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

eIF4F complex disruption causes protein synthesis inhibition during hypoxia in nerve growth factor (NGF)-differentiated PC12 cells

Poor oxygenation (hypoxia) influences important physiological and pathological situations, including development, ischemia, stroke and cancer. Hypoxia induces protein synthesis inhibition that is primarily regulated at the level of initiation step. This regulation generally takes place at two stages, the phosphorylation of the subunit α of the eukaryotic initiation factor (eIF) 2 and the inhibition of the eIF4F complex availability by dephosphorylation of the inhibitory protein 4E-BP1 (eukaryotic initiation factor 4E-binding protein 1). The contribution of each of them is mainly dependent of the extent of the oxygen deprivation. We have evaluated the regulation of hypoxia-induced translation inhibition in nerve growth factor (NGF)-differentiated PC12 cells subjected to a low oxygen concentration (0.1%) at several times. Our findings indicate that protein synthesis inhibition occurs primarily by the disruption of eIF4F complex through 4E-BP1 dephosphorylation, which is produced by the inhibition of the mammalian target of rapamycin (mTOR) activity via the activation of REDD1 (regulated in development and DNA damage 1) protein in a hypoxia-inducible factor 1 (HIF1)-dependent manner, as well as the translocation of eIF4E to the nucleus. In addition, this mechanism is reinforced by the increase in 4E-BP1 levels, mainly at prolonged times of hypoxia.

Detection of cytoplasmic and nuclear functions of mTOR by fractionation

Subcellular localization constitutes the environment in which proteins act. It tightly controls access to and availability of different types of molecular interacting partners and is therefore a major determinant of protein function and regulation. Originally thought to be a mere cytoplasmic kinase the mammalian target of rapamycin (mTOR) has recently been localized to various intracellular compartments including the nucleus and specific components of the endomembrane system such as lysosomes. The identification of essential binding partners and the structural and functional partitioning of mTOR into two distinct multiprotein complexes warrant the detailed investigation of the subcellular localization of mTOR as part of mTORC1 and mTORC2. Upon establishment of experimental conditions allowing cytoplasmic/nuclear fractionation at high purity and maximum mTOR complex recovery we have previously shown that the mTOR/raptor complex (mTORC1) is predominantly cytoplasmic whereas the mTOR/rictor complex (mTORC2) is abundant in both compartments. Moreover, the mTORC2 complex components rictor and sin1 are dephosphorylated and dynamically distributed between the cytoplasm and the nucleus upon long-term treatment with the mTOR-inhibitor rapamycin. These findings further demonstrate that the here presented and detailly described fractionation procedure is a valuable tool to study protein localization and cytoplasmic/nuclear protein shuttling in the context of expanding mTOR signalling.

Autophagy as a therapeutic target in diabetic nephropathy

Diabetic nephropathy is a serious complication of diabetes mellitus, and its prevalence has been increasing worldwide. Therefore, there is an urgent need to identify a new therapeutic target to prevent diabetic nephropathy. Autophagy is a major catabolic pathway involved in degrading and recycling macromolecules and damaged organelles to maintain intracellular homeostasis. The study of autophagy in mammalian systems is advancing rapidly and has revealed that it is involved in the pathogenesis of various metabolic or age-related diseases. The functional role of autophagy in the kidneys is also currently under intense investigation although, until recently, evidence showing the involvement of autophagy in the pathogenesis of diabetic nephropathy has been limited. We provide a systematic review of autophagy and discuss the therapeutic potential of autophagy in diabetic nephropathy to help future investigations in this field.

Concentration of antifungal agents within host cell membranes: a new paradigm governing the efficacy of prophylaxis

Posaconazole prophylaxis has proven highly effective in preventing invasive fungal infections, despite relatively low serum concentrations. However, high tissue levels of this agent have been reported in treated patients. We therefore hypothesized that the intracellular levels of antifungal agents are an important factor in determining the success of fungal prophylaxis. To examine the effect of host cell-associated antifungals on the growth of medically important molds, we exposed cells to antifungal agents and removed the extracellular drug prior to infection. Epithelial cells loaded with posaconazole and its parent molecule itraconazole, but not other antifungals, were able to inhibit fungal growth for at least 48 h and were protected from damage caused by infection. Cell-associated posaconazole levels were 40- to 50-fold higher than extracellular levels, and the drug was predominantly detected in cellular membranes. Fungistatic levels of posaconazole persisted within epithelial cells for up to 48 h. Therefore, the concentration of posaconazole in mammalian host cell membranes mediates its efficacy in prophylactic regimens and likely explains the observed discrepancy between serum antifungal levels and efficacy.

Protein kinases and phosphatases in the control of cell fate

Protein phosphorylation controls many aspects of cell fate and is often deregulated in pathological conditions. Several recent findings have provided an intriguing insight into the spatial regulation of protein phosphorylation across different subcellular compartments and how this can be finely orchestrated by specific kinases and phosphatases. In this review, the focus will be placed on (i) the phosphoinositide 3-kinase (PI3K) pathway, specifically on the kinases Akt and mTOR and on the phosphatases PP2a and PTEN, and on (ii) the PKC family of serine/threonine kinases. We will look at general aspects of cell physiology controlled by these kinases and phosphatases, highlighting the signalling pathways that drive cell division, proliferation, and apoptosis.

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

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

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

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

Frontier of epilepsy research - mTOR signaling pathway

Studies of epilepsy have mainly focused on the membrane proteins that control neuronal excitability. Recently, attention has been shifting to intracellular proteins and their interactions, signaling cascades and feedback regulation as they relate to epilepsy. The mTOR (mammalian target of rapamycin) signal transduction pathway, especially, has been suggested to play an important role in this regard. These pathways are involved in major physiological processes as well as in numerous pathological conditions. Here, involvement of the mTOR pathway in epilepsy will be reviewed by presenting; an overview of the pathway, a brief description of key signaling molecules, a summary of independent reports and possible implications of abnormalities of those molecules in epilepsy, a discussion of the lack of experimental data, and questions raised for the understanding its epileptogenic mechanism.

Nucleocytoplasmic localization of p70 S6K1, but not of its isoforms p85 and p31, is regulated by TSC2/mTOR

The tuberous sclerosis complex gene 2 (TSC2)/mammalian target of rapamycin (mTOR) pathway controls many cellular functions via phosphorylation of ribosomal protein S6 kinases (S6Ks). Alternative splicing and translation generate three S6K1 proteins. Although nuclear and cytoplasmic S6K targets are known, the nucleocytoplasmic localization of the S6K1 proteins has not been comparatively elucidated so far. We show that in primary fibroblasts p85 S6K1 is cytoplasmic, p70 can be found in both compartments and p31 is exclusively nuclear. As already known for p70 and p85, our data suggest that p31 is also a target of mTOR-mediated phosphorylation. Blocking mTOR kinase activity via rapamycin and its activation in TSC2(-/-) cells and via TSC2 small interfering RNAs revealed that it regulates the localization of p70, but not of p85 and p31. The mTOR-dependent phosphorylation of p70 S6K1 at T389 is essential for its nuclear localization and exclusively hyperphosphorylated p70 S6K1 can be found in the nucleus. We further demonstrate this mTOR-controlled p70 S6K1 localization to be growth factor dependent. During the cell-cycle phosphorylation and nuclear localization of p70 S6K1 occur in mid G1 phase. We report that the different S6K1 proteins exhibit different nucleocytoplasmic localizations and that the TSC2/mTOR cascade not only regulates p70 S6K1 activity, but also its localization. These findings provide new important insights into the temporal and spatial dynamics of TSC2/mTOR/S6K regulation.

Cancer cell survival following DNA damage-mediated premature senescence is regulated by mammalian target of rapamycin (mTOR)-dependent Inhibition of sirtuin 1

DNA-damaging agents can induce premature senescence in cancer cells, which contributes to the static effects of cancer. However, senescent cancer cells may re-enter the cell cycle and lead to tumor relapse. Understanding the mechanisms that control the viability of senescent cells may be helpful in eliminating these cells before they can regrow. Treating human squamous cell carcinoma (SCC) cells with the anti-cancer compounds, resveratrol and doxorubicin, triggered p53-independent premature senescence by invoking oxidative stress-mediated DNA damage. This process involved the mTOR-dependent phosphorylation of SIRT1 at serine 47, resulting in the inhibition of the deacetylase activity of SIRT1. SIRT1 phosphorylation caused concomitant increases in p65/RelA NF-κB acetylation and the expression of an anti-apoptotic Bfl-1/A1. SIRT1 physically interacts with the mTOR-Raptor complex, and a single amino acid substitution in the TOS (TOR signaling) motif in the SIRT1 prevented Ser-47 phosphorylation and Bfl-1/A1 induction. The pharmacologic and genetic inhibition of mTOR, unphosphorylatable S47A, or F474A TOS mutants restored SIRT1 deacetylase activity, blocked Bfl-1/A1 induction, and sensitized prematurely senescent SCC cells for apoptosis. We further show that the treatment of UVB-induced SCCs with doxorubicin transiently stabilized tumor growth but was followed by tumor regrowth upon drug removal in p53(+/-)/SKH-1 mice. The subsequent treatment of stabilized SCCs with rapamycin decreased tumor size and induced caspase-3 activation. These results demonstrate that the inhibition of SIRT1 by mTOR fosters survival of DNA damage-induced prematurely senescent SCC cells via Bfl-1/A1 in the absence of functional p53.

Reduction in ribosomal protein synthesis is sufficient to explain major effects on ribosome production after short-term TOR inactivation in Saccharomyces cerevisiae

Ribosome synthesis depends on nutrient availability, sensed by the target of rapamycin (TOR) signaling pathway in eukaryotes. TOR inactivation affects ribosome biogenesis at the level of rRNA gene transcription, expression of ribosomal proteins (r-proteins) and biogenesis factors, preribosome processing, and transport. Here, we demonstrate that upon TOR inactivation, levels of newly synthesized ribosomal subunits drop drastically before the integrity of the RNA polymerase I apparatus is severely impaired but in good correlation with a sharp decrease in r-protein production. Inhibition of translation by cycloheximide mimics the rRNA maturation defect observed immediately after TOR inactivation. Both cycloheximide addition and the depletion of individual r-proteins also reproduce TOR-dependent nucleolar entrapment of specific ribosomal precursor complexes. We suggest that shortage of newly synthesized r-proteins after short-term TOR inactivation is sufficient to explain most of the observed effects on ribosome production.

mTORC1 signaling: what we still don't know

The mammalian target of rapamycin (mTOR) is a protein kinase that plays key roles in cellular regulation. It forms complexes with additional proteins. The best-understood one is mTOR complex 1 (mTORC1). The regulation and cellular functions of mTORC1 have been the subjects of intense study; despite this, many questions remain to be answered. They include questions about the actual mechanisms by which mTORC1 signaling is stimulated by hormones and growth factors, which involves the small GTPase Rheb, and by amino acids, which involves other GTPase proteins. The control of Rheb and the mechanism by which it activates mTORC1 remain incompletely understood. Although it has been known for many years that rapamycin interferes with some functions of mTORC1, it is not known how it does this, or why only some functions of mTORC1 are affected. mTORC1 regulates diverse cellular functions. Several mTORC1 substrates are now known, although in several cases their physiological roles are poorly or incompletely understood. In the case of several processes, although it is clear that they are regulated by mTORC1, it is not known how mTORC1 does this. Lastly, mTORC1 is implicated in ageing, but again it is unclear what mechanisms account for this. Given the importance of mTORC1 signaling both for cellular functions and in human disease, it is a high priority to gain further insights into the control of mTORC1 signaling and the mechanisms by which it controls cellular functions and animal physiology.

New concepts in phospholipase D signaling in inflammation and cancer

Phospholipase D (PLD) catalyzes the hydrolysis of phosphatidylcholine to generate the lipid second messenger phosphatidic acid (PA) and choline. PLD regulation in cells falls into two major signaling categories. One is via growth factors/mitogens, such as EGF, PDGF, insulin, and serum, and implicates tyrosine kinases; the other is via the small GTPase proteins Arf and Rho. We summarize here our lab's and other groups' contributions to those pathways and introduce several novel concepts. For the mitogen-induced signaling, new data indicate that an increase in cell transformation in PLD2-overexpressing cells is due to an increase of de novo DNA synthesis induced by PLD2, with the specific tyrosine residues involved in those functions being Y and Y. Recent research has also implicated Grb2 in tyrosine phosphorylation of PLD2 that also involves Sos and the ERK pathway. The targets of phosphorylation within the PLD2 molecule that are key to its regulation have recently been precisely mapped. They are Y, Y, and Y and the responsible kinases are, respectively, EGFR, JAK3, and Src. Y is an inhibitory site and its phosphorylation explains the low PLD2 activity that exists in low-invasive MCF-7 breast cancer cells. Advances along the small GTPase front have implicated cell migration, as PLD1 and PLD2 cause an increase in chemotaxis of leukocytes and inflammation. PA is necessary for full chemotaxis. PA enriches the localization of the atypical guanine exchange factor (GEF), DOCK2, at the leading edge of polarized neutrophils. Further, extracellular PA serves as a neutrophil chemoattractant; PA enters the cell and activates the mTOR/S6K pathway (specifically, S6K). A clear connection between PLD with the mTOR/S6K pathway has been established, in that PA binds to mTOR and also binds to S6K independently of mTOR. Lastly, there is evidence in the upstream direction of cell signaling that mTOR and S6K keep PLD2 gene expression function down-regulated in basal conditions. In summary, the involvement of PLD2 in cell signaling continues to expand geometrically. It involves gene transcription, mitogenic and cell migration effects as seen in normal growth, tumor development, and inflammation.

Target of rapamycin (TOR)-like 1 kinase is involved in the control of polyphosphate levels and acidocalcisome maintenance in Trypanosoma brucei

Target of rapamycin (TOR) kinases are highly conserved protein kinases that integrate signals from nutrients and growth factors to coordinate cell growth and cell cycle progression. It has been previously described that two TOR kinases control cell growth in the protozoan parasite Trypanosoma brucei, the causative agent of African trypanosomiasis. Here we studied an unusual TOR-like protein named TbTOR-like 1 containing a PDZ domain and found exclusively in kinetoplastids. TbTOR-like 1 localizes to unique cytosolic granules. After hyperosmotic stress, the localization of the protein shifts to the cell periphery, different from other organelle markers. Ablation of TbTOR-like 1 causes a progressive inhibition of cell proliferation, producing parasites accumulating in the S/G(2) phase of the cell cycle. TbTOR-like 1 knocked down cells have an increased area occupied by acidic vacuoles, known as acidocalcisomes, and are enriched in polyphosphate and pyrophosphate. These results suggest that TbTOR-like 1 might be involved in the control of acidocalcisome and polyphosphate metabolism in T. brucei.

19-peptide, a fragment of tumstatin, inhibits the growth of poorly differentiated gastric carcinoma cells in vitro and in vivo

BACKGROUND AND AIM

This study investigated whether 19-peptide, a fragment of tumstatin, inhibited the growth of gastric tumor cells in vitro and in vivo.

METHODS

19-peptide was expressed in bacteria and purified with Sephadex G-15. SGC7901 gastric carcinoma cells and human umbilical-vein endothelial cells (HUVECs) were exposed to 19-peptide in vitro, and their viability was evaluated by biochemical and histopathological analysis. In vivo, pieces of solid tumor derived from SGC7901 cells were inoculated into the gastric serosa of 36 nude mice, with a biological glue to hold them in place. Twenty-eight days after injection of 19-peptide, the mice were killed. The tumors were measured and examined by western blotting, histopathology, and terminal deoxynucleotidyl transferase biotin-dUTP nick end labeling assay.

RESULTS

19-peptide induced apoptosis of many SGC7901 cells but few HUVECs in vitro. In vivo, after the application of 19-peptide, significant tumor cell apoptosis was observed in the center of the tumors, tumor volume was reduced significantly (P < 0.001), and the invasion and migration of cancer cells was reduced. PTEN was increased in the treatment group and phospho-Akt (pAkt) was decreased in the control group.

CONCLUSIONS

These results suggest that 19-peptide inhibits the growth and metastases of poorly differentiated gastric carcinoma cells, primarily by inducing apoptosis. The apoptotic mechanism could be related to anoikis and the PTEN/Akt pathway.

Requirement of the mTOR kinase for the regulation of Maf1 phosphorylation and control of RNA polymerase III-dependent transcription in cancer cells

The mammalian target of rapamycin (mTOR) regulates growth via promoting translation and transcription. Here, employing an mTOR active-site inhibitor WYE-125132 (WYE-132), we have performed quantitative phospho-proteomics and identified a Ser-75-containing phosphopeptide from Maf1, a known repressor of RNA polymerase III (Pol III) transcription. Treatment of cancer cells with WYE-132 or the rapamycin analog CCI-779 led to a rapid loss of the phosphorylation at Ser-75, whereas this effect was not seen in cells treated with cytotoxic agents or unrelated inhibitors. WYE-132-induced Maf1 dephosphorylation correlated with its accumulation in the nucleus and a marked decline in the cellular levels of pre-tRNAs. Depletion of cellular Maf1 via small interfering RNA increased basal pre-tRNA and rendered tRNA synthesis refractory to mTOR inhibitors. Maf1 mutant proteins carrying S75A alone or with S60A, T64A, and S68A (Maf1-S75A, Maf1-4A) progressively enhanced basal repression of tRNA in actively proliferating cells and attenuated amino acid-induced tRNA transcription. Gene alignment revealed conservation of all four Ser/Thr sites in high eukaryotes, further supporting a critical role of these residues in Maf1 function. Interestingly, mTOR inhibition led to an increase in the occupancy of Maf1 on a set of Pol III-dependent genes, with concomitant reduction in the binding of Pol III and Brf1. Unexpectedly, mTORC1 itself was also enriched at the same set of Pol III templates, but this association was not influenced by mTOR inhibitor treatment. Our results highlight a new and unique mode of regulation of Pol III transcription by mTOR and suggest that normalization of Pol III activity may contribute to the therapeutic efficacy of mTOR inhibitors.

A chemical genetic screen for mTOR pathway inhibitors based on 4E-BP-dependent nuclear accumulation of eIF4E

The signal transduction pathway wherein mTOR regulates cellular growth and proliferation is an active target for drug discovery. The search for new mTOR inhibitors has recently yielded a handful of promising compounds that hold therapeutic potential. This search has been limited by the lack of a high-throughput assay to monitor the phosphorylation of a direct rapamycin-sensitive mTOR substrate in cells. Here we describe a novel cell-based chemical genetic screen useful for efficiently monitoring mTOR signaling to 4E-BPs in response to stimuli. The screen is based on the nuclear accumulation of eIF4E, which occurs in a 4E-BP-dependent manner specifically upon inhibition of mTOR signaling. Using this assay in a small-scale screen, we have identified several compounds not previously known to inhibit mTOR signaling, demonstrating that this method can be adapted to larger screens.

Maf1 regulation: a model of signal transduction inside the nucleus

RNA polymerase III (Pol III) is responsible for the synthesis of 5S ribosomal RNA (rRNA) and transfer RNAs (tRNAs) essential for protein synthesis and cell growth. Pol III is tightly controlled by growth signals such as nutrients and deregulation of Pol III-dependent transcription can lead to oncogenic transformation. In response to extracellular stimuli, the target of rapamycin complex 1 (TORC1) regulates Pol III activity through Maf1, a key conserved Pol III repressor. Recent studies have unraveled intricate mechanisms by which Maf1 activity is controlled at multiple levels, including nuclear transport and phoshorylation at specific chromatin loci. These studies suggest an emerging mode of gene regulation by extracellular signals inside the nucleus.

Quantitative nuclear proteomics identifies mTOR regulation of DNA damage response

Cellular nutritional and energy status regulates a wide range of nuclear processes important for cell growth, survival, and metabolic homeostasis. Mammalian target of rapamycin (mTOR) plays a key role in the cellular responses to nutrients. However, the nuclear processes governed by mTOR have not been clearly defined. Using isobaric peptide tagging coupled with linear ion trap mass spectrometry, we performed quantitative proteomics analysis to identify nuclear processes in human cells under control of mTOR. Within 3 h of inhibiting mTOR with rapamycin in HeLa cells, we observed down-regulation of nuclear abundance of many proteins involved in translation and RNA modification. Unexpectedly, mTOR inhibition also down-regulated several proteins functioning in chromosomal integrity and up-regulated those involved in DNA damage responses (DDRs) such as 53BP1. Consistent with these proteomic changes and DDR activation, mTOR inhibition enhanced interaction between 53BP1 and p53 and increased phosphorylation of ataxia telangiectasia mutated (ATM) kinase substrates. ATM substrate phosphorylation was also induced by inhibiting protein synthesis and suppressed by inhibiting proteasomal activity, suggesting that mTOR inhibition reduces steady-state (abundance) levels of proteins that function in cellular pathways of DDR activation. Finally, rapamycin-induced changes led to increased survival after radiation exposure in HeLa cells. These findings reveal a novel functional link between mTOR and DDR pathways in the nucleus potentially operating as a survival mechanism against unfavorable growth conditions.

Cell signaling in protein synthesis ribosome biogenesis and translation initiation and elongation

Protein synthesis is a highly energy-consuming process that must be tightly regulated. Signal transduction cascades respond to extracellular and intracellular cues to phosphorylate proteins involved in ribosomal biogenesis and translation initiation and elongation. These phosphorylation events regulate the timing and rate of translation of both specific and total mRNAs. Alterations in this regulation can result in dysfunction and disease. While many signaling pathways intersect to control protein synthesis, the mTOR and MAPK pathways appear to be key players. This chapter briefly reviews the mTOR and MAPK pathways and then focuses on individual phosphorylation events that directly control ribosome biogenesis and translation.

PRAS40: target or modulator of mTORC1 signalling and insulin action?

Alterations in signalling via protein kinase B (PKB/Akt) and the mammalian target of rapamycin (mTOR) frequently occur in type 2 diabetes and various human malignancies. Proline-rich Akt substrate of 40-kDa (PRAS40) has a regulatory function at the intersection of these pathways. The interaction of PRAS40 with the mTOR complex 1 (mTORC1) inhibits the activity of mTORC1. Phosphorylation of PRAS40 by PKB/Akt and mTORC1 disrupts the binding between mTORC1 and PRAS40, and relieves the inhibitory constraint of PRAS40 on mTORC1 activity. This review summarizes the signalling pathways regulating PRAS40 phosphorylation, as well as the dual function of PRAS40 as substrate and inhibitor of mTORC1 in the physiological situation, and under pathological conditions, such as insulin resistance and cancer.

EGFR-dependent and independent activation of Akt/mTOR cascade in bone and soft tissue tumors

To gain the insight into the involvement of signaling mediated by the mammalian target of rapamycin (mTOR) in the phenotype and biological profiles of tumors and tumor-like lesions of the bone and soft tissue, we analyzed the expression and phosphorylation (activation) of mTOR and its correlation with the status of upstream and downstream modulator proteins Akt, p70S6-kinase (S6K), and eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), which we refer to collectively as mTOR cassette proteins. Immunohistochemical analysis of 140 cases showed activation of Akt in 55% (61% in malignant and 27% in benign), and mTOR expression in 61% (66% in malignant and 39% in benign). The preponderance of mTOR activation was found in tumors of peripheral nerve sheath (malignant peripheral nerve sheath tumor and schwannoma), skeletal muscle origin (rhabdomyosarcoma), and in those exhibiting epithelial nature (chordoma and synovial sarcoma). Together with the result of immunoblotting analysis, it was shown that many of those particular tumors with mTOR activation exhibited activation of Akt, S6K, and 4E-BP1, suggesting the constitutive activation of the Akt/mTOR pathway. In addition, although activation of the Akt/mTOR pathway was largely independent of activation of epidermal growth factor receptor (EGFR), mutation of EGFR was frequently accompanied by constitutive activation of Akt-mTOR-S6K/4E-BP1. By clinicopathological analysis, activation of Akt correlates with statistically higher probability of metastasis. We conclude that mTOR-mediated signaling proteins function not only in the proliferation of the tumor cells, but also in the differentiation and/or maintenance of morphological phenotypes in tumors of rhabdomyoblastic and nerve sheath cell origin. Furthermore, mTOR signaling may also modulate morphogenesis of tumors exhibiting epithelial nature. Additionally, activated Akt may have a function in metastasis. Overall, these results suggest that inhibitors of mTOR cassette may be useful as novel components of combined chemotherapy for a defined subset of bone and soft tissue sarcomas.

Inactivation of mammalian target of rapamycin increases STAT1 nuclear content and transcriptional activity in alpha4- and protein phosphatase 2A-dependent fashion

Target of rapamycin (TOR) is a highly conserved serine/threonine kinase that controls cell growth, primarily via regulation of protein synthesis. In Saccharomyces cerevisiae, TOR can also suppress the transcription of stress response genes by a mechanism involving Tap42, a serine/threonine phosphatase subunit, and the transcription factor Msn2. A physical association between mammalian TOR (mTOR) and the transcription factor signal transducer and activator of transcription-1 (STAT1) was recently identified in human cells, suggesting a similar role for mTOR in the transcription of interferon-gamma-stimulated genes. In the current study, we identified a macromolecular protein complex composed of mTOR, STAT1, the Tap42 homologue alpha4, and the protein phosphatase 2A catalytic subunit (PP2Ac). Inactivation of mTOR enhanced its association with STAT1 and increased STAT1 nuclear content in PP2Ac-dependent fashion. Depletion of alpha4, PP2A, or mTOR enhanced the induction of early (i.e. IRF-1) and late (i.e. caspase-1, hiNOS, and Fas) STAT1-dependent genes. The regulation of IRF-1 or caspase-1 by mTOR was independent of other known mTOR effectors p70 S6 kinase and Akt. These results describe a new role for mTOR and alpha4/PP2A in the control of STAT1 nuclear content, and the expression of interferon-gamma-sensitive genes involved in immunity and apoptosis.

TOR complex 2 controls gene silencing, telomere length maintenance, and survival under DNA-damaging conditions

The Target Of Rapamycin (TOR) kinase belongs to the highly conserved eukaryotic family of phosphatidylinositol-3-kinase-related kinases (PIKKs). TOR proteins are found at the core of two distinct evolutionarily conserved complexes, TORC1 and TORC2. Disruption of TORC1 or TORC2 results in characteristically dissimilar phenotypes. TORC1 is a major cell growth regulator, while the cellular roles of TORC2 are not well understood. In the fission yeast Schizosaccharomyces pombe, Tor1 is a component of the TORC2 complex, which is particularly required during starvation and various stress conditions. Our genome-wide gene expression analysis of Deltator1 mutants indicates an extensive similarity with chromatin structure mutants. Consistently, TORC2 regulates several chromatin-mediated functions, including gene silencing, telomere length maintenance, and tolerance to DNA damage. These novel cellular roles of TORC2 are rapamycin insensitive. Cells lacking Tor1 are highly sensitive to the DNA-damaging drugs hydroxyurea (HU) and methyl methanesulfonate, similar to mutants of the checkpoint kinase Rad3 (ATR). Unlike Rad3, Tor1 is not required for the cell cycle arrest in the presence of damaged DNA. Instead, Tor1 becomes essential for dephosphorylation and reactivation of the cyclin-dependent kinase Cdc2, thus allowing reentry into mitosis following recovery from DNA replication arrest. Taken together, our data highlight critical roles for TORC2 in chromatin metabolism and in promoting mitotic entry, most notably after recovery from DNA-damaging conditions. These data place TOR proteins in line with other PIKK members, such as ATM and ATR, as guardians of genome stability.

Mammalian target of rapamycin: a new target in prostate cancer

Molecular targets in prostate cancer are continually being explored, especially in the poor-prognosis androgen-independent phase of the disease, for which there are currently few therapeutic options. One such target is the mammalian target of rapamycin (mTOR) protein. Activation of mTOR results in sequential activation of downstream molecules, which ultimately results in cell division. In this review, we consider the rationale for pursuing mTOR as a therapeutic target in prostate cancer and summarize preclinical and clinical studies of mTOR inhibition in prostate cancer.

Sfp1 interaction with TORC1 and Mrs6 reveals feedback regulation on TOR signaling

Ribosome biogenesis drives cell growth, and the large transcriptional output underlying this process is tightly regulated. The Target of Rapamycin (TOR) kinase is part of a highly conserved signaling pathway linking nutritional and stress signals to regulation of ribosomal protein (RP) and ribosome biogenesis (Ribi) gene transcription. In Saccharomyces cerevisiae, one of the downstream effectors of TOR is Sfp1, a transcriptional activator that regulates both RP and Ribi genes. Here, we report that Sfp1 interacts directly with TOR complex 1 (TORC1) in a rapamycin-regulated manner, and that phosphorylation of Sfp1 by this kinase complex regulates its function. Sfp1, in turn, negatively regulates TORC1 phosphorylation of Sch9, another key TORC1 target that acts in parallel with Sfp1, revealing a feedback mechanism controlling the activity of these proteins. Finally, we show that the Sfp1-interacting protein Mrs6, a Rab escort protein involved in membrane trafficking, regulates both Sfp1 nuclear localization and TORC1 signaling.

S6K1 is involved in polyploidization through its phosphorylation at Thr421/Ser424

Studies on polyploidization of megakaryocytes have been hampered by the lack of synchronized polyploid megakaryocytes. In this study, a relatively synchronized polyploid cell model was successfully established by employing Dami cells treated with nocodazole. In nocodazole-induced cells, cyclin B expression oscillated normally as in diploid cells and polyploid megakaryocytes. By using the nocodazole-induced Dami cell model, we found that 4E-BP1 and Thr421/Ser424 of ribosomal S6 kinase 1(S6K1) were phosphorylated mostly at M-phase in cytoplasm and oscillated in nocodazole-induced polyploid Dami cells, concomitant with increased expression of p27 and cyclin D3. However, phosphorylation of 4E-BP1 and S6K1 on Thr421/Ser424 was significantly decreased in differentiated Dami cells induced by phorbol 12-myristate 13-acetate (PMA), concomitant with increased expression of cyclin D1 and p21 and cyclin D3. Overexpression of the kinase dead form of S6K1 containing the mutation Lys 100 --> Gln in PMA-induced Dami cells increased ploidy whereas overexpression of rapamycin-resistant form of S6K1 containing the mutations Thr421 --> Glu and Ser424 --> Asp significantly dephosphorylated 4E-BP1 and reduced expression of cyclin D1, cyclin D3, p21 and p27, and slightly decreased the ploidy of PMA-induced Dami cells, compared with treatment with PMA alone. Moreover, overexpression of rapamycin-resistant form of S6K1 significantly reversed polyploidization of nocodazole-induced Dami cells. Furthermore, MAP (a novel compound synthesized recently) partly blocked the phosphorylation of S6K1 on Thr421/Ser424 and decreased the expression of p27 and polyploidization in nocodazole-induced Dami cells. Taken together, these data suggested that S6K1/4E-BP1 pathway may play an important role in polyploidization of megakaryocytes.

p70 S6 kinase promotes epithelial to mesenchymal transition through snail induction in ovarian cancer cells

p70 S6 kinase (p70(S6K)) is a downstream effector of phosphatidylinositol 3-kinase and is frequently activated in human ovarian cancer. Here we show that p70(S6K) functions in epithelial to mesenchymal transition (EMT) responsible for the acquisition of invasiveness during tumor progression. This tumorigenic activity is associated with the ability of p70(S6K) to repress E-cadherin through the up-regulation of Snail. p70(S6K) activation induced phenotypic changes consistent with EMT in ovarian cancer cells: The cells lost epithelial cell morphology, acquired fibroblast-like properties, and showed reduced intercellular adhesion. Western blot showed that p70(S6K) activation led to decreased expression of the epithelial marker E-cadherin and increased expression of mesenchymal markers N-cadherin and vimentin. Inhibition of p70(S6K) by a specific inhibitor or small interfering RNA reversed the shift of EMT markers. Importantly, p70(S6K) activation also stimulated the expression of Snail, a repressor of E-cadherin and an inducer of EMT, but not other family members such as Slug. This induction of Snail was regulated at multiple levels by increasing transcription, inhibiting protein degradation, and enhancing nuclear localization of Snail. RNA interference-mediated knockdown of Snail suppressed p70(S6K)-induced EMT, confirming that the effect was Snail specific. Furthermore, phospho (active)-p70(S6K) staining correlated with higher tumor grade. We also showed a significant positive correlation between p70(S6K) activation and Snail expression in ovarian cancer tissues. These results indicate that p70(S6K) may play a critical role in tumor progression in ovarian cancer through the induction of EMT. Targeting p70(S6K) may thus be a useful strategy to impede cancer cell invasion and metastasis.

Rapamycin inhibits trypanosome cell growth by preventing TOR complex 2 formation

Target of rapamycin (TOR) kinases control cell growth through two functionally distinct multiprotein complexes. TOR complex 1 (TORC1) controls temporal cell growth and is sensitive to rapamycin, whereas TOR complex 2 (TORC2) is rapamycin resistant and regulates spatial cell growth. Here, we identified two TOR orthologues, TbTOR1 and TbTOR2, in the protozoan parasite Trypanosoma brucei, as well as orthologues of the well-known TORC1 and TORC2 partners, KOG1/raptor and AVO3/rictor. TbTOR proteins differ in their functions, subcellular localization, and rapamycin sensitivity. TbTOR1 controls cell growth by regulating cell cycle, nucleolus structure, and protein synthesis, whereas TbTOR2 coordinates cell polarization and cytokinesis. Rapamycin treatment of bloodstream trypanosomes resulted in a pronounced reduction of cell proliferation, with an EC(50) of 152 nM. Unique for a eukaryote, we observed that rapamycin acted exclusively by preventing TORC2 formation, with no effect on TORC1. Our findings on TOR signaling in this protozoan, which is located in a distal position in the eukaryotic cell lineage, highlight the clinical possibilities of rapamycin derivates and provide valuable insights into understanding rapamycin-mediated inhibition of TORC2.

Arginine stimulates cdx2-transformed intestinal epithelial cell migration via a mechanism requiring both nitric oxide and phosphorylation of p70 S6 kinase

In intestinal cells, arginine (Arg) is 1 of the 2 most potent amino acid activators of p70(s6k), a key regulator of 5'- terminal oligopyrimidine mRNA translation, a necessary condition for increased cell migration. To investigate the mechanism of response to Arg, we used the rat crypt cell line cdx2-transformed IEC-6 cells (cdx2-IEC) and measured cell migration, immunocytochemical analysis of p70(s6k) activation in response to Arg, and production of nitric oxide (NO). When treated with Arg, cdx2-IEC increased in phosphorylation on Thr-389 of p70(s6k) (pp70(s6k)) compared with control (P < 0.01). Phospho-Thr-421/Ser-424-p70(s6k) was located in the nucleus shortly after Arg treatment. Arg enhanced pp70(s6k), cell migration (55% wound coverage), and NO production. In comparison, the branched-chain amino acid leucine (Leu) activated pp70(s6k), was a weaker stimulator of migration (23% coverage), and did not increase NO. A total of 25 micromol/L DETA-NONOate (DETA/NO) did not significantly enhance phosphorylation of p70(s6k) but enhanced the rate of cell migration by approximately 25%. Wound coverage with Leu plus DETA/NO (25 micromol/L) was greater than coverage with DETA/NO alone (P < 0.01). These and our previous studies lead to a model in which Arg must stimulate both pp70(s6k) (in the nucleus) and NO release to enhance intestinal epithelial cell migration, which may be relevant to diseases that involve intestinal villous injury.

TOR1 and TOR2 have distinct locations in live cells

TOR is a structurally and functionally conserved Ser/Thr kinase found in two multiprotein complexes that regulate many cellular processes to control cell growth. Although extensively studied, the localization of TOR is still ambiguous, possibly because endogenous TOR in live cells has not been examined. Here, we examined the localization of green fluorescent protein (GFP) tagged, endogenous TOR1 and TOR2 in live S. cerevisiae cells. A DNA cassette encoding three copies of green fluorescent protein (3XGFP) was inserted in the TOR1 gene (at codon D330) or the TOR2 gene (at codon N321). The TORs were tagged internally because TOR1 or TOR2 tagged at the N or C terminus was not functional. The TOR1(D330-3XGFP) strain was not hypersensitive to rapamycin, was not cold sensitive, and was not resistant to manganese toxicity caused by the loss of Pmr1, all indications that TOR1-3XGFP was expressed and functional. TOR2-3XGFP was functional, as TOR2 is an essential gene and TOR2(N321-3XGFP) haploid cells were viable. Thus, TOR1 and TOR2 retain function after the insertion of 748 amino acids in a variable region of their noncatalytic domain. The localization patterns of TOR1-3XGFP and TOR2-3XGFP were documented by imaging of live cells. TOR1-3XGFP was diffusely cytoplasmic and concentrated near the vacuolar membrane. The TOR2-3XGFP signal was cytoplasmic but predominately in dots at the plasma membrane. Thus, TOR1 and TOR2 have distinct localization patterns, consistent with the regulation of cellular processes as part of two different complexes.

The switch I region of Rheb is critical for its interaction with FKBP38

The Ras-like small GTPase Rheb is an upstream activator of the mammalian target of rapamycin (mTOR). It has recently been shown that Rheb activates mTOR by binding to its endogenous inhibitor FKBP38 and preventing it from association with mTOR. The interaction of Rheb with FKBP38 is controlled by its guanine nucleotide binding states, which are responsive to growth factor and amino acid conditions. In this study, we show that Rheb interacts with FKBP38 through a section within its switch I region that is equivalent to the effector domain of other Ras-like small GTPases. We find that the ability for Rheb to interact with FKBP38 correlates with its activity for mTOR activation. Our findings suggest that FKBP38 is a bona fide effector of Rheb and that the ability to interact with FKBP38 is important for Rheb as an activator of mTOR.

Overexpression of Akt1 upregulates glycogen synthase activity and phosphorylation of mTOR in IRS-1 knockdown HepG2 cells

Insulin receptor substrate (IRS) proteins are important docking proteins in mediating the insulin signaling cascade. We have investigated the effect of short interfering RNA (siRNA) mediated knockdown of IRS-1 on insulin signaling cascade in primary human hepatocellular carcinoma HepG2 cell line and HepG2 cells overexpressing Akt1/PKB-alpha (HepG2-CA-Akt/PKB). IRS-1 knockdown in both cell lines resulted in reduction of insulin stimulated Akt1 phosphorylation at Ser 473. In parental HepG2 cells, IRS-1 knockdown resulted in reduction (ca. 50%) in the basal level of phosphorylated mTOR (Ser 2448) irrespective of insulin treatment. In contrast, HepG2-CA-Akt/PKB cells showed an upregulation in the basal level of phosphorylated mTOR (Ser 2448) (ca. 40%). Insulin mediated phosphorylation of mTOR was reduced. IRS-1 knockdown also reduced the cell proliferation of parental HepG2 cells by ca. 30% in the presence/absence of insulin, whereas in HepG2-CA-Akt/PKB the cell proliferation was reduced by 15% and treatment of insulin further reduced it to ca. 50% (vs. control). IRS-1 knockdown also reduced the glycogen synthase (GS) activity in parental HepG2 cells, however, it was upregulated in HepG2-CA-Akt/PKB cells. These results suggest that knockdown of IRS-1 abolished basal as well as insulin mediated phosphorylation/activity of proteins involved in cell proliferation or glycogen metabolism in the parental Hep2 cells. IRS-1 knockdown in cells overexpressing constitutively active Akt1/PKB-alpha either did not change or upregulated the basal levels of phosphorylated/active proteins. However, insulin mediated response was either not altered or downregulated in these cells.

Cytoplasmic and nuclear distribution of the protein complexes mTORC1 and mTORC2: rapamycin triggers dephosphorylation and delocalization of the mTORC2 components rictor and sin1

The mammalian target of rapamycin (mTOR) is part of two distinct complexes, mTORC1, containing raptor and mLST8, and mTORC2, containing rictor, mLST8 and sin1. Although great endeavors have already been made to elucidate the function and regulation of mTOR, the cytoplasmic nuclear distribution of the mTOR complexes is unknown. Upon establishment of the proper experimental conditions, we found mTOR, mLST8, rictor and sin1 to be less abundant in the nucleus than in the cytoplasm of non-transformed, non-immortalized, diploid human primary fibroblasts. Although raptor is also high abundant in the nucleus, the mTOR/raptor complex is predominantly cytoplasmic, whereas the mTOR/rictor complex is abundant in both compartments. Rapamycin negatively regulates the formation of both mTOR complexes, but the molecular mechanism of its effects on mTORC2 remained elusive. We describe that in primary cells short-term treatment with rapamycin triggers dephosphorylation of rictor and sin1 exclusively in the cytoplasm, but does not affect mTORC2 assembly. Prolonged drug treatment leads to complete dephosphorylation and cytoplasmic translocation of nuclear rictor and sin1 accompanied by inhibition of mTORC2 assembly. The distinct cytoplasmic and nuclear upstream and downstream effectors of mTOR are involved in many cancers and human genetic diseases, such as tuberous sclerosis, Peutz-Jeghers syndrome, von Hippel-Lindau disease, neurofibromatosis type 1, polycystic kidney disease, Alzheimer's disease, cardiac hypertrophy, obesity and diabetes. Accordingly, analogs of rapamycin are currently tested in many different clinical trials. Our data allow new insights into the molecular consequences of mTOR dysregulation under pathophysiological conditions and should help to optimize rapamycin treatment of human diseases.

Control of eIF4E cellular localization by eIF4E-binding proteins, 4E-BPs

Eukaryotic initiation factor (eIF) 4E, the mRNA 5'-cap-binding protein, mediates the association of eIF4F with the mRNA 5'-cap structure to stimulate cap-dependent translation initiation in the cytoplasm. The assembly of eIF4E into the eIF4F complex is negatively regulated through a family of repressor proteins, called the eIF4E-binding proteins (4E-BPs). eIF4E is also present in the nucleus, where it is thought to stimulate nuclear-cytoplasmic transport of certain mRNAs. eIF4E is transported to the nucleus via its interaction with 4E-T (4E-transporter), but it is unclear how it is retained in the nucleus. Here we show that a sizable fraction (approximately 30%) of 4E-BP1 is localized to the nucleus, where it binds eIF4E. In mouse embryo fibroblasts (MEFs) subjected to serum starvation and/or rapamycin treatment, nuclear 4E-BPs sequester eIF4E in the nucleus. A dramatic loss of nuclear 4E-BP1 occurs in c-Ha-Ras-expressing MEFs, which fail to show starvation-induced nuclear accumulation of eIF4E. Therefore, 4E-BP1 is a regulator of eIF4E cellular localization.

Constitutively active Rheb induces oncogenic transformation

Rheb (Ras-homolog enriched in brain) is a component of the phosphatidylinositol 3-kinase (PI3K) target of rapamycin (TOR) signaling pathway, functioning as a positive regulator of TOR. Constitutively active mutants of Rheb induce oncogenic transformation in cell culture. The transformed cells are larger and contain more protein than their normal counterparts. They show constitutive phosphorylation of the ribosomal protein S6 kinase and the eukaryotic initiation factor 4E-binding protein 1, two downstream targets of TOR. The TOR-specific inhibitor rapamycin strongly interferes with transformation induced by constitutively active Rheb, suggesting that TOR activity is essential for the oncogenic effects of mutant Rheb. Rheb-induced transformation is also dependent on a C-terminal farnesylation signal that mediates localization to a cellular membrane. An engineered N-terminal myristylation signal can substitute for the farnesylation. Immunofluorescence localizes wild-type and mutant Rheb to vesicular structures in the cytoplasm, overlapping with the endoplasmic reticulum.

Nutritional control via Tor signaling in Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae senses and responds to nutrients by adapting its growth rate and undergoing morphogenic transitions to ensure survival. The Tor pathway is a major integrator of nutrient-derived signals that in coordination with other signaling pathways orchestrates cell growth. Recent advances have identified novel Tor kinase substrates and established the protein trafficking membranous network and the nucleus as platforms for Tor signaling. These and other recent findings delineate distinct signaling branches emanating from membrane-associated Tor complexes to control cell growth.

Integration of protein kinases mTOR and extracellular signal-regulated kinase 5 in regulating nucleocytoplasmic localization of NFATc4

The target of rapamycin (TOR) signaling regulates the nucleocytoplasmic shuttling of transcription factors in yeast. Whether the mammalian counterpart of TOR (mTOR) also regulates nucleocytoplasmic shuttling is not known. Using a phospho-specific monoclonal antibody, we demonstrate that mTOR phosphorylates Ser(168,170) of endogenous NFATc4, which are conserved gate-keeping Ser residues that control NFAT subcellular distribution. The mTOR acts as a basal kinase during the resting state to maintain NFATc4 in the cytosol. Inactivation and nuclear export of NFATc4 are mediated by rephosphorylation of Ser(168,170), which can be a nuclear event. Kinetic analyses demonstrate that rephosphorylation of Ser(168,170) of endogenous NFATc4 is mediated by mTOR and, surprisingly, by extracellular signal-regulated kinase 5 (ERK5) mitogen-activated protein kinase as well. Ablation of ERK5 in the Erk5(-/-) cells ascertains defects in NFATc4 rephosphorylation and nucleocytoplasmic shuttling. In addition, phosphorylation of NFATc4 by ERK5 primes subsequent phosphorylation mediated by CK1alpha. These results demonstrate that distinct protein kinases are integrated to phosphorylate the gate-keeping residues Ser(168,170) of NFATc4, to regulate subcellular distribution. These data also expand the repertoire of physiological substrates of mTOR and ERK5.

Target of rapamycin and LST8 proteins associate with membranes from the endoplasmic reticulum in the unicellular green alga Chlamydomonas reinhardtii

The highly conserved target of rapamycin (TOR) kinase is a central controller of cell growth in all eukaryotes. TOR exists in two functionally and structurally distinct complexes, termed TOR complex 1 (TORC1) and TORC2. LST8 is a TOR-interacting protein that is present in both TORC1 and TORC2. Here we report the identification and characterization of TOR and LST8 in large protein complexes in the model photosynthetic green alga Chlamydomonas reinhardtii. We demonstrate that Chlamydomonas LST8 is part of a rapamycin-sensitive TOR complex in this green alga. Biochemical fractionation and indirect immunofluorescence microscopy studies indicate that TOR and LST8 exist in high-molecular-mass complexes that associate with microsomal membranes and are particularly abundant in the peri-basal body region in Chlamydomonas cells. A Saccharomyces cerevisiae complementation assay demonstrates that Chlamydomonas LST8 is able to functionally and structurally replace endogenous yeast LST8 and allows us to propose that binding of LST8 to TOR is essential for cell growth.

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.

Hypoxia-inducible factor 1alpha is regulated by the mammalian target of rapamycin (mTOR) via an mTOR signaling motif

Tumors that form as a result of heightened mammalian target of rapamycin (mTOR) signaling are highly vascularized. This process of angiogenesis is regulated through hypoxia-inducible factor (HIF)-mediated transcription of angiogenic factors. It is recognized that inhibition of mTOR with rapamycin can diminish the process of angiogenesis. Our work shows that activation of mTOR by Ras homologue enriched in brain (Rheb) overexpression potently enhances the activity of HIF1alpha and vascular endothelial growth factor (VEGF)-A secretion during hypoxia, which is reversed with rapamycin. Mutants of Rheb, which do not bind guanine nucleotide (D60K, D60V, N119I, and D122N) and are unable to activate mTOR, inhibit the activity of HIF when overexpressed. We show that regulatory associated protein of mTOR (Raptor) interacts with HIF1alpha and requires an mTOR signaling (TOS) motif located in the N terminus of HIF1alpha. Furthermore, a mutant of HIF1alpha lacking this TOS motif dominantly impaired HIF activity during hypoxia and was unable to bind to the co-activator CBP/p300. Rapamycin treatments do not affect the stability of HIF1alpha and modulate HIF activity via a Von Hippel-Lindau (VHL)-independent mechanism. We demonstrate that the high levels of HIF activity in cells devoid of TSC2 can be reversed by treatments with rapamycin or the readdition of TSC2. Our work explains why human cancers with aberrant mTOR signaling are prone to angiogenesis and suggests that inhibition of mTOR with rapamycin might be a suitable therapeutic strategy.

Differential Pi3K-pathway activation in cortical tubers and focal cortical dysplasias with balloon cells

Balloon cells of distinct focal cortical dysplasias type IIb (FCD(IIb)) and giant cells of cortical tubers in tuberous sclerosis (TSC) constitute neuropathological hallmarks and cytological similarities. In TSC, frequent mutations in the TSC1 or TSC2 genes result in mTOR-signaling activity. Here, we addressed whether Pi3K-pathway activation differentiates balloon cells from giant cells. We used immunohistochemistry with antibodies against p-PDK1 (S241), p-Akt (S473), p-tuberin (T1462), p-p70(S6K) (T389), p-p70(S6K) (T229) and phalloidin-staining to analyze stress fiber formation in balloon cells of FCD(IIb) (n = 23) compared with cortical tuber giant cells (n = 5) and adjacent normal CNS tissue as control. We have further established an in vitro assay to assess potential phosphorylation between Akt and S6. We observed phosphorylated (p-)PDK1, p-Akt, p-tuberin, and p-p70-kDa S6-kinase (p-p70(S6K); residue T229) in balloon cells, whereas giant cells showed only equivalent levels of p-tuberin, p-p70(S6K) and stress fibers. Furthermore, Pi3K-cascade activity in balloon cells may reflect pathway "cross-talk". An in vitro assay revealed S6, a major target of p70(S6K), to increase phosphorylation of Akt. Our data suggest recruitment of different Pi3K-cascade factors in the molecular pathogenesis of giant cells in cortical tubers vs. balloon cells in FCD(IIb) and provides new implications for the development of treatment strategies for these cortical malformations.

Nuclear/cytoplasmic localization of Akt activity in the cell cycle

The serine/threonine protein kinase Akt (also known as PKB) is a proto-oncogene and one of the most frequently hyperactivated kinases in human cancer. Its activation downstream of growth-factor-stimulated phosphatidylinositide-3'-OH kinase activity plays a role in the control of cell cycle, cell growth, apoptosis and cell energy metabolism. Akt phosphorylates some thousand downstream substrates, including typical cytoplasmic as well as nuclear proteins. Accordingly, it is not surprising that Akt activity can be found in both, the cytoplasm and the nucleus. Here we report the cell cycle regulation of nuclear and cytoplasmic Akt activity in mammalian cells. These data provide new insights into the regulation of Akt activity and have implications for future studies on the regulation of the wide variety of different nuclear and cytoplasmic Akt substrates.

An activated mTOR/p70S6K signaling pathway in esophageal squamous cell carcinoma cell lines and inhibition of the pathway by rapamycin and siRNA against mTOR

mTOR/p70S6K pathway is considered a central regulator in various malignant tumors, but its roles in esophageal squamous cell carcinoma (ESCC), which is a common cause of mortality in China, remain unknown. Here, we identify that the mTOR/p70S6K pathway is activated in ESCC; rapamycin and siRNA against mTOR rapidly inhibited expression of mTOR and the phosphorylation of its major downstream effectors, p70S6K and 4E-BP1, arrested cells in the G(0)/G(1) phase and induced apoptosis of ESCC cells. The findings may lay a foundation for making further investigations on the mTOR/p70S6K pathway as a potential target for ESCC therapy.

Effect of Sirolimus Treatment on Gene Expression in Renal Transplant Patients

BACKGROUND

Sirolimus (rapamycin), a strong immunosuppressive agent, is administered to renal transplant patients to prevent rejection. The rapamycin signaling pathway [mammalian target of rapamycin (mTOR)] has been implicated in transcriptional regulation.

METHODS

We used high-density oligonucleotide human microarrays to evaluate the effects of sirolimus treatment on gene expression in renal transplant patients. With this technique, we assessed selected genes in the rapamycin signaling, immunosuppression, insulin signaling, and triglyceride metabolism pathways.

RESULTS

Filtered data from both treated and untreated patients showed variability within each group. Significant fold changes were observed in genes from the immunosuppression and insulin signaling pathways but not the rapamycin signaling pathway. The triglyceride metabolism pathway revealed a significant reduction of message levels in lipoprotein and triglyceride synthesis genes.

CONCLUSIONS

These results show that using oligonucleotide microarrays to analyze the effects of sirolimus treatment in patients with renal transplant is an effective way to evaluate gene message levels in multiple pathways.