that the observed shift in redox balance is regulated via mTOR kinase signalling.

Muscle fiber type diversification during exercise and regeneration

The plasticity of skeletal muscle can be traced down to extensive metabolic, structural and molecular remodeling at the single fiber level. Skeletal muscle is comprised of different fiber types that are the basis of muscle plasticity in response to various functional demands. Resistance and endurance exercises are two external stimuli that differ in their duration and intensity of contraction and elicit markedly different responses in muscles adaptation. Further, eccentric contractions that are associated with exercise-induced injuries, elicit varied muscle adaptation and regenerative responses. Most adaptive changes are fiber type-specific and are highly influenced by diverse structural, metabolic and functional characteristics of individual fiber types. Regulation of signaling pathways by reactive oxygen species (ROS) and oxidative stress also plays an important role in muscle fiber adaptation during exercise. This review focuses on cellular and molecular responses that regulate the adaptation of skeletal muscle to exercise and exercise-related injuries.

Gad8 Protein Is Found in the Nucleus Where It Interacts with the MluI Cell Cycle Box-binding Factor (MBF) Transcriptional Complex to Regulate the Response to DNA Replication Stress

The target of rapamycin (TOR) kinase is found at the core of two evolutionarily conserved complexes known as TOR complexes 1 and 2 (TORC1 and TORC2). In fission yeast, TORC2 is dispensable for proliferation under optimal growth conditions but is required for starvation and stress responses. We have previously reported that loss of function of TORC2 renders cells highly sensitive to DNA replication stress; however, the mechanism underlying this sensitivity is unknown. TORC2 has one known direct substrate, the kinase Gad8, which is related to AKT in human cells. Here we show that both TORC2 and its substrate Gad8 are found in the nucleus and are bound to the chromatin. We also demonstrate that Gad8 physically interacts with the MluI cell cycle box-binding factor (MBF) transcription complex that regulates the G1/S progression and the response to DNA stress. In mutant cells lacking TORC2 or Gad8, the binding of the MBF complex to its cognate promoters is compromised, and the induction of MBF target genes in response to DNA replication stress is reduced. Consistently, the protein levels of Cdt2 and Cig2, two MBF target genes, are reduced in the absence of TORC2-Gad8 signaling. Taken together, our findings highlight critical functions of TORC2 in the nucleus and suggest a role in surviving DNA replication stress via transcriptional regulation of MBF target genes.

cAMP-Induced Histones H3 Dephosphorylation Is Independent of PKA and MAP Kinase Activations and Correlates With mTOR Inactivation

cAMP is a second messenger well documented to be involved in the phosphorylation of PKA, MAP kinase, and histone H3 (H3). Early, we reported that cAMP also induced H3 dephosphorylation in a variety of proliferating cell lines. Herein, it is shown that cAMP elicits a biphasic H3 dephosphorylation independent of PKA activation in cycling cells. H89, a potent inhibitor of PKA catalytic sub-unite, could not abolish this effect. Additionally, H89 induces a rapid and biphasic H3 serine 10 dephosphorylation, while a decline in the basal phosphorylation of CREB/ATF-1 is observed. Rp-cAMPS, an analog of cAMP and specific inhibitor of PKA, is unable to suppress cAMP-mediated H3 dephosphorylation, whereas Rp-cAMPS effectively blocks CREB/ATF-1 hyper-phosphorylation by cAMP and its inducers. Interestingly, cAMP exerts a rapid and profound H3 dephosphorylation at much lower concentration (50-fold lower, 0.125 mM) than the concentration required for maximal CREB/ATF-1 phosphorylation (5 mM). Much higher cAMP concentration is required to fully induce CREB/ATF-1 gain in phosphate (5 mM), which correlates with the inhibition of H3 dephosphorylation. Also, the dephosphorylation of H3 does not overlap at onset of MAP kinase phosphorylation pathways, p38 and ERK. Surprisingly, rapamycin (an mTOR inhibitor), cAMP, and its natural inducer isoproterenol, elicit identical dephosphorylation kinetics on both S6K1 ribosomal kinase (a downstream mTOR target) and H3. Finally, cAMP-induced H3 dephosphorylation is PP1/2-dependent. The results suggest that a pathway, requiring much lower cAMP concentration to that required for CREB/ATF-1 hyper-phosphorylation, is responsible for histone H3 dephosphorylation and may be linked to mTOR down regulation.

The metabolic and temporal basis of muscle hypertrophy in response to resistance exercise

Constituting ∼40% of body mass, skeletal muscle has essential locomotory and metabolic functions. As such, an insight into the control of muscle mass is of great importance for maintaining health and quality-of-life into older age, under conditions of cachectic disease and with rehabilitation. In healthy weight-bearing individuals, muscle mass is maintained by the equilibrium between muscle protein synthesis (MPS) and muscle protein breakdown; when this balance tips in favour of MPS hypertrophy occurs. Despite considerable research into pharmacological/nutraceutical interventions, resistance exercise training (RE-T) remains the most potent stimulator of MPS and hypertrophy (in the majority of individuals). However, the mechanism(s) and time course of hypertrophic responses to RE-T remain poorly understood. We would suggest that available data are very much in favour of the notion that the majority of hypertrophy occurs in the early phases of RE-T (though still controversial to some) and that, for the most part, continued gains are hard to come by. Whilst the mechanisms of muscle hypertrophy represent the culmination of mechanical, auto/paracrine and endocrine events, the measurement of MPS remains a cornerstone for understanding the control of hypertrophy - mainly because it is the underlying driving force behind skeletal muscle hypertrophy. Development of sophisticated isotopic techniques (i.e. deuterium oxide) that lend to longer term insight into the control of hypertrophy by sustained RE-T will be paramount in providing insights into the metabolic and temporal regulation of hypertrophy. Such technologies will have broad application in muscle mass intervention for both athletes and for mitigating disease/age-related cachexia and sarcopenia, alike.

Novel mechanism of transcriptional regulation of cell matrix protein through CREB

The transcription mechanism(s) of renal cell matrix accumulation in diabetes does not explored. Phosphorylation of the transcription factor cAMP-responsive element binding protein (CREB) significantly increased in cells treated with high glucose (HG) compared to cell grown in normal glucose (NG). Cells pretreated with rapamycin before exposure to HG showed significant decrease phosphorylation of CREB, increase in AMPK activity and decrease protein/mRNA and promoter activity of fibronectin. In addition, cells transfected with siRNA against CREB showed significant increase in AMPK activity, decrease in protein/mRNA and promoter activity of fibronectin. Cells treated with HG showed nuclear localization of p-CREB while pretreated cells with rapamycin reversed HG effect. Moreover, gel shift analysis shows increase binding of CREB to fibronectin promoter in cells treated with HG while cells pretreated with rapamycin reversed the effect of HG. Furthermore, db/db mice treated with rapamycin showed significant increase in AMPK activity, decrease in expression of p-CREB and protein/mRNA of fibronectin. Strong staining of fibronectin and p-CREB was detected in kidney cortex of db/db mice while treated mice with rapamycin reversed hyperglycemia effect. In summary, our data provide a novel mechanism of transcriptional regulation of fibronectin through CREB that may be used as therapeutic approach to prevent diabetes complications.

mTOR pathway activation in multiple myeloma cell lines and primary tumour cells: pomalidomide enhances cytoplasmic-nuclear shuttling of mTOR protein

mTOR is a protein kinase that plays a central role in regulating critical cellular processes. We evaluated the activation and cellular localization of the mTOR pathway in multiple myeloma (MM) and analyzed the role of pomalidomide in regulating mTOR. By immunohistochemistry cytoplasmic p-mTOR stained positive in 57 out 101 (57.6%) cases with a nuclear p-mTOR localization in 14 out 101 samples (13.8%). In the 70 MM samples analyzed for the entire pathway, p-mTOR expression significantly correlated with p-AKT, p-P70S6K, and p-4E-BP1 suggesting that the AKT/mTOR pathway is activated in a subset of MM patients. Immunofluorescence assays demonstrated that mTOR protein is distributed throughout the cytoplasm and the nucleus at baseline in MM cell lines and in plasma cells of 13 MM patients and that pomalidomide facilitated the shift of the mTOR protein in the nucleus. By western blotting, treatment with pomalidomide increased nuclear mTOR and p-mTOR expression levels in the nucleus with a concomitant decrease of the cytoplasmic fractions while does not seem to affect significantly AKT phosphorylation status. In MM cells the anti-myeloma activity of pomalidomide may be mediated by the regulation of the mTOR pathway.

Cellular metabolism and lysosomal mTOR signaling

Over the last few years extensive studies have linked the activity of mTORC1 to lysosomal function. These observations propose an intriguing integration of cellular catabolism, sustained by lysosomes, with anabolic processes, largely controlled by mTORC1. Interestingly, lysosomal function directly affects mTORC1 activity and is regulated by ZKSCAN3 and TFEB, two transcription factors and substrates of mTORC1. Thus, the lysosomal mTOR signaling complex represents a hub of cellular energy metabolism, and its dysregulation may lead to a number of human diseases. Here, we discuss the recent developments and highlight the open questions in this growing field.

Amino acid residues required for Gtr1p-Gtr2p complex formation and its interactions with the Ego1p-Ego3p complex and TORC1 components in yeast

The yeast Ras-like GTPases Gtr1p and Gtr2p form a heterodimer, are implicated in the regulation of TOR complex 1 (TORC1) and play pivotal roles in cell growth. Gtr1p and Gtr2p bind Ego1p and Ego3p, which are tethered to the endosomal and vacuolar membranes where TORC1 functions are regulated through a relay of amino acid signaling interactions. The mechanisms by which Gtr1p and Gtr2p activate TORC1 remain obscure. We probed the interactions of the Gtr1p-Gtr2p complex with the Ego1p-Ego3p complex and TORC1 subunits. Mutations in the region (179-220 a.a.) following the nucleotide-binding region of Gtr1p and Gtr2p abrogated their mutual interaction and resulted in a loss in function, suggesting that complex formation between Gtr1p and Gtr2p was indispensable for TORC1 function. A modified yeast two-hybrid assay showed that Gtr1p-Gtr2p complex formation is important for its interaction with the Ego1p-Ego3p complex. GTP-bound Gtr1p interacted with the region containing the HEAT repeats of Kog1p and the C-terminal region of Tco89p. The GTP-bound Gtr2p suppressed a Kog1p mutation. Our findings indicate that the interactions of the Gtr1p-Gtr2p complex with the Ego1p-Ego3p complex and TORC1 components Kog1p and Tco89p play a role in TORC1 function.

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.

Mechanistic target of rapamycin complex 1 is critical for invariant natural killer T-cell development and effector function

The mechanisms that control invariant natural killer T (iNKT)-cell development and function are still poorly understood. The mechanistic or mammalian target of rapamycin (mTOR) integrates various environmental signals/cues to regulate cell growth, proliferation, metabolism, and survival. We report here that ablation of mTOR complex 1 (mTORC1) signaling by conditionally deleting Raptor causes severe defects in iNKT-cell development at early stages, leading to drastic reductions in iNKT-cell numbers in the thymus and periphery. In addition, loss of Raptor impairs iNKT-cell proliferation and production of cytokines upon α-galactosylceramide stimulation in vitro and in vivo, and inhibits liver inflammation in an iNKT cell-mediated hepatitis model. Furthermore, Raptor deficiency and rapamycin treatment lead to aberrant intracellular localization and functional impairment of promyelocytic leukemia zinc-finger, a transcription factor critical for iNKT-cell development and effector programs. Our findings define an essential role of mTORC1 to direct iNKT-cell lineage development and effector function.

Environmental signaling through the mechanistic target of rapamycin complex 1: mTORC1 goes nuclear

Mechanistic target of rapamycin complex 1 (mTORC1) is a well-known regulator of cell growth and proliferation in response to environmental stimuli and stressors. To date, the majority of mTORC1 studies have focused on its function as a cytoplasmic effector of translation regulation. However, recent studies have identified additional, nuclear-specific roles for mTORC1 signaling related to transcription of the ribosomal DNA (rDNA) and ribosomal protein (RP) genes, mitotic cell cycle control, and the regulation of epigenetic processes. As this area of study is still in its infancy, the purpose of this review to highlight these significant findings and discuss the relevance of nuclear mTORC1 signaling dysregulation as it pertains to health and disease.

mRNA encoding WAVE-Arp2/3-associated proteins is co-localized with foci of active protein synthesis at the leading edge of MRC5 fibroblasts during cell migration

During cell spreading, mammalian cells migrate using lamellipodia formed from a large dense branched actin network which produces the protrusive force required for leading edge advancement. The formation of lamellipodia is a dynamic process and is dependent on a variety of protein cofactors that mediate their local regulation, structural characteristics and dynamics. In the present study, we show that mRNAs encoding some structural and regulatory components of the WAVE [WASP (Wiskott-Aldrich syndrome protein) verprolin homologous] complex are localized to the leading edge of the cell and associated with sites of active translation. Furthermore, we demonstrate that steady-state levels of ArpC2 and Rac1 proteins increase at the leading edge during cell spreading, suggesting that localized protein synthesis has a pivotal role in controlling cell spreading and migration.

Proline-rich Akt substrate of 40-kDa contains a nuclear export signal

The proline-rich Akt substrate of 40-kDa (PRAS40) has been linked to the regulation of the activity of the mammalian target of rapamycin complex 1 as well as insulin action. Despite these cytosolic functions, PRAS40 was originally identified as nuclear phosphoprotein in Hela cells. This study aimed to detail mechanisms and consequences of the nucleocytosolic trafficking of PRAS40. Sequence analysis identified a potential leucine-rich nuclear export signal (NES) within PRAS40. Incubation of A14 fibroblasts overexpressing human PRAS40 (hPRAS40) resulted in nuclear accumulation of the protein. Furthermore, mutation of the NES mimicked the effects of leptomycin B, a specific inhibitor of nuclear export, on the subcellular localization of hPRAS40. Finally, A14 cells expressing the NES-mutant showed impaired activation of components of the Akt-pathway as well as of the mTORC1 substrate p70 S6 kinase after insulin stimulation. This impaired insulin signaling could be ascribed to reduced protein levels of insulin receptor substrate 1 in cells expressing mutant NES. In conclusion, PRAS40 contains a functional nuclear export signal. Furthermore, enforced nuclear accumulation of PRAS40 impairs insulin action, thereby substantiating the function of this protein in the regulation of insulin sensitivity.

Molecular and genetic crosstalks between mTOR and ERRα are key determinants of rapamycin-induced nonalcoholic fatty liver

mTOR and ERRα are key regulators of common metabolic processes, including lipid homeostasis. However, it is currently unknown whether these factors cooperate in the control of metabolism. ChIP-sequencing analyses of mouse liver reveal that mTOR occupies regulatory regions of genes on a genome-wide scale including enrichment at genes shared with ERRα that are involved in the TCA cycle and lipid biosynthesis. Genetic ablation of ERRα and rapamycin treatment, alone or in combination, alter the expression of these genes and induce the accumulation of TCA metabolites. As a consequence, both genetic and pharmacological inhibition of ERRα activity exacerbates hepatic hyperlipidemia observed in rapamycin-treated mice. We further show that mTOR regulates ERRα activity through ubiquitin-mediated degradation via transcriptional control of the ubiquitin-proteasome pathway. Our work expands the role of mTOR action in metabolism and highlights the existence of a potent mTOR/ERRα regulatory axis with significant clinical impact.

An N-terminal nuclear export signal regulates trafficking and aggregation of Huntingtin (Htt) protein exon 1

Huntington disease is a dominantly inherited neurodegenerative condition caused by polyglutamine expansion in the N terminus of the huntingtin protein (Htt). The first 17 amino acids (N17) of Htt play a key role in regulating its toxicity and aggregation. Both nuclear export and cytoplasm retention functions have been ascribed to N17. We have determined that N17 acts as a nuclear export sequence (NES) within Htt exon and when fused to yellow fluorescent protein. We have defined amino acids within N17 that constitute the nuclear export sequence (NES). Mutation of any of the conserved residues increases nuclear accumulation of Htt exon 1. Nuclear export of Htt is sensitive to leptomycin B and is reduced by knockdown of exportin 1. In HEK293 cells, NES mutations decrease overall Htt aggregation but increase the fraction of cells with nuclear inclusions. In primary cultured neurons, NES mutations increase nuclear accumulation and increase overall aggregation. This work defines a bona fide nuclear export sequence within N17 and links it to effects on protein aggregation. This may help explain the important role of N17 in controlling Htt toxicity.

mTOR direct interactions with Rheb-GTPase and raptor: sub-cellular localization using fluorescence lifetime imaging

BACKGROUND

The mammalian target of rapamycin (mTOR) signalling pathway has a key role in cellular regulation and several diseases. While it is thought that Rheb GTPase regulates mTOR, acting immediately upstream, while raptor is immediately downstream of mTOR, direct interactions have yet to be verified in living cells, furthermore the localisation of Rheb has been reported to have only a cytoplasmic cellular localization.

RESULTS

In this study a cytoplasmic as well as a significant sub-cellular nuclear mTOR localization was shown , utilizing green and red fluorescent protein (GFP and DsRed) fusion and highly sensitive single photon counting fluorescence lifetime imaging microscopy (FLIM) of live cells. The interaction of the mTORC1 components Rheb, mTOR and raptor, tagged with EGFP/DsRed was determined using fluorescence energy transfer-FLIM. The excited-state lifetime of EGFP-mTOR of ~2400 ps was reduced by energy transfer to ~2200 ps in the cytoplasm and to 2000 ps in the nucleus when co-expressed with DsRed-Rheb, similar results being obtained for co-expressed EGFP-mTOR and DsRed-raptor. The localization and distribution of mTOR was modified by amino acid withdrawal and re-addition but not by rapamycin.

CONCLUSIONS

The results illustrate the power of GFP-technology combined with FRET-FLIM imaging in the study of the interaction of signalling components in living cells, here providing evidence for a direct physical interaction between mTOR and Rheb and between mTOR and raptor in living cells for the first time.

The role of mTORC1 in regulating protein synthesis and skeletal muscle mass in response to various mechanical stimuli

Skeletal muscle plays a fundamental role in mobility, disease prevention, and quality of life. Skeletal muscle mass is, in part, determined by the rates of protein synthesis, and mechanical loading is a major regulator of protein synthesis and skeletal muscle mass. The mammalian/mechanistic target of rapamycin (mTOR), found in the multi-protein complex, mTORC1, is proposed to play an essential role in the regulation of protein synthesis and skeletal muscle mass. The purpose of this review is to examine the function of mTORC1 in relation to protein synthesis and cell growth, the current evidence from rodent and human studies for the activation of mTORC1 signaling by different types of mechanical stimuli, whether mTORC1 signaling is necessary for changes in protein synthesis and skeletal muscle mass that occur in response to different types of mechanical stimuli, and the proposed molecular signaling mechanisms that may be responsible for the mechanical activation of mTORC1 signaling.

Beyond control of protein translation: what we have learned about the non-canonical regulation and function of mammalian target of rapamycin (mTOR)

Mammalian target of rapamycin (mTOR) is a serine-threonine kinase involved in almost every aspect of mammalian cell function. This kinase was initially believed to control protein translation in response to amino acids and trophic factors, and this function has become a canonical role for mTOR. However, mTOR can form two separate protein complexes (mTORCs). Recent advances clearly demonstrate that both mTORCs can respond to various stimuli and change myriad cellular processes. Therefore, our current view of the cellular roles of TORCs has rapidly expanded and cannot be fully explained without appreciating recent findings about the new modes of mTOR regulation and identification of non-canonical effectors of mTOR that contribute to transcription, cytoskeleton dynamics, and membrane trafficking. This review discusses the molecular details of these newly discovered non-canonical functions that allow mTORCs to control the cellular environment at multiple levels. This article is part of a Special Issue entitled: Inhibitors of Protein Kinases (2012).

Mammalian target of rapamycin (mTOR) signaling network in skeletal myogenesis

Mammalian (or mechanistic) target of rapamycin (mTOR) regulates a wide range of cellular and developmental processes by coordinating signaling responses to mitogens, nutrients, and various stresses. Over the last decade, mTOR has emerged as a master regulator of skeletal myogenesis, controlling multiple stages of the myofiber formation process. In this minireview, we present an emerging view of the signaling network underlying mTOR regulation of myogenesis, which contrasts with the well established mechanisms in the regulation of cell and muscle growth. Current questions for future studies are also highlighted.

Staying alive: metabolic adaptations to quiescence

Quiescence is a state of reversible cell cycle arrest that can grant protection against many environmental insults. In some systems, cellular quiescence is associated with a low metabolic state characterized by a decrease in glucose uptake and glycolysis, reduced translation rates and activation of autophagy as a means to provide nutrients for survival. For cells in multiple different quiescence model systems, including Saccharomyces cerevisiae, mammalian lymphocytes and hematopoietic stem cells, the PI3Kinase/TOR signaling pathway helps to integrate information about nutrient availability with cell growth rates. Quiescence signals often inactivate the TOR kinase, resulting in reduced cell growth and biosynthesis. However, quiescence is not always associated with reduced metabolism; it is also possible to achieve a state of cellular quiescence in which glucose uptake, glycolysis and flux through central carbon metabolism are not reduced. In this review, we compare and contrast the metabolic changes that occur with quiescence in different model systems.

Translational control of cell fate: from integration of environmental signals to breaching anticancer defense

Despite their genetic diversity, different cancers manifest common features at the protein pathway level. They share a core group of perturbed pathways that converge upon a few regulatory hubs linking the cellular signaling network with the basic metabolic machinery. Available evidence indicates that one such hub is the eIF4F-mediated cap-dependent mRNA translation initiation apparatus, whose integrity is required for physiological control of growth, proliferation and viability. However, when hyperactivated by upstream oncogenic signaling, eIF4F selectively stimulates the translation of a group of mRNAs required for cancer genesis and progression. Here, we describe a model that links the pro-neoplastic function of eIF4F to its ability to disable oncogene-activated tumor surveillance programs and propose a novel therapeutic strategy for cancer based upon targeting aberrant eIF4F with small-molecule antagonists.

Regulation of karyopherin α1 and nuclear import by mammalian target of rapamycin

Under conditions of reduced mitogen or nutritional substrate levels, the serine/threonine kinase target of rapamycin can augment the nuclear content of distinct transcription factors and promote the induction of stress response genes. In its latent (i.e., unphosphorylated) form, the transcription factor STAT1 regulates a subset of genes involved in immune modulation and apoptosis. Based on previous work indicating a functional relationship between mammalian target of rapamycin (mTOR) and the nuclear content of latent STAT1, we investigated the mechanism by which mTOR controls STAT1 nuclear import. By fluorescence confocal microscopy, inactivation of mTOR with rapamycin promoted the nuclear translocation of unphosphorylated STAT1, but not that of a STAT1 mutant incapable of binding its nuclear import adaptor karyopherin-α1 (KPNA1). By immunoprecipitation, KPNA1 was physically associated with mTOR and STAT1 in a complex that translocated to the nucleus in response to rapamycin. Although mTOR is not a kinase for KPNA1, the mTOR-associated phosphatase protein phosphatase 2A catalytic interacted directly with KPNA1 and regulated nuclear import of the mTOR-KPNA1 complex. KPNA1, or its interaction with STAT1, was required for the nuclear import of latent STAT1, transcriptional induction of the STAT1 gene, and caspase-3 activation under conditions of reduced mTOR activity (i.e. rapamycin, glucose starvation, serum withdrawal). Therefore, at low mitogen or nutrient levels, mTOR and protein phosphatase 2A catalytically control the constitutive nuclear import of latent STAT1 by KPNA1, which are key modulators of STAT1 expression and apoptosis.

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.