Prolonged Rapamycin Treatment Inhibits mTORC2 Assembly and Akt/PKB

The pro-apoptotic kinase Mst1 and its caspase cleavage products are direct inhibitors of Akt1

Akt kinases mediate cell growth and survival. Here, we report that a pro-apoptotic kinase, Mst1/STK4, is a physiological Akt1 interaction partner. Mst1 was identified as a component of an Akt1 multiprotein complex isolated from lipid raft-enriched fractions of LNCaP human prostate cancer cells. Endogenous Mst1, along with its paralog, Mst2, acted as inhibitors of endogenous Akt1. Surprisingly, mature Mst1 as well as both of its caspase cleavage products, which localize to distinct subcellular compartments and are not structurally homologous, complexed with and inhibited Akt1. cRNAs encoding full-length Mst1, and N- and C-terminal caspase Mst1 cleavage products, reverted an early lethal phenotype in zebrafish development induced by expression of membrane-targeted Akt1. Mst1 and Akt1 localized to identical subcellular sites in human prostate tumors. Mst1 levels declined with progression from clinically localized to hormone refractory disease, coinciding with an increase in Akt activation with transition from hormone naïve to hormone-resistant metastases. These results position Mst1/2 within a novel branch of the phosphoinositide 3-kinase/Akt pathway and suggest an important role in cancer progression.

Development and characterization of gemcitabine-resistant pancreatic tumor cells

BACKGROUND

Pancreatic cancer is an exceptionally lethal disease with an annual mortality nearly equivalent to its annual incidence. This dismal rate of survival is due to several factors including late presentation with locally advanced, unresectable tumors, early metastatic disease, and rapidly arising chemoresistance. To study the mechanisms of chemoresistance in pancreatic cancer we developed two gemcitabine-resistant pancreatic cancer cell lines.

METHODS

Resistant cells were obtained by culturing L3.6pl and AsPC-1 cells in serially increasing concentrations of gemcitabine. Stable cultures were obtained that were 40- to 50-fold increased in resistance relative to parental cells. Immunofluorescent staining was performed to examine changes in beta-catenin and E-cadherin localization. Protein expression was determined by immunoblotting. Migration and invasion were determined by modified Boyden chamber assays. Fluorescence-activated cell sorting (FACS) analyses were performed to examine stem cell markers.

RESULTS

Gemcitabine-resistant cells underwent distinct morphological changes, including spindle-shaped morphology, appearance of pseudopodia, and reduced adhesion characteristic of transformed fibroblasts. Gemcitabine-resistant cells were more invasive and migratory. Gemcitabine-resistant cells were increased in vimentin and decreased in E-cadherin expression. Immunofluorescence and immunoblotting revealed increased nuclear localization of total beta-catenin. These alterations are hallmarks of epithelial-to-mesenchymal transition (EMT). Resistant cells were activated in the receptor protein tyrosine kinase, c-Met and increased in expression of the stem cell markers CD (cluster of differentiation)24, CD44, and epithelial-specific antigen (ESA).

CONCLUSIONS

Gemcitabine-resistant pancreatic tumor cells are associated with EMT, a more-aggressive and invasive phenotype in numerous solid tumors. The increased phosphorylation of c-Met may also be related to chemoresistance and EMT and presents as an attractive adjunctive chemotherapeutic target in pancreatic cancer.

Rapamycin regulates the phosphorylation of rictor

The mammalian target of rapamycin (mTOR) is a central regulator of cell growth. mTOR exists in two functional complexes, mTORC1 and mTORC2. mTORC1 is rapamycin-sensitive, and results in phosphorylation of 4E-BP1 and S6K1. mTORC2 is proposed to regulate Akt Ser473 phosphorylation and be rapamycin-insensitive. mTORC2 consists of mTOR, mLST8, sin1, Protor/PRR5, and the rapamycin insensitive companion of mTOR (rictor). Here, we show that rapamycin regulates the phosphorylation of rictor. Rapamycin-mediated rictor dephosphorylation is time and concentration dependent, and occurs at physiologically relevant rapamycin concentrations. siRNA knockdown of mTOR also leads to rictor dephosphorylation, suggesting that rictor phosphorylation is mediated by mTOR or one of its downstream targets. Rictor phosphorylation induced by serum, insulin and insulin-like growth factor is blocked by rapamycin. Rictor dephosphorylation is not associated with dephosphorylation of Akt Ser473. Further work is needed to better characterize the mechanism of rictor regulation and its role in rapamycin-mediated growth inhibition.

Targeting the mTOR signaling network in cancer

The mammalian target of rapamycin (mTOR) is an unconventional protein kinase that is centrally involved in the control of cancer cell metabolism, growth and proliferation. The mTOR pathway has attracted broad scientific and clinical interest, particularly in light of the ongoing clinical cancer trials with mTOR inhibitors. The mixed clinical results to date reflect the complexity of both cancer as a disease target, and the mTOR signaling network, which contains two functionally distinct mTOR complexes, parallel regulatory pathways, and feedback loops that contribute to the variable cellular responses to the current inhibitors. In this review, we discuss the regulatory pathways that govern mTOR activity, and highlight clinical results obtained with the first generation of mTOR inhibitors to reach the oncology clinics.

Akt phosphorylation regulates the tumour-suppressor merlin through ubiquitination and degradation

The neurofibromatosis-2 (NF2) tumour-suppressor gene encodes an intracellular membrane-associated protein, called merlin, whose growth-suppressive function is dependent on its ability to form interactions through its intramolecular amino-terminal domain (NTD) and carboxy-terminal domain (CTD). Merlin phosphorylation plays a critical part in dictating merlin NTD/CTD interactions as well as in controlling binding to its effector proteins. Merlin is partially regulated by phosphorylation of Ser 518, such that hyperphosphorylated merlin is inactive and fails to form productive intramolecular and intermolecular interactions. Here, we show that the protein kinase Akt directly binds to and phosphorylates merlin on residues Thr 230 and Ser 315, which abolishes merlin NTD/CTD interactions and binding to merlin's effector protein PIKE-L and other binding partners. Furthermore, Akt-mediated phosphorylation leads to merlin degradation by ubiquitination. These studies demonstrate that Akt-mediated merlin phosphorylation regulates the function of merlin in the absence of an inactivating mutation.

An improved bioluminescence resonance energy transfer strategy for imaging intracellular events in single cells and living subjects

Bioluminescence resonance energy transfer (BRET) is currently used for monitoring various intracellular events, including protein-protein interactions, in normal and aberrant signal transduction pathways. However, the BRET vectors currently used lack adequate sensitivity for imaging events of interest from both single living cells and small living subjects. Taking advantage of the critical relationship of BRET efficiency and donor quantum efficiency, we report generation of a novel BRET vector by fusing a GFP(2) acceptor protein with a novel mutant Renilla luciferase donor selected for higher quantum yield. This new BRET vector shows an overall 5.5-fold improvement in the BRET ratio, thereby greatly enhancing the dynamic range of the BRET signal. This new BRET strategy provides a unique platform to assay protein functions from both single live cells and cells located deep within small living subjects. The imaging utility of the new BRET vector is shown by constructing a sensor using two mammalian target of rapamycin pathway proteins (FKBP12 and FRB) that dimerize only in the presence of rapamycin. This new BRET vector should facilitate high-throughput sensitive BRET assays, including studies in single live cells and small living subjects. Applications will include anticancer therapy screening in cell culture and in small living animals.

Regulation of macroautophagy by mTOR and Beclin 1 complexes

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

The molecular bases of training adaptation

Skeletal muscle is a malleable tissue capable of altering the type and amount of protein in response to disruptions to cellular homeostasis. The process of exercise-induced adaptation in skeletal muscle involves a multitude of signalling mechanisms initiating replication of specific DNA genetic sequences, enabling subsequent translation of the genetic message and ultimately generating a series of amino acids that form new proteins. The functional consequences of these adaptations are determined by training volume, intensity and frequency, and the half-life of the protein. Moreover, many features of the training adaptation are specific to the type of stimulus, such as the mode of exercise. Prolonged endurance training elicits a variety of metabolic and morphological changes, including mitochondrial biogenesis, fast-to-slow fibre-type transformation and substrate metabolism. In contrast, heavy resistance exercise stimulates synthesis of contractile proteins responsible for muscle hypertrophy and increases in maximal contractile force output. Concomitant with the vastly different functional outcomes induced by these diverse exercise modes, the genetic and molecular mechanisms of adaptation are distinct. With recent advances in technology, it is now possible to study the effects of various training interventions on a variety of signalling proteins and early-response genes in skeletal muscle. Although it cannot presently be claimed that such scientific endeavours have influenced the training practices of elite athletes, these new and exciting technologies have provided insight into how current training techniques result in specific muscular adaptations, and may ultimately provide clues for future and novel training methodologies. Greater knowledge of the mechanisms and interaction of exercise-induced adaptive pathways in skeletal muscle is important for our understanding of the aetiology of disease, maintenance of metabolic and functional capacity with aging, and training for athletic performance. This article highlights the effects of exercise on molecular and genetic mechanisms of training adaptation in skeletal muscle.

The mammalian target of rapamycin pathway as a potential target for cancer chemoprevention

The mammalian target of rapamycin (mTOR) is a key signaling node coordinating cell cycle progression and cell growth in response to genetic, epigenetic, and environmental conditions. Pathways involved in mTOR signaling are dysregulated in precancerous human tissues. These findings, together with the intriguing possibility that mTOR suppression may be associated with antitumor actions of caloric restriction, suggest that mTOR signaling may be an important target for chemopreventive drugs.

Inhibition of mammalian target of rapamycin induces phosphatidylinositol 3-kinase-dependent and Mnk-mediated eukaryotic translation initiation factor 4E phosphorylation

The initiation factor eukaryotic translation initiation factor 4E (eIF4E) plays a critical role in initiating translation of mRNAs, including those encoding oncogenic proteins. Therefore, eIF4E is considered a survival protein involved in cell cycle progression, cell transformation, and apoptotic resistance. Phosphorylation of eIF4E (usually at Ser209) increases its binding affinity for the cap of mRNA and may also favor its entry into initiation complexes. Mammalian target of rapamycin (mTOR) inhibitors suppress cap-dependent translation through inhibition of the phosphorylation of eIF4E-binding protein 1. Paradoxically, we have shown that inhibition of mTOR signaling increases eIF4E phosphorylation in human cancer cells. In this study, we focused on revealing the mechanism by which mTOR inhibition increases eIF4E phosphorylation. Silencing of either mTOR or raptor could mimic mTOR inhibitors' effects to increase eIF4E phosphorylation. Moreover, knockdown of mTOR, but not rictor or p70S6K, abrogated rapamycin's ability to increase eIF4E phosphorylation. These results indicate that mTOR inhibitor-induced eIF4E phosphorylation is secondary to mTOR/raptor inhibition and independent of p70S6K. Importantly, mTOR inhibitors lost their ability to increase eIF4E phosphorylation only in cells where both Mnk1 and Mnk2 were knocked out, indicating that mTOR inhibitors increase eIF4E phosphorylation through a Mnk-dependent mechanism. Given that mTOR inhibitors failed to increase Mnk and eIF4E phosphorylation in phosphatidylinositol 3-kinase (PI3K)-deficient cells, we conclude that mTOR inhibition increases eIF4E phosphorylation through a PI3K-dependent and Mnk-mediated mechanism. In addition, we also suggest an effective therapeutic strategy for enhancing mTOR-targeted cancer therapy by cotargeting mTOR signaling and Mnk/eIF4E phosphorylation.

The mammalian target of rapamycin signaling pathway: twists and turns in the road to cancer therapy

The immunosuppressive drug rapamycin played a key role in the functional characterization of mammalian target of rapamycin (mTOR), an unusual protein kinase that coordinates growth factor and nutrient availability with cell growth and proliferation. Several rapamycin-related compounds are now in various stages of clinical development as anticancer agents. This article highlights recent advances in our understanding of the mTOR signaling pathway and the implications of these findings for the clinical application of mTOR inhibitors in cancer patients.

Identification of Protor as a novel Rictor-binding component of mTOR complex-2

The mTOR (mammalian target of rapamycin) protein kinase is an important regulator of cell growth. Two complexes of mTOR have been identified: complex 1, consisting of mTOR-Raptor (regulatory associated protein of mTOR)-mLST8 (termed mTORC1), and complex 2, comprising mTOR-Rictor (rapamycininsensitive companion of mTOR)-mLST8-Sin1 (termed mTORC2). mTORC1 phosphorylates the p70 ribosomal S6K (S6 kinase) at its hydrophobic motif (Thr389), whereas mTORC2 phosphorylates PKB (protein kinase B) at its hydrophobic motif (Ser473). In the present study, we report that widely expressed isoforms of unstudied proteins termed Protor-1 (protein observed with Rictor-1) and Protor-2 interact with Rictor and are components of mTORC2. We demonstrate that immunoprecipitation of Protor-1 or Protor-2 results in the co-immunoprecipitation of other mTORC2 subunits, but not Raptor, a specific component of mTORC1. We show that detergents such as Triton X-100 or n-octylglucoside dissociate mTOR and mLST8 from a complex of Protor-1, Sin1 and Rictor. We also provide evidence that Rictor regulates the expression of Protor-1, and that Protor-1 is not required for the assembly of other mTORC2 subunits into a complex. Protor-1 is a novel Rictor-binding subunit of mTORC2, but further work is required to establish its role.

Reelin signals through phosphatidylinositol 3-kinase and Akt to control cortical development and through mTor to regulate dendritic growth

Reelin is an extracellular matrix protein with various functions during development and in the mature brain. It activates different signaling cascades in target cells, one of which is the phosphatidylinositol 3-kinase (PI3K) pathway, which we investigated further using pathway inhibitors and in vitro brain slice and neuronal cultures. We show that the mTor (mammalian target of rapamycin)-S6K1 (S6 kinase 1) pathway is activated by Reelin and that this depends on Dab1 (Disabled-1) phosphorylation and activation of PI3K and Akt (protein kinase B). PI3K and Akt are required for the effects of Reelin on the organization of the cortical plate, but their downstream partners mTor and glycogen synthase kinase 3beta (GSK3beta) are not. On the other hand, mTor, but not GSK3beta, mediates the effects of Reelin on the growth and branching of dendrites of hippocampal neurons. In addition, PI3K fosters radial migration of cortical neurons through the intermediate zone, an effect that is independent of Reelin and Akt.

The growing role of mTOR in neuronal development and plasticity

Neuronal development and synaptic plasticity are highly regulated processes in which protein kinases play a key role. Recently, increasing attention has been paid to a serine/threonine protein kinase called mammalian target of rapamycin (mTOR) that has well-known functions in cell proliferation and growth. In neuronal cells, mTOR is implicated in multiple processes, including transcription, ubiquitin-dependent proteolysis, and microtubule and actin dynamics, all of which are crucial for neuronal development and long-term modification of synaptic strength. The aim of this article is to present our current understanding of mTOR functions in axon guidance, dendritic tree development, formation of dendritic spines, and in several forms of long-term synaptic plasticity. We also aim to present explanation for the mTOR effects on neurons at the level of mTORregulated genes and proteins.

Rapamycin limits formation of active eukaryotic initiation factor 4F complex following meal feeding in rat hearts

Feeding promotes protein synthesis in cardiac muscle through a stimulation of the messenger RNA translation initiation phase of protein synthesis by enhancing assembly of active eukaryotic initiation factor (eIF)4F complex. The experiments reported herein examined the potential role for a rapamycin-sensitive signaling pathway in increasing formation of active eIF4G-eIF4E complex during meal feeding. Hearts from male Sprague-Dawley rats fed a meal consisting of rat nonpurified diet were sampled prior to and 3 h following the meal in the presence or absence of treatment with rapamycin, an inhibitor of the mammalian target of rapamycin (mTOR) complex 1. Rapamycin prevented the meal feeding-induced stimulation of myocardial protein synthesis. Inhibition of mTOR with rapamycin decreased the association of rapamycin-associated TOR protein with mTOR and prevented the feeding-induced assembly of eIF4G-eIF4E complex. In contrast, the abundance of eIF4E binding protein-1 (4E-BP1)-eIF4E complex was unaffected by either meal feeding or rapamycin. Pretreatment with rapamycin completely prevented the feeding-induced phosphorylation of eIF4G(Ser(1108)), whereas the inhibitor only partially attenuated meal feeding-induced 70-kDa ribosomal protein S6 kinase1(Thr(389)) phosphorylation and extent of 4E-BP1 in the gamma-form. Meal feeding-induced phosphorylation of protein kinase B on either Ser(473) or Thr(308) was unaffected by rapamycin. These findings suggest the extent of phosphorylation of eIF4G following meal feeding occurs by a rapamycin-sensitive mechanism in cardiac muscle. Furthermore, the rapamycin-sensitive reductions in phosphorylation of eIF4G may also lead to decreased formation of active eIF4G-eIF4E complex.

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

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

Late-breaking news from the "4th International Meeting on Inflammatory Bowel Diseases" Capri, 2006

At the "4th International Meeting on Inflammatory Bowel Diseases: on the Way to New Therapies," Capri, 2006, genetics, bacteria-host interactions, immunomodulation, and tissue response were discussed deeply in order to understand, rationalize, and develop novel therapies. About genetics, the importance of a better understanding of the nature of known loci and of the putative associations was stressed. It was confirmed that genotype-phenotype associations in inflammatory bowel disease (IBD) have important clinical and therapeutic implications. The importance of the search for dominant bacterial antigens in chronic immune-mediated intestinal inflammation emerged, as well as knowledge of cellular and molecular mechanisms of bacterial-host interactions. It was discussed how innate and adaptive immunity signaling events can perpetuate chronic inflammation. Signal transduction pathways provide an intracellular mechanism by which cells respond and adapt to environmental stress. The identification of these signals have led to a greater understanding of the pathogenesis of IBD and pointed to potential therapeutic targets. It was shown that immune homeostasis is lost in IBD, resulting in a complex tissue response involving the action of immune and nonimmune cells. The nonimmune tissue response in IBD could be regarded as a new target for control of chronic intestinal inflammation. The changing role of biotherapy in IBD was widely discussed and in particular the anti-TNF-alpha monoclonal antibodies. Granulocyte-colony stimulating factor (GM-CSF) and stem cells therapies were also discussed. The risk-to-benefit ratio of the novel therapies was analyzed in detail. Finally, future directions for basic science and the unmet needs for clinical practice were presented.

Interferon-gamma induces human vascular smooth muscle cell proliferation and intimal expansion by phosphatidylinositol 3-kinase dependent mammalian target of rapamycin raptor complex 1 activation

Interferon (IFN)-gamma, a cytokine characteristically expressed in arteriosclerotic diseases, acts directly on vascular smooth muscle cells to induce cellular proliferation and intimal expansion. Signaling by the mammalian target of rapamycin raptor complex, known as mTORC1, is associated with cell growth and is active within arteriosclerotic lesions but is not known to be triggered by proinflammatory factors in vascular smooth muscle cells. We investigated the mechanisms for the proarteriosclerotic effects of IFN-gamma in the absence of leukocytes by exploiting the species specificity of this cytokine in a chimeric model of immunodeficient mouse recipients bearing human coronary artery grafts and intravenously inoculated with adenovirus encoding a human IFN-gamma transgene. We found that IFN-gamma-mediated vascular smooth muscle cell proliferation and intimal expansion were associated with phosphorylation of the mTORC1 effector ribosomal protein S6 kinase 1, that the graft morphological changes and S6 kinase 1 activation were inhibited by the mTORC1 inhibitor rapamycin in vivo, and that IFN-gamma-induced mTORC1 signaling was dependent on phosphatidylinositol 3-kinase activity under serum-free conditions in vitro. Our work establishes an immunologic stimulus for mTORC1 signaling in vascular smooth muscle cells, emphasizes that mTORC1 activation is critical in immune-mediated vascular remodeling, and provides further mechanistic insight into the successful clinical application of rapamycin therapy for atherosclerosis and graft arteriosclerosis.

Endothelial Akt signaling is rate-limiting for rapamycin inhibition of mouse mammary tumor progression

Chronic activation of Akt signaling in the endothelium recapitulates the salient features of a tumor vasculature and can be inhibited by rapamycin, an inhibitor of mammalian target of rapamycin. This led to the hypothesis that the antitumor efficacy of rapamycin may be partially dependent on its ability to inhibit endothelial Akt signaling, making rapamycin an antiangiogenic agent and endothelial Akt pathway inhibitor. Dose-response studies with rapamycin showed that primary human endothelial cells and fibroblasts had a bimodal Akt response with effective reductions in phosphorylated Akt (pAkt) achieved at 10 ng/mL. In contrast, rapamycin increased pAkt levels in tumor cell lines. When tumor-bearing mice were treated with rapamycin doses comparable to those used clinically in transplant patients, we observed strong inhibition of mammary tumor growth. To test whether Akt activation in the endothelium was rate-limiting for this antitumor response, we engineered mouse mammary tumor virus-polyoma virus middle T antigen mice with endothelial cell-specific expression of constitutively activated Akt. We observed that the antitumor efficacy of rapamycin was reduced in the presence of elevated endothelial Akt activation. Just as we observed in MCF7 cells in vitro, rapamycin doses that were antiangiogenic resulted in increased pAkt levels in total mouse mammary tumor virus-polyoma virus middle T antigen tumor lysates, suggesting that tumor cells had an opposite Akt response following mammalian target of rapamycin inhibition compared with tumor endothelial cells. Together, these data support the hypothesis that endothelial Akt signaling in the tumor vasculature is an important target of the novel anticancer drug rapamycin.

[Pulmonary lymphangioleiomyomatosis with or without tuberous sclerosis]

INTRODUCTION

Pulmonary lymphangioleiomyomatosis (LAM) is a rare disease affecting young women and presenting with recurrent pneumothorax.

BACKGROUND

Other lesions such as chylothorax or renal angiomyolipoma may suggest the diagnosis. The condition is related to a proliferation of abnormal smooth muscle cells staining for the monoclonal antibody HMB45. LAM can appear sporadically or be associated with tuberous sclerosis with abnormalities of the TSC2 suppressor gene. High resolution thoracic CT scanning shows bilateral, thin walled pulmonary cysts. Pulmonary function tests reveal bronchial obstruction and over-inflation with a reduced DLCO being the earliest abnormality.

VIEWPOINT

Although there are non-progressive forms, LAM usually leads to chronic respiratory insufficiency within a few, or ten or so years. In the absence of a controlled clinical trial hormone therapy has not been shown to be effective. Lung transplantation is the last therapeutic resort; recurrences in the transplanted lung have been occasionally reported.

CONCLUSIONS

Analysis of the molecular mechanisms induced by mutations of the TSC2 suppressor gene and the demonstration of the migratory properties of smooth muscle cells, whose origin may be extra-thoracic, reveal new specific antiproliferative therapeutic options.

Regulation of hepatitis B virus replication by the phosphatidylinositol 3-kinase-akt signal transduction pathway

The phosphatidylinositol 3-kinase (PI3K)-protein kinase B (Akt) signaling pathway is one of the major oncogenic pathways and is activated in many types of human cancers, including hepatocellular carcinoma. It can also be activated by the hepatitis C virus (HCV) nonstructural 5A (NS5A) protein. In the present study, we set out to determine the regulatory effects of this pathway on the replication of hepatitis B virus (HBV). Our results demonstrate that the expression of a constitutively active Akt1 profoundly inhibited HBV RNA transcription and consequently reduced HBV DNA replication in HepG2 cells. This suppression of HBV gene transcription was apparently mediated by the activation of mTOR, as it was abolished by the mTOR inhibitor rapamycin. Moreover, treatment of HBV-expressing HepG2.2.15 cells with inhibitors of PI3K, Akt, and mTOR increased the transcription of 3.5-kb and 2.4-kb viral RNA as well as the replication of HBV DNA. This observation implies that the basal level activation of this pathway in HepG2 cells regulated HBV replication. Consistent with previous reports showing that the HCV NS5A protein could bind to the p85 subunit of PI3K and activate the PI3K-Akt signal transduction pathway, our results showed that expression of this protein could inhibit HBV RNA transcription and reduce HBV DNA replication in HepG2 cells. Taken together, our results suggest that the activation of the PI3K-Akt pathway during liver oncogenesis may be at least partially responsible for the elimination of HBV replication from tumor cells and may also provide an explanation for the observed suppression of HBV replication by HCV coinfection.

PRAS40 Regulates mTORC1 Kinase Activity by Functioning as a Direct Inhibitor of Substrate Binding

Mammalian target of rapamycin (mTOR) functions in two distinct signaling complexes, mTORC1 and mTORC2. In response to insulin and nutrients, mTORC1, consisting of mTOR, raptor (regulatory-associated protein of mTOR), and mLST8, is activated and phosphorylates eukaryotic initiation factor 4E-binding protein (4EBP) and p70 S6 kinase to promote protein synthesis and cell size. Previously we found that activation of mTOR kinase in response to insulin was associated with increased 4EBP1 binding to raptor. Here we identify prolinerich Akt substrate 40 (PRAS40) as a binding partner for mTORC1. A putative TOR signaling motif, FVMDE, is identified in PRAS40 and shown to be required for interaction with raptor. Insulin stimulation markedly decreases the level of PRAS40 bound by mTORC1. Recombinant PRAS40 inhibits mTORC1 kinase activity in vivo and in vitro, and this inhibition depends on PRAS40 association with raptor. Furthermore, decreasing PRAS40 expression by short hairpin RNA enhances 4E-BP1 binding to raptor, and recombinant PRAS40 competes with 4E-BP1 binding to raptor. We, therefore, propose that PRAS40 regulates mTORC1 kinase activity by functioning as a direct inhibitor of substrate binding.

Defining the role of mTOR in cancer

The mammalian target of rapamycin (mTOR) has emerged as a critical effector in cell-signaling pathways commonly deregulated in human cancers. This has led to the prediction that mTOR inhibitors may be useful in oncology, and derivatives of one such molecule, rapamycin (from which mTOR derives its name), are currently in clinical development. In this review, we discuss recent progress in understanding mTOR signaling, paying particular attention to its relevance in cancer. We further discuss the use of rapamycin in oncology and conclude with a discussion on the future of mTOR-targeted therapy.

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

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

The effects of mTOR-Akt interactions on anti-apoptotic signaling in vascular endothelial cells

Recent studies have determined that mTOR mediates the activation of the protein kinase Akt in several cell types, but little is known about the association between mTOR and Akt in vascular endothelial cells. Furthermore, the functional significance of mTOR/Akt signaling has not been characterized in the endothelium. In these studies we treated endothelial cells with the mTOR inhibitor rapamycin, and we found that it decreases Akt phosphorylation and activity, as determined by phosphorylation of its substrate glycogen synthase kinase-3. This effect of rapamycin on Akt phosphorylation could not be demonstrated in endothelial cells transfected with a rapamycin-resistant mTOR construct. Also, in the presence of rapamycin, vascular endothelial growth factor, tumor necrosis factor, and insulin failed to phosphorylate Akt, further indicating that mTOR regulates Akt activation in endothelial cells. The activation of Akt is well established to mediate pro-survival signals. In part this is mediated via the phosphorylation and inactivation of the pro-apoptotic Akt substrates Foxo1 and Foxo3a. We find that rapamycin totally blocks vascular endothelial growth factor and Akt-inducible phosophorylation of these transcription factors in endothelial cells. Furthermore, inhibition of Akt activity by rapamycin increased the number of endothelial cells undergoing apoptosis after serum withdrawal as well as after stimulation by vascular endothelial growth factor or tumor necrosis factor. Taken together these observations demonstrate first, that mTOR regulates the phosphorylation and activation of Akt in endothelial cells and, second, that a major effect of mTOR inhibition in endothelial cells is to suppress Akt-inducible pro-survival signals.

S6 kinase deletion suppresses muscle growth adaptations to nutrient availability by activating AMP kinase

S6 kinase (S6K) deletion in metazoans causes small cell size, insulin hypersensitivity, and metabolic adaptations; however, the underlying molecular mechanisms are unclear. Here we show that S6K-deficient skeletal muscle cells have increased AMP and inorganic phosphate levels relative to ATP and phosphocreatine, causing AMP-activated protein kinase (AMPK) upregulation. Energy stress and muscle cell atrophy are specifically triggered by the S6K1 deletion, independent of S6K2 activity. Two known AMPK-dependent functions, mitochondrial biogenesis and fatty acid beta-oxidation, are upregulated in S6K-deficient muscle cells, leading to a sharp depletion of lipid content, while glycogen stores are spared. Strikingly, AMPK inhibition in S6K-deficient cells restores cell growth and sensitivity to nutrient signals. These data indicate that S6K1 controls the energy state of the cell and the AMPK-dependent metabolic program, providing a mechanism for cell mass accumulation under high-calorie diet.

Potential therapeutic applications of autophagy

Autophagy is a dynamic process of subcellular degradation, which has recently sparked great interest as it is now recognized to be involved in various developmental processes and various diseases including cancer and neurodegeneration. Autophagy can function as a cytoprotective mechanism; however, it also has the capacity to cause cell death. A better understanding of autophagy is needed to allow its manipulation for therapeutic purposes, and new insights into the molecular mechanisms of autophagy are now leading to the discovery of exciting new potential drug targets.

The Akt/Mammalian Target of Rapamycin Signal Transduction Pathway Is Activated in High-Risk Myelodysplastic Syndromes and Influences Cell Survival and Proliferation

The Akt/mammalian target of rapamycin (mTOR) signaling pathway is important for both cell growth and survival. In particular, an impaired regulation of the Akt/mTOR axis has been strongly implicated in mechanisms related to neoplastic transformation, through enhancement of cell proliferation and survival. Myelodysplastic syndromes (MDS) are a group of heterogeneous hematopoietic stem cell disorders characterized by ineffective hematopoiesis and by a high risk of evolution into acute myelogenous leukemia (AML). The pathogenesis of the MDS evolution into AML is still unclear, although some recent studies indicate that aberrant activation of survival signaling pathways could be involved. In this investigation, done by means of immunofluorescent staining, we report an activation of the Akt/mTOR pathway in high-risk MDS patients. Interestingly, not only mTOR was activated but also its downstream targets, 4E-binding protein 1 and p70 ribosomal S6 kinase. Treatment with the selective mTOR inhibitor, rapamycin, significantly increased apoptotic cell death of CD33(+) (but not CD33(-)) cells from high-risk MDS patients. Rapamycin was ineffective in cells from healthy donors or low-risk MDS. Moreover, incubation of high-risk MDS patient CD34(+) cells with rapamycin decreased the in vitro clonogenic capability of these cells. In contrast, the phosphoinositide 3-kinase inhibitor, LY294002, did not significantly affect the clonogenic activity of high-risk MDS cells. Taken together, our results indicate that the Akt/mTOR pathway is critical for cell survival and proliferation in high-risk MDS patients. Therefore, this signaling network could become an interesting therapeutic target for treating more advanced MDS cases.

Akt blocks ligand binding and protects against expanded polyglutamine androgen receptor toxicity

Spinal and bulbar muscular atrophy (SBMA) is a progressive neurodegenerative disease caused by an expansion of the polyglutamine tract in the androgen receptor (AR). Here, we investigated the regulation of AR phosphorylation in order to understand factors that may modify SBMA disease progression. We show that expanded polyglutamine AR is phosphorylated by Akt. Substitution of the AR at two Akt consensus sites, S215 and S792, with aspartate, which mimics phosphorylation, reduces ligand binding, ligand-dependent nuclear translocation, transcriptional activation and toxicity of expanded polyglutamine AR. Co-expression of constitutively active Akt and the AR has similar consequences, which are blocked by alanine substitutions at residues 215 and 792. Furthermore, in motor neuron-derived MN-1 cells toxicity associated with polyglutamine-expanded AR is rescued by co-expression with Akt. Insulin-like growth factor-1 (IGF-1) stimulation, which activates several cell survival promoting pathways, also reduces toxicity of the expanded polyglutamine AR in MN-1 cells, in a manner dependent upon phospho-inositol-3-kinase. IGF-1 rescue of AR toxicity is diminished by alanine substitutions at the Akt consensus sites. These results highlight potential targets for therapeutic intervention in SBMA.

Mammalian target of rapamycin in the human placenta regulates leucine transport and is down-regulated in restricted fetal growth

Pathological fetal growth is associated with perinatal morbidity and the development of diabetes and cardiovascular disease later in life. Placental nutrient transport is a primary determinant of fetal growth. In human intrauterine growth restriction (IUGR) the activity of key placental amino acid transporters, such as systems A and L, is decreased. However the mechanisms regulating placental nutrient transporters are poorly understood. We tested the hypothesis that the mammalian target of rapamycin (mTOR) signalling pathway regulates amino acid transport in the human placenta and that the activity of the placental mTOR pathway is reduced in IUGR. Using immunohistochemistry and culture of trophoblast cells, we show for the first time that the mTOR protein is expressed in the transporting epithelium of the human placenta. We further demonstrate that placental mTOR regulates activity of the l-amino acid transporter, but not system A or taurine transporters, by determining the mediated uptake of isotope-labelled leucine, methylaminoisobutyric acid and taurine in primary villous fragments after inhibition of mTOR using rapamycin. The protein expression of placental phospho-S6K1 (Thr-389), a measure of the activity of the mTOR signalling pathway, was markedly reduced in placentas obtained from pregnancies complicated by IUGR. These data identify mTOR as an important regulator of placental amino acid transport, and provide a mechanism for the changes in placental leucine transport in IUGR previously demonstrated in humans. We propose that mTOR functions as a placental nutrient sensor, matching fetal growth with maternal nutrient availability by regulating placental nutrient transport.

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

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

mTOR pathway as a target in tissue hypertrophy

Recent work has shown that the mTOR (mammalian target of rapamycin) pathway is an integral cell growth regulator. The mTOR pathway involves two functional complexes, TORC1 and TORC2, which have been defined by both their association with raptor or rictor, respectively, and their sensitivity to short-term rapamycin inhibition. Loss of tumor suppressors TSC1 or TSC2 leads to aberrant activation of TORC1, which has been implicated in the control of cell size. As a result, both physiologic and pathologic tissue hypertrophy are associated with TORC1 activation. Some clinical examples include skeletal and cardiac muscle hypertrophy, vascular restenosis, and compensatory nephrotic hypertrophy. Clarification of the mTOR pathway may lead to increased understanding of both the etiology and consequences of aberrant cell size regulation. This review covers some of the biochemical regulation of the mTOR pathway that may be important to the regulation of cell size, and it will present several potential clinical applications where the control of cell size may be biologically significant.

The Two TORCs and Akt

The regulatory circuits that control the activities of the two distinct target of rapamycin (TOR) complexes, TORC1 and TORC2, and of Akt have been a focus of intense research in recent years. It has become increasingly evident that these regulatory circuits control some of the most fundamental aspects of metabolism, cell growth, proliferation, survival, and differentiation at both the cellular and organismal levels. As such, they also play a pivotal role in the genesis of diseases including cancer, diabetes, aging, and degenerative diseases. This review highlights recent developments aimed at deciphering the interplay between Akt and mTORCs as well as their role in embryonic development and in cancer.

Identification of a highly effective rapamycin schedule that markedly reduces the size, multiplicity, and phenotypic progression of tobacco carcinogen-induced murine lung tumors

PURPOSE

Human and murine preneoplastic lung lesions induced by tobacco exposure are characterized by increased activation of the Akt/mammalian target of rapamycin (mTOR) pathway, suggesting a role for this pathway in lung cancer development. To test this, we did studies with rapamycin, an inhibitor of mTOR, in A/J mice that had been exposed to the tobacco-specific carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK).

EXPERIMENTAL DESIGN

Tumorigenesis was induced by i.p. injection of NNK, and rapamycin was administered 1 or 26 weeks after NNK administration. Biomarkers associated with mTOR inhibition were assessed in lung and/or surrogate tissues using immunohistochemistry and immunoblotting. Rapamycin levels were measured using mass spectroscopy.

RESULTS

Rapamycin was administered on a daily (5 of 7 days) regimen beginning 26 weeks after NNK decreased tumor size, proliferative rate, and mTOR activity. Multiplicity was not affected. Comparing this regimen with an every-other-day (qod) regimen revealed that rapamycin levels were better maintained with qod administration, reaching a nadir of 16.4 ng/mL, a level relevant in humans. When begun 1 week after NNK, this regimen was well tolerated and decreased tumor multiplicity by 90%. Tumors that did develop showed decreased phenotypic progression and a 74% decrease in size that correlated with decreased proliferation and inhibition of mTOR.

CONCLUSIONS

Tobacco carcinogen-induced lung tumors in A/J mice are dependent upon mTOR activity because rapamycin markedly reduced the development and growth of tumors. Combined with the Food and Drug Administration approval of rapamycin and broad clinical experience, these studies provide a rationale to assess rapamycin in trials with smokers at high risk to develop lung cancer.

Signalling to translation: how signal transduction pathways control the protein synthetic machinery

Recent advances in our understanding of both the regulation of components of the translational machinery and the upstream signalling pathways that modulate them have provided important new insights into the mechanisms by which hormones, growth factors, nutrients and cellular energy status control protein synthesis in mammalian cells. The importance of proper control of mRNA translation is strikingly illustrated by the fact that defects in this process or its control are implicated in a number of disease states, such as cancer, tissue hypertrophy and neurodegeneration. Signalling pathways such as those involving mTOR (mammalian target of rapamycin) and mitogen-activated protein kinases modulate the phosphorylation of translation factors, the activities of the protein kinases that act upon them and the association of RNA-binding proteins with specific mRNAs. These effects contribute both to the overall control of protein synthesis (which is linked to cell growth) and to the modulation of the translation or stability of specific mRNAs. However, important questions remain about both the contributions of individual regulatory events to the control of general protein synthesis and the mechanisms by which the translation of specific mRNAs is controlled.

PHLPP and a Second Isoform, PHLPP2, Differentially Attenuate the Amplitude of Akt Signaling by Regulating Distinct Akt Isoforms

Akt/protein kinase B controls cell growth, proliferation, and survival. We recently discovered a novel phosphatase PHLPP, for PH domain leucine-rich repeat protein phosphatase, which terminates Akt signaling by directly dephosphorylating and inactivating Akt. Here we describe a second family member, PHLPP2, which also inactivates Akt, inhibits cell-cycle progression, and promotes apoptosis. These phosphatases control the amplitude of Akt signaling: depletion of either isoform increases the magnitude of agonist-evoked Akt phosphorylation by almost two orders of magnitude. Although PHLPP1 and PHLPP2 both dephosphorylate the same residue (hydrophobic phosphorylation motif) on Akt, they differentially terminate Akt signaling by regulating distinct Akt isoforms. Knockdown studies reveal that PHLPP1 specifically modulates the phosphorylation of HDM2 and GSK-3alpha through Akt2, whereas PHLPP2 specifically modulates the phosphorylation of p27 through Akt3. Our data unveil a mechanism to selectively terminate Akt-signaling pathways through the differential inactivation of specific Akt isoforms by specific PHLPP isoforms.

PDGFRs are critical for PI3K/Akt activation and negatively regulated by mTOR

The receptor tyrosine kinase/PI3K/Akt/mammalian target of rapamycin (RTK/PI3K/Akt/mTOR) pathway is frequently altered in tumors. Inactivating mutations of either the TSC1 or the TSC2 tumor-suppressor genes cause tuberous sclerosis complex (TSC), a benign tumor syndrome in which there is both hyperactivation of mTOR and inhibition of RTK/PI3K/Akt signaling, partially due to reduced PDGFR expression. We report here that activation of PI3K or Akt, or deletion of phosphatase and tensin homolog (PTEN) in mouse embryonic fibroblasts (MEFs) also suppresses PDGFR expression. This was a direct effect of mTOR activation, since rapamycin restored PDGFR expression and PDGF-sensitive Akt activation in Tsc1-/- and Tsc2-/- cells. Akt activation in response to EGF in Tsc2-/- cells was also reduced. Furthermore, Akt activation in response to each of EGF, IGF, and PMA was reduced in cells lacking both PDGFRalpha and PDGFRbeta, implying a role for PDGFR in transmission of growth signals downstream of these stimuli. Consistent with the reduction in PI3K/Akt signaling, in a nude mouse model both Tsc1-/- and Tsc2-/- cells had reduced tumorigenic potential in comparison to control cells, which was enhanced by expression of either active Akt or PDGFRbeta. In conclusion, PDGFR is a major target of negative feedback regulation in cells with activated mTOR, which limits the growth potential of TSC tumors.

High sirolimus levels may induce focal segmental glomerulosclerosis de novo

Sirolimus has been associated with high-range proteinuria when used in replacement of calcineurin inhibitors in renal transplant recipients with chronic allograft nephropathy (CAN). Primary FSGS was demonstrated previously in some such patients, but the coexistence of CAN lesions made the interpretation uneasy. However, nephrotic syndrome and FSGS were observed recently in three patients who received sirolimus de novo, without medical history of primary FSGS or CAN. Markers of podocyte differentiation were studied in kidney biopsies of the three patients who received sirolimus de novo and of five patients who switched to sirolimus. All patients developed FSGS lesions of classic type (not otherwise specified), but only switched patients exhibited advanced sclerotic lesions. Immunohistochemistry showed that some podocytes in FSGS lesions had absent or diminished expression of the podocyte-specific epitopes synaptopodin and p57, reflecting dedifferentiation, and had acquired expression of cytokeratin and PAX2, reflecting a immature fetal phenotype. Such a pattern of epitope expression provides evidence for podocyte dysregulation. Moreover, a decrease in vascular endothelial growth factor expression was observed in some glomeruli. In conclusion, sirolimus induces FSGS that is responsible for proteinuria in some transplant patients.

Ablation in mice of the mTORC components raptor, rictor, or mLST8 reveals that mTORC2 is required for signaling to Akt-FOXO and PKCalpha, but not S6K1

The mTOR kinase controls cell growth, proliferation, and survival through two distinct multiprotein complexes, mTORC1 and mTORC2. mTOR and mLST8 are in both complexes, while raptor and rictor are part of only mTORC1 and mTORC2, respectively. To investigate mTORC1 and mTORC2 function in vivo, we generated mice deficient for raptor, rictor, or mLST8. Like mice null for mTOR, those lacking raptor die early in development. However, mLST8 null embryos survive until e10.5 and resemble embryos missing rictor. mLST8 is necessary to maintain the rictor-mTOR, but not the raptor-mTOR, interaction, and both mLST8 and rictor are required for the hydrophobic motif phosphorylation of Akt/PKB and PKCalpha, but not S6K1. Furthermore, insulin signaling to FOXO3, but not to TSC2 or GSK3beta, requires mLST8 and rictor. Thus, mTORC1 function is essential in early development, mLST8 is required only for mTORC2 signaling, and mTORC2 is a necessary component of the Akt-FOXO and PKCalpha pathways.

Endoplasmic reticulum and Golgi localization sequences for mammalian target of rapamycin

Mammalian target of rapamycin (mTOR) forms two complexes, mTORC1 and mTORC2, that play central roles in cell growth and functions. Only mTORC1 is directly inhibited by the immunosuppressive drug rapamycin. Despite recent progress in identifying new components and functions of the mTOR pathway, relatively little is known about the spatial arrangement of mTOR signaling and the underlying mechanisms. In a previous study, we showed that a large proportion of mTOR is localized to the endoplasmic reticulum (ER) and Golgi in many common cell lines. Here, we report the identification of an internal mTOR sequence that contains two HEAT (HT) repeats, HT18 and HT19, and two intervening interunit spacers (IUSs), IUS17 and IUS18, which is sufficient to target enhanced green fluorescent protein to the Golgi. Surprisingly, deletion of IUS17 from this Golgi localization sequence (GLS) converts it to an ER localization sequence (ELS). Deletion of HT19, a common element of both GLS and ELS from the full-length mTOR, causes delocalization of mTOR and inhibits the ability of mTOR to promote S6 phosphorylation. Moreover, overexpression of GLS and ELS inhibits both mTOR complexes. Together, our results reveal unusual ER- and Golgi-targeting sequences and suggest that anchoring to these organelles is important for the functions of mTOR complexes.

Rapid turnover of mcl-1 couples translation to cell survival and apoptosis

Inhibition of translation plays a role in apoptosis induced by a variety of stimuli, but the mechanism by which it promotes apoptosis has not been established. We have investigated the hypothesis that selective degradation of anti-apoptotic regulatory protein(s) is responsible for apoptosis resulting from translation inhibition. Induction of apoptosis by cycloheximide was detected within 2-4 h and blocked by proteasome inhibitors, indicating that degradation of short-lived protein(s) was required. Caspase inhibition and overexpression of Bcl-x(L) blocked cycloheximide-induced apoptosis. In addition, cycloheximide induced rapid activation of Bak and Bax, which required proteasome activity. Mcl-1 was degraded by the proteasome with a half-life of approximately 30 min following inhibition of protein synthesis, preceding Bak/Bax activation and the onset of apoptosis. Overexpression of Mcl-1 blocked apoptosis induced by cycloheximide, whereas RNA interference knockdown of Mcl-1 induced apoptosis. Knockdown of Bim and Bak, downstream targets of Mcl-1, inhibited cycloheximide-induced apoptosis, as did knockdown of Bax. Apoptosis resulting from inhibition of translation thus involves the rapid degradation of Mcl-1, leading to activation of Bim, Bak, and Bax. Because of its rapid turnover, Mcl-1 may serve as a convergence point for signals that affect global translation, coupling translation to cell survival and the apoptotic machinery.

FoxOs Are Critical Mediators of Hematopoietic Stem Cell Resistance to Physiologic Oxidative Stress

To understand the role of FoxO family members in hematopoiesis, we conditionally deleted FoxO1, FoxO3, and FoxO4 in the adult hematopoietic system. FoxO-deficient mice exhibited myeloid lineage expansion, lymphoid developmental abnormalities, and a marked decrease of the lineage-negative Sca-1+, c-Kit+ (LSK) compartment that contains the short- and long-term hematopoietic stem cell (HSC) populations. FoxO-deficient bone marrow had defective long-term repopulating activity that correlated with increased cell cycling and apoptosis of HSC. Notably, there was a marked context-dependent increase in reactive oxygen species (ROS) in FoxO-deficient HSC compared with wild-type HSC that correlated with changes in expression of genes that regulate ROS. Furthermore, in vivo treatment with the antioxidative agent N-acetyl-L-cysteine resulted in reversion of the FoxO-deficient HSC phenotype. Thus, FoxO proteins play essential roles in the response to physiologic oxidative stress and thereby mediate quiescence and enhanced survival in the HSC compartment, a function that is required for its long-term regenerative potential.

Role of the phosphatidylinositol 3-kinase and mTOR pathways in the regulation of renal fibroblast function and differentiation

Tubulointerstitial fibrosis is largely mediated by (myo)fibroblasts present in the interstitium. In this study, we investigated the role of mTOR and phosphatidylinositol 3-kinase in the regulation of fibroblast kinetics, fibroblast differentiation, and collagen synthesis. Rat renal fibroblasts were propagated from kidneys 3 days post-ureteric obstruction and specific inhibitors of mTOR (RAD) and phosphatidylinositol 3-kinase (LY294002) were used to examine the regulation of fibrogenesis. LY294002 but not RAD completely inhibited phosphorylation of Akt, while both inhibitors decreased phosphorylation of the S6 ribosomal protein. RAD and LY decreased foetal calf serum stimulated proliferation and DNA synthesis. In addition to their individual effects, treatment with both RAD and LY294002 decreased serum-induced fibroblast proliferation and DNA synthesis significantly more than either drug alone. TUNEL positive cells (apoptosis) in RAD and LY294002 treated groups were not different from control groups. In addition to their effect on proliferation, both inhibitors also reduced total collagen synthesis. Differentiation studies indicated an increase in alpha-smooth muscle actin expression relative to beta-actin (western blotting), with cytochemistry confirming that all doses of RAD and LY294002 increased the proportion of alpha-smooth muscle actin positive cells, and hence myofibroblasts. Effects were independent of cell toxicity. These results highlight the potential significance of PI3K and mTOR, in the regulation of renal (myo)fibroblast activity. The synergistic effects of LY and RAD on proliferation suggest that mTOR signalling involves pathways other than phosphatidylinositol 3-kinase. These results provide a novel insight into the mechanisms of fibroblast regulation during fibrogenesis.

Methods for studying signal-dependent regulation of translation factor activity

The translational machinery of mammalian cells is regulated through the phosphorylation of a number of its components, especially translation factor proteins. These include factors involved in the initiation and elongation stages of translation, and proteins that modify their activity. Examples include eukaryotic initiation factor (eIF) 4E, eukaryotic elongation factor (eEF) 2, and eIF4E-binding protein 1 (4E-BP1). Their phosphorylation is mediated by protein kinases that, in turn, are regulated by specific intracellular signaling pathways. These pathways include those mediated via the mammalian target of rapamycin (mTOR), the ERK and p38 MAP kinase pathways, and protein kinase B (Akt). These pathways are activated by hormones (e.g., insulin), growth factors, mitogens, and other extracellular stimuli. In some cases, amino acids also modulate the pathway (e.g., mTOR). Procedures are described for determining the states of phosphorylation and/or activity of several translation factors, and of kinases that phosphorylate them. We also outline procedures for assessing the states of activation of relevant signaling pathways. In addition, we provide guidelines on using small molecule inhibitors to assess the involvement of specific signaling pathways in controlling translation factors and protein synthesis.

Insulin delays the progression of Drosophila cells through G2/M by activating the dTOR/dRaptor complex

In Drosophila and mammals, insulin signalling can increase growth, progression through G1/S, cell size and tissue size. Here, we analyse the way insulin affects cell size and cell-cycle progression in two haemocyte-derived Drosophila cell lines. Surprisingly, we find that although insulin increases cell size, it slows the rate at which these cells increase in number. By using BrdU pulse-chase to label S-phase cells and follow their progression through the cell cycle, we show that insulin delays progression through G2/M, thereby slowing cell division. The ability of insulin to slow progression through G2/M is independent of its ability to stimulate progression through G1/S, so is not a consequence of feedback by the cell-cycle machinery to maintain cell-cycle length. Insulin's effects on progression through G2/M are mediated by dTOR/dRaptor signalling. Partially inhibiting dTOR/dRaptor signalling by dsRNAi or mild rapamycin treatment can increase cell number in cultured haemocytes and the Drosophila wing, respectively. Thus, insulin signalling can influence cell number depending on a balance between its ability to accelerate progression through G1/S and delay progression through G2/M.

Rapamycin derivatives reduce mTORC2 signaling and inhibit AKT activation in AML

The mTOR complex 2 (mTORC2) containing mTOR and rictor is thought to be rapamycin insensitive and was recently shown to regulate the prosurvival kinase AKT by phosphorylation on Ser473. We investigated the molecular effects of mTOR inhibition by the rapamycin derivatives (RDs) temsirolimus (CCI-779) and everolimus (RAD001) in acute myeloid leukemia (AML) cells. Unexpectedly, RDs not only inhibited the mTOR complex 1 (mTORC1) containing mTOR and raptor with decreased p70S6K, 4EPB1 phosphorylation, and GLUT1 mRNA, but also blocked AKT activation via inhibition of mTORC2 formation. This resulted in suppression of phosphorylation of the direct AKT substrate FKHR and decreased transcription of D-cyclins in AML cells. Similar observations were made in samples from patients with hematologic malignancies who received RDs in clinical studies. Our study provides the first evidence that rapamycin derivatives inhibit AKT signaling in primary AML cells both in vitro and in vivo, and supports the therapeutic potential of mTOR inhibition strategies in leukemias.

Targeting mammalian target of rapamycin (mTOR) for health and diseases

The macrolide rapamycin is used clinically to treat graft rejection and restenosis. Mammalian target of rapamycin (mTOR) is a central controller of cellular and organism growth that integrates nutrient and hormonal signals, and regulates diverse cellular processes. New studies have linked mTOR to several human diseases including cancer, diabetes, obesity, cardiovascular diseases and neurological disorders. Recent data have also revealed that mTOR is involved in the regulation of lifespan and in age-related diseases. These findings demonstrate the importance of growth control in the pathology of major diseases and overall human health, and underscore the therapeutic potential of the mTOR pathway.

Aspects of mTOR biology and the use of mTOR inhibitors in non-Hodgkin's lymphoma

The mammalian target of rapamycin (mTOR) is a large and highly conserved kinase that integrates growth factor stimulation, energy and nutrient availability to modulate translation of proteins responsible for cellular growth and proliferation. Its importance in malignant cells provides strong rationale for the development of mTOR inhibitors (mTORi) in a broad variety of solid tumors and hematological malignancies. However several questions regarding mTOR biology and its interaction with pharmacological inhibitors remain unanswered and are relevant for further development of this novel family of cancer drugs. Nevertheless, mTORi have demonstrated activity in lymphoma cells either alone or in combination with cytotoxic agents. The most promising results have been seen in mantle cell lymphoma (MCL), likely because of its dependence on Cyclin D, the translation of which is largely regulated by mTOR activity. The currently knowledge of mTOR biology will here be reviewed along with the status of clinical development of mTORi in non-Hodgkin's lymphomas.