Mammalian target of rapamycin-dependent phosphorylation of PHAS-I in four (S/T)P sites detected by phospho-specific antibodies

Tumor Necrosis Factor Receptor-2 Signals Clear-Cell Renal Carcinoma Proliferation via Phosphorylated 4E Binding Protein-1 and Mitochondrial Gene Translation

Clear-cell renal cell carcinoma (ccRCC), a tubular epithelial malignancy, secretes tumor necrosis factor (TNF), which signals ccRCC cells in an autocrine manner via two cell surface receptors, TNFR1 and TNFR2, to activate shared and distinct signaling pathways. Selective ligation of TNFR2 drives cell cycle entry of malignant cells via a signaling pathway involving epithelial tyrosine kinase, vascular endothelial cell growth factor receptor type 2, phosphatidylinositol-3-kinase, Akt, pSer727-Stat3, and mammalian target of rapamycin. In this study, phosphorylated 4E binding protein-1 (4EBP1) serine 65 (pSer65-4EBP1) was identified as a downstream target of this TNFR2 signaling pathway. pSer65-4EBP1 expression was significantly elevated relative to total 4EBP1 in ccRCC tissue compared with that in normal kidneys, with signal intensity increasing with malignant grade. Selective ligation of TNFR2 with the TNFR2-specific mutein increased pSer65-4EBP1 expression in organ cultures that co-localized with internalized TNFR2 in mitochondria and increased expression of mitochondrially encoded COX (cytochrome c oxidase subunit) Cox1, as well as nuclear-encoded Cox4/5b subunits. Pharmacologic inhibition of mammalian target of rapamycin reduced both TNFR2-specific mutein-mediated phosphorylation of 4EBP1 and cell cycle activation in tumor cells while increasing cell death. These results signify the importance of pSer65-4EBP1 in mediating TNFR2-driven cell-cycle entry in tumor cells in ccRCC and implicate a novel relationship between the TNFR2/pSer65-4EBP1/COX axis and mitochondrial function.

X-ray Diffraction Analysis to Explore Molecular Traces of Eccentric Contraction on Rat Skeletal Muscle Parallelly Evaluated with Signal Protein Phosphorylation Levels

We performed X-ray diffraction analyses on rat plantaris muscle to determine if there are strain-specific structural changes at the molecular level after eccentric contraction (ECC). ECC was elicited in situ by supramaximal electrical stimulation through the tibial nerve. One hour after a series of ECC sessions, the structural changes that remained in the sarcomere were evaluated using X-ray diffraction. Proteins involved in cell signaling pathways in the muscle were also examined. ECC elicited by 100, 75, and 50 Hz stimulation respectively developed peak tension of 1.34, 1.12 and 0.79 times the isometric maximal tetanus tension. The series of ECC sessions phosphorylated the forkhead box O proteins (FoxO) in a tension-time integral-dependent manner, as well as phosphorylated the mitogen-activated protein kinases (MAPK) and a protein in the mammalian target of rapamycin (mTOR) pathway in a maximal tension dependent manner. Compared to isometric contractions, ECC was more efficient in phosphorylating the signaling proteins. X-ray diffraction revealed that the myofilament lattice was preserved even after intense ECC stimulation at 100 Hz. Additionally, ECC < 75 Hz preserved the molecular alignment of myoproteins along the myofilaments, while 75-Hz stimulation induced a slight but significant decrease in the intensity of meridional troponin reflection at 1/38 nm⁻¹, and of myosin reflection at 1/14.4 nm⁻¹. These two reflections demonstrated no appreciable decrease with triple repetitions of the standard series of ECC sessions at 50 Hz, suggesting that the intensity decrease depended on the instantaneous maximal tension development rather than the total load of contraction, and was more likely linked with the phosphorylation of MAPK and mTOR signaling proteins.

mTORC1 promotes TOP mRNA translation through site-specific phosphorylation of LARP1

LARP1 is a key repressor of TOP mRNA translation. It binds the m7Gppp cap moiety and the adjacent 5'TOP motif of TOP mRNAs, thus impeding the assembly of the eIF4F complex on these transcripts. mTORC1 controls TOP mRNA translation via LARP1, but the details of the mechanism are unclear. Herein we elucidate the mechanism by which mTORC1 controls LARP1's translation repression activity. We demonstrate that mTORC1 phosphorylates LARP1 in vitro and in vivo, activities that are efficiently inhibited by rapamycin and torin1. We uncover 26 rapamycin-sensitive phospho-serine and -threonine residues on LARP1 that are distributed in 7 clusters. Our data show that phosphorylation of a cluster of residues located proximally to the m7Gppp cap-binding DM15 region is particularly sensitive to rapamycin and regulates both the RNA-binding and the translation inhibitory activities of LARP1. Our results unravel a new model of translation control in which the La module (LaMod) and DM15 region of LARP1, both of which can directly interact with TOP mRNA, are differentially regulated: the LaMod remains constitutively bound to PABP (irrespective of the activation status of mTORC1), while the C-terminal DM15 'pendular hook' engages the TOP mRNA 5'-end to repress translation, but only in conditions of mTORC1 inhibition.

Differential responses to kinase inhibition in FGFR2-addicted triple negative breast cancer cells: a quantitative phosphoproteomics study

Fibroblast Growth Factor (FGF) dependent signalling is frequently activated in cancer by a variety of different mechanisms. However, the downstream signal transduction pathways involved are poorly characterised. Here a quantitative differential phosphoproteomics approach, SILAC, is applied to identify FGF-regulated phosphorylation events in two triple- negative breast tumour cell lines, MFM223 and SUM52, that exhibit amplified expression of FGF receptor 2 (FGFR2) and are dependent on continued FGFR2 signalling for cell viability. Comparative Gene Ontology proteome analysis revealed that SUM52 cells were enriched in proteins associated with cell metabolism and MFM223 cells enriched in proteins associated with cell adhesion and migration. FGFR2 inhibition by SU5402 impacts a significant fraction of the observed phosphoproteome of these cells. This study expands the known landscape of FGF signalling and identifies many new targets for functional investigation. FGF signalling pathways are found to be flexible in architecture as both shared, and divergent, responses to inhibition of FGFR2 kinase activity in the canonical RAF/MAPK/ERK/RSK and PI3K/AKT/PDK/mTOR/S6K pathways are identified. Inhibition of phosphorylation-dependent negative-feedback pathways is observed, defining mechanisms of intrinsic resistance to FGFR2 inhibition. These findings have implications for the therapeutic application of FGFR inhibitors as they identify both common and divergent responses in cells harbouring the same genetic lesion and pathways of drug resistance.

LARP1 on TOP of ribosome production

The ribosome is an essential unit of all living organisms that commands protein synthesis, ultimately fuelling cell growth (accumulation of cell mass) and cell proliferation (increase in cell number). The eukaryotic ribosome consists of 4 ribosomal RNAs (rRNAs) and 80 ribosomal proteins (RPs). Despite its fundamental role in every living organism, our present understanding of how higher eukaryotes produce the various ribosome components is incomplete. Uncovering the mechanisms utilized by human cells to generate functional ribosomes will likely have far-reaching implications in human disease. Recent biochemical and structural studies revealed La-related protein 1 (LARP1) as a key new player in RP production. LARP1 is an RNA-binding protein that belongs to the LARP superfamily; it controls the translation and stability of the mRNAs that encode RPs and translation factors, which are characterized by a 5' terminal oligopyrimidine (5'TOP) motif and are thus known as TOP mRNAs. The activity of LARP1 is regulated by the mammalian target of rapamycin complex 1 (mTORC1): a eukaryotic protein kinase complex that integrates nutrient sensing with mRNA translation, particularly that of TOP mRNAs. In this review, we provide an overview of the role of LARP1 in the control of ribosome production in multicellular eukaryotes. This article is categorized under: Translation > Translation Regulation RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications RNA Processing > Capping and 5' End Modifications.

Activation of mTORC1 by leucine is potentiated by branched-chain amino acids and even more so by essential amino acids following resistance exercise

Protein synthesis is stimulated by resistance exercise and intake of amino acids, in particular leucine. Moreover, activation of mammalian target of rapamycin complex 1 (mTORC1) signaling by leucine is potentiated by the presence of other essential amino acids (EAA). However, the contribution of the branched-chain amino acids (BCAA) to this effect is yet unknown. Here we compare the stimulatory role of leucine, BCAA, and EAA ingestion on anabolic signaling following exercise. Accordingly, eight trained volunteers completed four sessions of resistance exercise during which they ingested either placebo, leucine, BCAA, or EAA (including the BCAA) in random order. Muscle biopsies were taken at rest, immediately after exercise, and following 90 and 180 min of recovery. Following 90 min of recovery the activity of S6 kinase 1 (S6K1) was greater than at rest in all four trials (Placebo<Leucine<BCAA<EAA; P < 0.05 time × supplement), with a ninefold increase in the EAA trial. At this same time point, phosphorylation of eukaryotic translation initiation factor 4E (eIF4E)-binding protein 1 (4E-BP1) at Thr(37/46) was unaffected by supplementation, while that of Thr(46) alone exhibited a pattern similar to that of S6K1, being 18% higher with EAA than BCAA. However, after 180 min of recovery this difference between EAA and BCAA had disappeared, although with both these supplements the increases were still higher than with leucine (40%, P < 0.05) and placebo (100%, P < 0.05). In summary, EAA ingestion appears to stimulate translation initiation more effectively than the other supplements, although the results also suggest that this effect is primarily attributable to the BCAA.

Exploring the phosphoproteome profiles during Xenopus egg activation by calcium stimulation using a fully automated phosphopeptide purification system

To explore the phosphoproteome profiles duringXenopusegg activation by Ca(2+)-stimulation, an automated phosphopeptide purification system involving a titania column was improved by introducing 4-step elution with phosphate buffers. The number of detected phosphopeptides in the tryptic digest of aXenopusegg cytosol fraction on mass spectrometry (MS) was increased 1.5-fold and the percentage of multiply phosphorylated peptides increased from 17 to 24% with introduction of the 4-step elution method. Phosphopeptides were purified by the improved method from tryptic digests of cytosol fractions ofXenopuseggs without and with a Ca(2+)-stimulus, and then, analysed by MS. One thousand three hundred and seventy-five and 994 phosphopeptides were reproducibly detected on duplicate MS, respectively. They included 818 and 437 phosphopeptides specific to each digest, respectively. A method involving isobaric tags for relative and absolute quantitation (iTRAQ) was also applied to compare the phosphorylation levels inXenopuseggs without and with a Ca(2+)-stimulus, the ratios for 112 phosphopeptides in tryptic digests of these egg cytosol fractions being obtained. It was suggested from all the results that the phosphorylation sites and levels change duringXenopusegg activation for many known and unknown sites on structural proteins, signalling related proteins, cell cycle-related proteins and others.

Morphological and molecular changes in the murine placenta exposed to normobaric hypoxia throughout pregnancy

Key points

Exposure of pregnant mice to chronic hypoxia at 13% O₂ induces fetal growth restriction but increases placental weight.

Sex dimorphism induces differential responses in placental weight to hypoxia. The male placenta is heavier than the female and is associated with less severe fetal growth restriction.

Increases in maternal arterial/venous blood spaces and higher protein kinase B (Akt)‐mechanistic target of rapamycin growth signalling could contribute to the heavier hypoxic placenta.

Placental endoplasmic reticulum stress is elevated equally in both sexes in response to hypoxia. In comparison, oxidative stress is only apparent in female placentas.

Chronic hypoxia induces down‐regulation of placental mitochondrial electron transport chain complexes protein subunits but does not cause intracellular energy depletion.

Abstract

Chronic hypoxia is a common complication of pregnancy, arising through malperfusion of the placenta or pregnancy at high altitude. The present study investigated the effects of hypoxia on the growth of the placenta, which is the organ that interfaces between the mother and her fetus. Mice were housed in an hypoxic environment for the whole of gestation. An atmosphere of 13% oxygen induced fetal growth restriction (1182 ± 9 mg, n = 90 vs. 1044 ± 11 mg, n = 62, P < 0.05) but enhanced placental weight (907 ± 11 mg, n = 90 vs. 998 ± 15 mg, n = 62,P < 0.05). Stereological analyses revealed an increase in the volume of maternal blood spaces in the placenta, consistent with increased flow. At the molecular level, we observed activation of the protein kinase B (Akt)‐mechanistic target of rapamycin growth and proliferation pathway. Chronic hypoxia also triggered mild endoplasmic reticulum stress, a conserved homeostatic response that mediates translational arrest through phosphorylation of eukaryotic initiation factor 2 subunit α. Surprisingly, although subunits of the mitochondrial electron transport chain complexes were reduced at the protein level, there was no evidence of intracellular energy depletion. Finally, we demonstrated sex‐specific placental responses to chronic hypoxia. Placentas from male fetuses were heavier (1082 ± 2 mg, n = 30 vs. 928 ± 2 mg, n = 34, P < 0.05) and less susceptible to hypoxia‐induced oxidative stress than those from females. Their capacity to adapt may explain why male fetuses were significantly less growth restricted at embryonic day 18.5 than their female counterparts. These findings are consistent with the concept that male fetuses are more aggressive with respect to their nutrient demands, which may place them at greater risk of adverse outcomes under limiting conditions.

Exposure of pregnant mice to chronic hypoxia at 13% O₂ induces fetal growth restriction but increases placental weight.

Sex dimorphism induces differential responses in placental weight to hypoxia. The male placenta is heavier than the female and is associated with less severe fetal growth restriction.

Increases in maternal arterial/venous blood spaces and higher protein kinase B (Akt)‐mechanistic target of rapamycin growth signalling could contribute to the heavier hypoxic placenta.

Placental endoplasmic reticulum stress is elevated equally in both sexes in response to hypoxia. In comparison, oxidative stress is only apparent in female placentas.

Chronic hypoxia induces down‐regulation of placental mitochondrial electron transport chain complexes protein subunits but does not cause intracellular energy depletion.

Computational analysis of an autophagy/translation switch based on mutual inhibition of MTORC1 and ULK1

We constructed a mechanistic, computational model for regulation of (macro)autophagy and protein synthesis (at the level of translation). The model was formulated to study the system-level consequences of interactions among the following proteins: two key components of MTOR complex 1 (MTORC1), namely the protein kinase MTOR (mechanistic target of rapamycin) and the scaffold protein RPTOR; the autophagy-initiating protein kinase ULK1; and the multimeric energy-sensing AMP-activated protein kinase (AMPK). Inputs of the model include intrinsic AMPK kinase activity, which is taken as an adjustable surrogate parameter for cellular energy level or AMP:ATP ratio, and rapamycin dose, which controls MTORC1 activity. Outputs of the model include the phosphorylation level of the translational repressor EIF4EBP1, a substrate of MTORC1, and the phosphorylation level of AMBRA1 (activating molecule in BECN1-regulated autophagy), a substrate of ULK1 critical for autophagosome formation. The model incorporates reciprocal regulation of mTORC1 and ULK1 by AMPK, mutual inhibition of MTORC1 and ULK1, and ULK1-mediated negative feedback regulation of AMPK. Through analysis of the model, we find that these processes may be responsible, depending on conditions, for graded responses to stress inputs, for bistable switching between autophagy and protein synthesis, or relaxation oscillations, comprising alternating periods of autophagy and protein synthesis. A sensitivity analysis indicates that the prediction of oscillatory behavior is robust to changes of the parameter values of the model. The model provides testable predictions about the behavior of the AMPK-MTORC1-ULK1 network, which plays a central role in maintaining cellular energy and nutrient homeostasis.

Alcohol intoxication following muscle contraction in mice decreases muscle protein synthesis but not mTOR signal transduction

BACKGROUND

Alcohol (ethanol [EtOH]) intoxication antagonizes stimulation of muscle protein synthesis and mammalian target of rapamycin (mTOR) signaling. However, whether the anabolic response can be reversed when alcohol is consumed after the stimulus is unknown.

METHODS

A single bout of electrically stimulated muscle contractions (10 sets of 6 contractions) was induced in fasted male C57BL/6 mice 2 hours prior to alcohol intoxication. EtOH was injected intraperitoneally (3 g/kg), and the gastrocnemius/plantaris muscle complex was collected 2 hours later from the stimulated and contralateral unstimulated control leg.

RESULTS

Muscle contraction increased protein synthesis 28% in control mice, while EtOH abolished this stimulation-induced increase. Further, EtOH suppressed the rate of synthesis ~75% compared to control muscle irrespective of stimulation. This decrease was associated with impaired protein elongation as EtOH increased the phosphorylation of eEF2 Thr(56) . In contrast, stimulation-induced increases in mTOR protein complex-1 (mTORC1) (S6K1 Thr(421) /Ser(424) , S6K1 Thr(389) , rpS6 Ser(240/244) , and 4E-BP1 Thr(37/46) ) and mitogen-activated protein kinase (MAPK) (JNK Thr(183) /Tyr(185) , p38 Thr(180) /Tyr(182) , and rpS6S(235/236) ) signaling were not reversed by acute EtOH.

CONCLUSIONS

These data suggest that EtOH-induced decreases in protein synthesis in fasted mice may be independent of mTORC1 and MAPK signaling following muscle contraction and instead due to the antagonistic actions of EtOH on mRNA translation elongation. Therefore, EtOH suppresses the contraction-induced increase in protein synthesis, and over time has the potential to prevent skeletal muscle hypertrophy induced by repeated muscle contraction.

Signaling crosstalk between the mTOR complexes

mTOR is a protein kinase which integrates a variety of environmental and intracellular stimuli to positively regulate many anabolic processes of the cell, including protein synthesis. It exists within two highly conserved multi-protein complexes known as mTORC1 and 2 mTORC2. Each of these complexes phosphorylates different downstream targets, and play roles in different cellular functions. They also show distinctive sensitivity to the mTOR inhibitor rapamycin. Nevertheless, despite their biochemical and functional differences, recent studies have suggested that the regulation of these complexes is tightly linked to each other. For instance, both mTORC1 and 2 share some common upstream signaling molecules, such as PI3K and tuberous sclerosis complex TSC, which control their activation. Stimulation of the mTOR complexes may also trigger both positive and negative feedback mechanisms, which then in turn either further enhance or suppress their activation. Here, we summarize some recently discovered features relating to the crosstalk between mTORC1 and 2. We then discuss how aberrant mTOR complex crosstalk mechanisms may have an impact on the development of human diseases and drug resistance.

Lysophosphatidic acid acyltransferase beta regulates mTOR signaling

Lysophosphatidic acid acyltransferase (LPAAT-β) is a phosphatidic acid (PA) generating enzyme that plays an essential role in triglyceride synthesis. However, LPAAT-β is now being studied as an important regulator of cell growth and differentiation and as a potential therapeutic target in cancer since PA is necessary for the activity of key proteins such as Raf, PKC-ζ and mTOR. In this report we determine the effect of LPAAT-β silencing with siRNA in pancreatic adenocarcinoma cell lines. We show for the first time that LPAAT-β knockdown inhibits proliferation and anchorage-independent growth of pancreatic cancer cells. This is associated with inhibition of signaling by mTOR as determined by levels of mTORC1- and mTORC2-specific phosphorylation sites on 4E-BP1, S6K and Akt. Since PA regulates the activity of mTOR by modulating its binding to FKBP38, we explored the possibility that LPAAT-β might regulate mTOR by affecting its association with FKBP38. Coimmunoprecipitation studies of FKBP38 with mTOR show increased levels of FKBP38 associated with mTOR when LPAAT-β protein levels are knocked down. Furthermore, depletion of LPAAT-β results in increased Lipin 1 nuclear localization which is associated with increased nuclear eccentricity, a nuclear shape change that is dependent on mTOR, further confirming the ability of LPAAT-β to regulate mTOR function. Our results provide support for the hypothesis that PA generated by LPAAT-β regulates mTOR signaling. We discuss the implications of these findings for using LPAAT-β as a therapeutic target.

Activation of mammalian target of rapamycin contributes to pain nociception induced in rats by BmK I, a sodium channel-specific modulator

The mammalian target of rapamycin (mTOR) pathway is essential for maintenance of the sensitivity of certain adult sensory neurons. Here, we investigated whether the mTOR cascade is involved in scorpion envenomation-induced pain hypersensitivity in rats. The results showed that intraplantar injection of a neurotoxin from Buthus martensii Karsch, BmK I (10 μg), induced the activation of mTOR, as well as its downstream molecules p70 ribosomal S6 protein kinase (p70 S6K) and eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), in lumbar 5-6 dorsal root ganglia neurons on both sides in rats. The activation peaked at 2 h and recovered 1 day after injection. Compared with the control group, the ratios of p-mTOR/p-p70 S6K/p-4EBP1 in three types of neurons changed significantly. The cell typology of p-mTOR/p-p70 S6K/p-4E-BP1 immuno-reactive neurons also changed. Intrathecal administration of deforolimus, a specific inhibitor of mTOR, attenuated BmK I-induced pain responses (spontaneous flinching, paroxysmal pain-like behavior, and mechanical hypersensitivity). Together, these results imply that the mTOR signaling pathway is mobilized by and contributes to experimental scorpion sting-induced pain.

Activation of mammalian target of rapamycin mediates rat pain-related responses induced by BmK I, a sodium channel-specific modulator

The mammalian target of rapamycin (mTOR) is known to regulate cell proliferation and growth by controlling protein translation. Recently, it has been shown that mTOR signaling pathway is involved in long-term synaptic plasticity. However, the role of mTOR under different pain conditions is less clear. In this study, the spatiotemporal activation of mTOR that contributes to pain-related behaviors was investigated using a novel animal inflammatory pain model induced by BmK I, a sodium channel-specific modulator purified from scorpion venom. In this study, intraplantar injections of BmK I were found to induce the activation of mTOR, p70 ribosomal S6 protein kinase (p70 S6K) and eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) in rat L5-L6 spinal neurons. In the spinal cord, mTOR, p70 S6K and 4E-BP1 were observed to be activated in the ipsilateral and contralateral regions, peaking at 1-2 h and recovery at 24 h post-intraplantar (i.pl.) BmK I administration. In addition, intrathecal (i.t.) injection of rapamycin - a specific inhibitor of mTOR - was observed to result in the reduction of spontaneous pain responses and the attenuation of unilateral thermal and bilateral mechanical hypersensitivity elicited by BmK I. Thus, these results indicate that the mTOR signaling pathway is mobilized in the induction and maintenance of pain-activated hypersensitivity.

Threonine affects intestinal function, protein synthesis and gene expression of TOR in Jian carp (Cyprinus carpio var. Jian)

This study aimed to investigate the effects of threonine (Thr) on the digestive and absorptive ability, proliferation and differentiation of enterocytes, and gene expression of juvenile Jian carp (Cyprinus carpio var. Jian). First, seven isonitrogenous diets containing graded levels of Thr (7.4-25.2 g/kg diet) were fed to the fishes for 60 days. Second, enterocyte proliferation and differentiation were assayed by culturing enterocytes with graded levels of Thr (0-275 mg/l) in vitro. Finally, enterocytes were cultured with 0 and 205 mg/l Thr to determine protein synthesis. The percent weight gain (PWG), specific growth rate, feed intake, feed efficiency, protein retention value, activities of trypsin, lipase and amylase, weights and protein contents of hepatopancreas and intestine, folds heights, activities of alkaline phosphatase (AKP), γ- glutamyl transpeptidase and Na(+)/K(+)-ATPase in all intestinal segments, glutamate-oxaloacetate transaminase (GOT) and glutamate-pyruvate transaminase (GPT) activities in hepatopancreas, and 4E-BP2 gene expression in muscle, hepatopancreas and intestinal segments were significantly enhanced by Thr (p<0.05). However, the plasma ammonia concentration and TOR gene expression decreased (p<0.05). In vitro, Thr supplement significantly increased cell numbers, protein content, the activities of GOT, GPT, AKP and Na(+)/K(+)-ATPase, and protein synthesis rate of enterocytes, and decreased LDH activity and ammonia content in cell medium (p<0.05). In conclusion, Thr improved growth, digestive and absorptive capacity, enterocyte proliferation and differentiation, and protein synthesis and regulated TOR and 4E-BP2 gene expression in juvenile Jian carp. The dietary Thr requirement of juvenile Jian carp was 16.25 g/kg diet (51.3 g/kg protein) based on quadratic regression analysis of PWG.

Mechanical stimulation induces mTOR signaling via an ERK-independent mechanism: implications for a direct activation of mTOR by phosphatidic acid

Signaling by mTOR is a well-recognized component of the pathway through which mechanical signals regulate protein synthesis and muscle mass. However, the mechanisms involved in the mechanical regulation of mTOR signaling have not been defined. Nevertheless, recent studies suggest that a mechanically-induced increase in phosphatidic acid (PA) may be involved. There is also evidence which suggests that mechanical stimuli, and PA, utilize ERK to induce mTOR signaling. Hence, we reasoned that a mechanically-induced increase in PA might promote mTOR signaling via an ERK-dependent mechanism. To test this, we subjected mouse skeletal muscles to mechanical stimulation in the presence or absence of a MEK/ERK inhibitor, and then measured several commonly used markers of mTOR signaling. Transgenic mice expressing a rapamycin-resistant mutant of mTOR were also used to confirm the validity of these markers. The results demonstrated that mechanically-induced increases in p70(s6k) T389 and 4E-BP1 S64 phosphorylation, and unexpectedly, a loss in total 4E-BP1, were fully mTOR-dependent signaling events. Furthermore, we determined that mechanical stimulation induced these mTOR-dependent events, and protein synthesis, through an ERK-independent mechanism. Similar to mechanical stimulation, exogenous PA also induced mTOR-dependent signaling via an ERK-independent mechanism. Moreover, PA was able to directly activate mTOR signaling in vitro. Combined, these results demonstrate that mechanical stimulation induces mTOR signaling, and protein synthesis, via an ERK-independent mechanism that potentially involves a direct interaction of PA with mTOR. Furthermore, it appears that a decrease in total 4E-BP1 may be part of the mTOR-dependent mechanism through which mechanical stimuli activate protein synthesis.

Global quantitative phosphoproteome analysis of human tumor xenografts treated with a CD44 antagonist

The cell surface glycoprotein CD44 plays an important role in the development and progression of various tumor types. RG7356 is a humanized antibody targeting the constant region of CD44 that shows antitumor efficacy in mice implanted with CD44-expressing tumors such as MDA-MB-231 breast cancer cells. CD44 receptor seems to function as the main receptor for hyaluronic acid and osteopontin, serving as coreceptor for growth factor pathways like cMet, EGFR, HER-2, and VEGFR and by cytoskeletal modulation via ERM and Rho kinase signaling. To assess the direct impact of RG7356 binding to the CD44 receptor, a global mass spectrometry-based phosphoproteomics approach was applied to freshly isolated MDA-MB-231 tumor xenografts. Results from a global phosphoproteomics screen were further corroborated by Western blot and ELISA analyses of tumor lysates from CD44-expressing tumors. Short-term treatment of tumor-bearing mice with RG7356 resulted in modifications of the MAPK pathway in the responsive model, although no effects on downstream phosphorylation were observed in a nonresponsive xenograft model. Taken together, our approach augments the value of other high throughput techniques to identify biomarkers for clinical development of targeted agents.

Analysis of the pharmacodynamic activity of the mTOR inhibitor ridaforolimus (AP23573, MK-8669) in a phase 1 clinical trial

PURPOSE

As part of a phase 1 dose-escalation trial, the pharmacodynamic activity of the mammalian target of rapamycin (mTOR) inhibitor ridaforolimus was assessed in multiple tissues by measuring levels of phosphorylated 4E binding protein-1 (p-4E-BP1) or S6, two downstream markers of mTOR activity.

METHODS

32 patients (pts) were dosed intravenously with ridaforolimus once daily for 5 consecutive days (QD × 5) every 2 weeks. The pharmacodynamic activity of ridaforolimus was assessed in peripheral blood mononuclear cells (PBMCs; 32 pts), skin (28 pts), and tumor specimens (3 pts) collected before and after dosing by measuring levels of p-4E-BP1 by immunoblot analysis or pS6 by immunohistochemistry. Levels of these markers were assessed in up to 19, 5, and 2 pre- and post-dose time points in PBMC, skin, and tumor specimens, respectively.

RESULTS

In preclinical models, ridaforolimus induced a dose-dependent inhibition of p-4E-BP1 in PBMCs that was associated with antitumor activity. Rapid and potent inhibition of mTOR was observed in PBMCs from all 32 pts dosed, with a median level of inhibition of 96% observed within 1 h after the first dose. Inhibition of mTOR (>90%) was sustained during the entire QD × 5 dosing period, and substantial inhibition was still observed after the 9-day holiday between dosing courses. Evidence of mTOR inhibition was also obtained in skin in pts from all dose cohorts, although it did not persist through the break between courses. After two to three doses of ridaforolimus, inhibition of mTOR was detected in the tumor from one of three pts analyzed.

CONCLUSIONS

Ridaforolimus was shown to inhibit its intended target, mTOR, in PBMCs, skin, and tumors. In PBMCs and skin, inhibition was observed at all dose levels tested, thus supporting but not driving the selection of a recommended phase 2 dose.

Leucine and arginine regulate trophoblast motility through mTOR-dependent and independent pathways in the preimplantation mouse embryo

Uterine implantation is a critical element of mammalian reproduction and is a tightly and highly coordinated event. An intricate and reciprocal uterine-embryo dialog exists to synchronize uterine receptivity with the concomitant activation of the blastocyst, maximizing implantation success. While a number of pathways involved in regulating uterine receptivity have been identified in the mouse, less is understood about blastocyst activation, the process by which the trophectoderm (TE) receives extrinsic cues that initiate new characteristics essential for implantation. Amino acids (AA) have been found to regulate blastocyst activation and TE motility in vitro. In particular, we find that arginine and leucine alone are necessary and sufficient to induce TE motility. Both arginine and leucine act individually and additively to propagate signals that are dependent on the activity of the mammalian target of rapamycin complex 1 (mTORC1). The activities of the well-established downstream targets of mTORC1, p70S6K and 4EBP, do not correlate with trophoblast motility, suggesting that an independent-rapamycin-sensitive pathway operates to induce trophoblast motility, or that other, parallel amino acid-dependent pathways are also involved. We find that endogenous uterine factors act to induce mTORC1 activation and trophoblast motility at a specific time during pregnancy, and that this uterine signal is later than the previously defined signal that induces the attachment reaction. In vivo matured blastocysts exhibit competence to respond to an 8-hour AA stimulus by activating mTOR and subsequently undergoing trophoblast outgrowth by the morning of day 4.5 of pregnancy, but not on day 3.5. By the late afternoon of day 4.5, the embryos no longer require any exposure to AA to undergo trophoblast outgrowth in vitro, demonstrating the existence and timing of an equivalent in vivo signal. These results suggest that there are two separate uterine signals regulating implantation, one that primes the embryo for the attachment reaction and another that activates mTOR and initiates invasive behavior.

mTOR complex 1 regulates lipin 1 localization to control the SREBP pathway

The nutrient- and growth factor-responsive kinase mTOR complex 1 (mTORC1) regulates many processes that control growth, including protein synthesis, autophagy, and lipogenesis. Through unknown mechanisms, mTORC1 promotes the function of SREBP, a master regulator of lipo- and sterolgenic gene transcription. Here, we demonstrate that mTORC1 regulates SREBP by controlling the nuclear entry of lipin 1, a phosphatidic acid phosphatase. Dephosphorylated, nuclear, catalytically active lipin 1 promotes nuclear remodeling and mediates the effects of mTORC1 on SREBP target gene, SREBP promoter activity, and nuclear SREBP protein abundance. Inhibition of mTORC1 in the liver significantly impairs SREBP function and makes mice resistant, in a lipin 1-dependent fashion, to the hepatic steatosis and hypercholesterolemia induced by a high-fat and -cholesterol diet. These findings establish lipin 1 as a key component of the mTORC1-SREBP pathway.

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.

Pushing the envelope in the mTOR pathway: the second generation of inhibitors

The phosphatidylinositol-3-kinase (PI3K)/mTOR pathway has been a major focus of attention for cancer researchers in the past decade. A preliminary and incomplete understanding of the molecular biology of this complex network has importantly conditioned not only the development of the first generation of mTOR inhibitors, but also the biomarker studies designed to identify the best responders to these agents. Most recently, research in this pathway has focused on the dual nature of mTOR that is integrated by the mTOR complex 1 and complex 2. These two complexes are formed and regulated by different proteins and are also driven by multiple different compensatory feedback loops. This deeper understanding has allowed the development of a promising second generation of inhibitors, which are able to block simultaneously both complexes due to their catalytic activity over mTOR. Moreover, some of them also exert an inhibitory effect over PI3K that is a key player in the feedback loops. This article reviews the newest insights in the signaling of the mTOR pathway and then focuses on the development of the new wave of mTOR inhibitors.

Mutational analysis of the EMCV 2A protein identifies a nuclear localization signal and an eIF4E binding site

Cardioviruses have a unique 2A protein (143 aa). During genome translation, the encephalomyocarditis virus (EMCV) 2A is released through a ribosome skipping event mitigated through C-terminal 2A sequences and by subsequent N-terminal reaction with viral 3C(pro). Although viral replication is cytoplasmic, mature 2A accumulates in nucleoli shortly after infection. Some protein also transiently associates with cytoplasmic 40S ribosomal subunits, an activity contributing to inhibition of cellular cap-dependent translation. Cardiovirus sequences predict an eIF4E binding site (aa 126-134) and a nuclear localization signal (NLS, aa 91-102), within 2A, both of which are functional during EMCV infection. Point mutations preventing eIF4E:2A interactions gave small-plaque phenotype viruses, but still inhibited cellular cap-dependent translation. Deletions within the NLS motif relocalized 2A to the cytoplasm and abrogated the inhibition of cap-dependent translation. A fusion protein linking the 2A NLS to eGFP was sufficient to redirect the reporter to the nucleus but not into nucleoli.

Rapamycin inhibits cytoskeleton reorganization and cell motility by suppressing RhoA expression and activity

The mammalian target of rapamycin (mTOR) functions in cells at least as two complexes, mTORC1 and mTORC2. Intensive studies have focused on the roles of mTOR in the regulation of cell proliferation, growth, and survival. Recently we found that rapamycin inhibits type I insulin-like growth factor (IGF-1)-stimulated lamellipodia formation and cell motility, indicating involvement of mTOR in regulating cell motility. This study was set to further elucidate the underlying mechanism. Here we show that rapamycin inhibited protein synthesis and activities of small GTPases (RhoA, Cdc42, and Rac1), crucial regulatory proteins for cell migration. Disruption of mTORC1 or mTORC2 by down-regulation of raptor or rictor, respectively, inhibited the activities of these proteins. However, only disruption of mTORC1 mimicked the effect of rapamycin, inhibiting their protein expression. Ectopic expression of rapamycin-resistant and constitutively active S6K1 partially prevented rapamycin inhibition of RhoA, Rac1, and Cdc42 expression, whereas expression of constitutively hypophosphorylated 4E-BP1 (4EBP1-5A) or down-regulation of S6K1 by RNA interference suppressed expression of the GTPases, suggesting that both mTORC1-mediated S6K1 and 4E-BP1 pathways are involved in protein synthesis of the GTPases. Expression of constitutively active RhoA, but not Cdc42 and Rac1, conferred resistance to rapamycin inhibition of IGF-1-stimulated lamellipodia formation and cell migration. The results suggest that rapamycin inhibits cell motility at least in part by down-regulation of RhoA protein expression and activity through mTORC1-mediated S6K1 and 4E-BP1-signaling pathways.

New hierarchical phosphorylation pathway of the translational repressor eIF4E-binding protein 1 (4E-BP1) in ischemia-reperfusion stress

Eukaryotic initiation factor (eIF) 4E-binding protein 1 (4E-BP1) is a translational repressor that is characterized by its capacity to bind specifically to eIF4E and inhibit its interaction with eIF4G. Phosphorylation of 4E-BP1 regulates eIF4E availability, and therefore, cap-dependent translation, in cell stress. This study reports a physiological study of 4E-BP1 regulation by phosphorylation using control conditions and a stress-induced translational repression condition, ischemia-reperfusion (IR) stress, in brain tissue. In control conditions, 4E-BP1 was found in four phosphorylation states that were detected by two-dimensional gel electrophoresis and Western blotting, which corresponded to Thr⁶⁹-phosphorylated alone, Thr⁶⁹- and Thr³⁶/Thr⁴⁵-phosphorylated, all these plus Ser⁶⁴ phosphorylation, and dephosphorylation of the sites analyzed. In control or IR conditions, no Thr³⁶/Thr⁴⁵ phosphorylation alone was detected without Thr⁶⁹ phosphorylation, and neither was Ser⁶⁴ phosphorylation without Thr³⁶/Thr⁴⁵/Thr⁶⁹ phosphorylation detected. Ischemic stress induced 4E-BP1 dephosphorylation at Thr⁶⁹, Thr³⁶/Thr⁴⁵, and Ser⁶⁴ residues, with 4E-BP1 remaining phosphorylated at Thr⁶⁹ alone or dephosphorylated. In the subsequent reperfusion, 4E-BP1 phosphorylation was induced at Thr³⁶/Thr⁴⁵ and Ser⁶⁴, in addition to Thr⁶⁹. Changes in 4E-BP1 phosphorylation after IR were according to those found for Akt and mammalian target of rapamycin (mTOR) kinases. These results demonstrate a new hierarchical phosphorylation for 4E-BP1 regulation in which Thr⁶⁹ is phosphorylated first followed by Thr³⁶/Thr⁴⁵ phosphorylation, and Ser⁶⁴ is phosphorylated last. Thr⁶⁹ phosphorylation alone allows binding to eIF4E, and subsequent Thr³⁶/Thr⁴⁵ phosphorylation was sufficient to dissociate 4E-BP1 from eIF4E, which led to eIF4E-4G interaction. These data help to elucidate the physiological role of 4E-BP1 phosphorylation in controlling protein synthesis.

Targeting TORC2 in multiple myeloma with a new mTOR kinase inhibitor

Although preclinical work with rapalogs suggests potential in treatment of multiple myeloma (MM), they have been less successful clinically. These drugs allostearically inhibit the mammalian target of rapamycin kinase primarily curtailing activity of the target of rapamycin complex (TORC)1. To assess if the mammalian target of rapamycin within the TORC2 complex could be a better target in MM, we tested a new agent, pp242, which prevents activation of TORC2 as well as TORC1. Although comparable to rapamycin against phosphorylation of the TORC1 substrates p70S6kinase and 4E-BP-1, pp242 could also inhibit phosphorylation of AKT on serine 473, a TORC2 substrate, while rapamycin was ineffective. pp242 was also more effective than rapamycin in achieving cytoreduction and apoptosis in MM cells. In addition, pp242 was an effective agent against primary MM cells in vitro and growth of 8226 cells in mice. Knockdown of the TORC2 complex protein, rictor, was deleterious to MM cells further supporting TORC2 as the critical target for pp242. TORC2 activation was frequently identified in primary specimens by immunostaining for AKT phosphorylation on serine 473. Potential mechanisms of up-regulated TORC2 activity in MM were stimulation with interleukin-6 or insulin-like growth factor 1, and phosphatase and tensin homolog or RAS alterations. Combining pp242 with bortezomib led to synergistic anti-MM effects. These results support TORC2 as a therapeutic target in MM.

mTORC1 directly phosphorylates and regulates human MAF1

mTORC1 is a central regulator of growth in response to nutrient availability, but few direct targets have been identified. RNA polymerase (pol) III produces a number of essential RNA molecules involved in protein synthesis, RNA maturation, and other processes. Its activity is highly regulated, and deregulation can lead to cell transformation. The human phosphoprotein MAF1 becomes dephosphorylated and represses pol III transcription after various stresses, but neither the significance of the phosphorylations nor the kinase involved is known. We find that human MAF1 is absolutely required for pol III repression in response to serum starvation or TORC1 inhibition by rapamycin or Torin1. The protein is phosphorylated mainly on residues S60, S68, and S75, and this inhibits its pol III repression function. The responsible kinase is mTORC1, which phosphorylates MAF1 directly. Our results describe molecular mechanisms by which mTORC1 controls human MAF1, a key repressor of RNA polymerase III transcription, and add a new branch to the signal transduction cascade immediately downstream of TORC1.

Resveratrol modulates tumor cell proliferation and protein translation via SIRT1-dependent AMPK activation

Resveratrol functions as an agonist for estrogen receptor (ER)-mediated transcription. However, other researchers have reported that resveratrol decreases proliferation of breast cancer cells that are either ER-positive or ER-negative, which suggests that the interaction of resveratrol with the ER may not fully explain its inhibitory effect on proliferation. Similar to those effects associated with caloric restriction (CR), resveratrol has multiple beneficial activities, such as increased life span and delay in the onset of diseases associated with aging. One key enzyme thought to be activated during CR is the AMP-activated kinase (AMPK), a sensor of cellular energy levels. The suppression of nonessential energy expenditure by activated AMPK along with the CR mimetic and antiproliferative properties of resveratrol has led us to hypothesize that resveratrol activity might have an important role in the activation of AMPK. Here, we show that resveratrol activated AMPK in both ER-positive and ER-negative breast cancer cells. Once activated, AMPK inhibited 4E-BP1 signaling and mRNA translation via mammalian target of rapamycin (mTOR). Moreover, we also found that AMPK activity mediated by resveratrol in cancer cells was due to inducing the expression of Sirtuin type 1 (SIRT1) via elevation in the cellular NAD(+)/NADH in ER-positive cells. To our knowledge, we demonstrate here for the first time that resveratrol induces the expression of SIRT1 protein in human cancer cells. These observations raise the possibility that SIRT1 functions as a novel upstream regulator for AMPK signaling and may additionally modulate tumor cell proliferation. Targeting SIRT1/AMPK signaling by resveratrol may have potential therapeutic implications for cancer and age-related diseases.

EGCG inhibits protein synthesis, lipogenesis, and cell cycle progression through activation of AMPK in p53 positive and negative human hepatoma cells

In the previous studies, (-)-epigallocatechin-3-gallate (EGCG) has been shown to have anticarcinogenic effects via modulation in protein expression of p53. Using p53 positive Hep G2 and p53 negative Hep 3B cells, we found that treatment of EGCG resulted in dose-dependent inhibition of cellular proliferation, which suggests that the interaction of EGCG with p53 may not fully explain its inhibitory effect on proliferation. Caloric restriction (CR) reduces the incidence and progression of spontaneous and induced tumors in laboratory rodents. EGCG has multiple beneficial activities similar to those associated with CR. One key enzyme thought to be activated during CR is AMP-activated kinase (AMPK), a sensor of cellular energy levels. Here, we showed that EGCG activated AMPK in both p53 positive and negative human hepatoma cells. The activation of AMPK suppressed downstream substrates, such as mammalian target of rapamycin (mTOR) and eukaryotic initiation factor 4E-binding protein-1 (4E-BP1) and a general decrease in mRNA translation. Moreover, EGCG activated AMPK decreases the activity and/or expression of lipogenic enzymes, such as fatty acid synthase (FASN) and acetyl-CoA carboxylase (ACC). Interestingly, the decision between apoptosis and growth arrest following AMPK activation is greatly influenced by p53 status. In p53 positive Hep G2 cells, EGCG blocked the progression of cell cycle at G1 phase by inducing p53 expression and further up-regulating p21 expression. However, EGCG inducted apoptosis in p53 negative Hep 3B cells. Based on these results, we have demonstrated that EGCG has a potential to be a chemoprevention and anti-lipogenesis agent for human hepatoma cells.

Regulation of mRNA translation in renal physiology and disease

Translation, a process of generating a peptide from the codons present in messenger RNA, can be a site of independent regulation of protein synthesis; it has not been well studied in the kidney. Translation occurs in three stages (initiation, elongation, and termination), each with its own set of regulatory factors. Mechanisms controlling translation include small inhibitory RNAs such as microRNAs, binding proteins, and signaling reactions. Role of translation in renal injury in diabetes, endoplasmic reticulum stress, acute kidney injury, and, in physiological adaptation to loss of nephrons is reviewed here. Contribution of mRNA translation to physiology and disease is not well understood. Because it is involved in such diverse areas as development and cancer, it should prove a fertile field for investigation in renal science.

Intracellular parasitism with Toxoplasma gondii stimulates mammalian-target-of-rapamycin-dependent host cell growth despite impaired signalling to S6K1 and 4E-BP1

The Ser/Thr kinase mammalian-target-of-rapamycin (mTOR) is a central regulator of anabolism, growth and proliferation. We investigated the effects of Toxoplasma gondii on host mTOR signalling. Toxoplasma invasion of multiple cell types rapidly induced sustained mTOR activation that was restricted to infected cells, as determined by rapamycin-sensitive phosphorylation of ribosomal protein S6; however, phosphorylation of the growth-associated mTOR substrates 4E-BP1 and S6K1 was not detected. Infected cells still phosphorylated S6K1 and 4E-BP1 in response to insulin, although the S6K1 response was blunted. Parasite-induced S6 phosphorylation was independent of S6K1 and did not require activation of canonical mTOR-inducing pathways mediated by phosphatidylinositol 3-kinase-Akt and ERK. Host mTOR was localized in a vesicular pattern surrounding the parasitophorous vacuole, suggesting potential activation by phosphatidic acid in the vacuolar membrane. In spite of a failure to phosphorylate 4E-BP1 and S6K1, intracellular T. gondii triggered host cell cycle progression in an mTOR-dependent manner and progression of infected cells displayed increased sensitivity to rapamycin. Moreover, normal cell growth was maintained during parasite-induced cell cycle progression, as indicated by total cellular S6 levels. The Toxoplasma-infected cell provides a unique example of non-canonical mTOR activation supporting growth that is independent of signalling through either S6K1 or 4E-BP1.

Active-site inhibitors of mTOR target rapamycin-resistant outputs of mTORC1 and mTORC2

The mammalian target of rapamycin (mTOR) regulates cell growth and survival by integrating nutrient and hormonal signals. These signaling functions are distributed between at least two distinct mTOR protein complexes: mTORC1 and mTORC2. mTORC1 is sensitive to the selective inhibitor rapamycin and activated by growth factor stimulation via the canonical phosphoinositide 3-kinase (PI3K)→Akt→mTOR pathway. Activated mTORC1 kinase up-regulates protein synthesis by phosphorylating key regulators of mRNA translation. By contrast, mTORC2 is resistant to rapamycin. Genetic studies have suggested that mTORC2 may phosphorylate Akt at S473, one of two phosphorylation sites required for Akt activation; this has been controversial, in part because RNA interference and gene knockouts produce distinct Akt phospho-isoforms. The central role of mTOR in controlling key cellular growth and survival pathways has sparked interest in discovering mTOR inhibitors that bind to the ATP site and therefore target both mTORC2 and mTORC1. We investigated mTOR signaling in cells and animals with two novel and specific mTOR kinase domain inhibitors (TORKinibs). Unlike rapamycin, these TORKinibs (PP242 and PP30) inhibit mTORC2, and we use them to show that pharmacological inhibition of mTOR blocks the phosphorylation of Akt at S473 and prevents its full activation. Furthermore, we show that TORKinibs inhibit proliferation of primary cells more completely than rapamycin. Surprisingly, we find that mTORC2 is not the basis for this enhanced activity, and we show that the TORKinib PP242 is a more effective mTORC1 inhibitor than rapamycin. Importantly, at the molecular level, PP242 inhibits cap-dependent translation under conditions in which rapamycin has no effect. Our findings identify new functional features of mTORC1 that are resistant to rapamycin but are effectively targeted by TORKinibs. These potent new pharmacological agents complement rapamycin in the study of mTOR and its role in normal physiology and human disease.

Growth factor pathways are required for normal development but are often inappropriately activated in many cancers. One growth-factor–sensitive pathway of increasing interest to cancer researchers relies on the mammalian target of rapamycin (mTOR), a kinase that (like all kinases) delivers phosphate groups from ATP to amino acid residues of downstream proteins. TOR proteins were first discovered in yeast as the cellular targets of rapamycin, a small, naturally occurring molecule derived from bacteria that is widely used as an immunosuppressant and more recently in some cancer therapies. The study of TOR proteins has relied heavily on the use of rapamycin, but rapamycin does not directly inhibit TOR kinase activity; rather, rapamycin influences TOR's enzymatic activities by binding to a domain far from the kinase's active site. Some mTOR functions are resistant to rapamycin, as a result of the kinase activity of one kind of multiprotein complex, the mTOR complex 2 (mTORC2), whereas rapamycin-sensitive functions of mTOR are due to the mTOR complex 1 (mTORC1). We have developed new inhibitors of mTOR that bind to the ATP-binding site of mTOR and inhibit the catalytic activity of both mTORC1 and mTORC2 without inhibiting other kinases. Unexpectedly, these inhibitors had profound effects on protein synthesis and cell proliferation due to their inhibition of mTORC1 rather than mTORC2. We found that the phosphorylation of a protein that controls protein synthesis, the mTORC1 substrate 4E binding protein (4EBP) is partially resistant to rapamycin but fully inhibited by our new inhibitors. The finding that 4EBP phosphorylation is resistant to rapamycin suggests that active-site inhibitors may be more effective than rapamycin in the treatment of cancer and may explain why rapamycin is so well tolerated when taken for immunosuppression.

Cells rely on the mammalian target of rapamycin kinase (mTOR) to sense growth factors. Inhibition of all forms of mTOR using newly developed inhibitors of its active site reveals new insights into the function of two mTOR-containing protein complexes and their potential as therapeutic targets.

Mammalian target of rapamycin complex 1 (mTORC1) activity is associated with phosphorylation of raptor by mTOR

mTORC1 contains multiple proteins and plays a central role in cell growth and metabolism. Raptor (regulatory-associated protein of mammalian target of rapamycin (mTOR)), a constitutively binding protein of mTORC1, is essential for mTORC1 activity and critical for the regulation of mTORC1 activity in response to insulin signaling and nutrient and energy sufficiency. Herein we demonstrate that mTOR phosphorylates raptor in vitro and in vivo. The phosphorylated residues were identified by using phosphopeptide mapping and mutagenesis. The phosphorylation of raptor is stimulated by insulin and inhibited by rapamycin. Importantly, the site-directed mutation of raptor at one phosphorylation site, Ser(863), reduced mTORC1 activity both in vitro and in vivo. Moreover, the Ser(863) mutant prevented small GTP-binding protein Rheb from enhancing the phosphorylation of S6 kinase (S6K) in cells. Therefore, our findings indicate that mTOR-mediated raptor phosphorylation plays an important role on activation of mTORC1.

Specific activation of mTORC1 by Rheb G-protein in vitro involves enhanced recruitment of its substrate protein

Rheb G-protein plays critical roles in the TSC/Rheb/mTOR signaling pathway by activating mTORC1. The activation of mTORC1 by Rheb can be faithfully reproduced in vitro by using mTORC1 immunoprecipitated by the use of anti-raptor antibody from mammalian cells starved for nutrients. The low in vitro kinase activity against 4E-BP1 of this mTORC1 preparation is dramatically increased by the addition of recombinant Rheb. On the other hand, the addition of Rheb does not activate mTORC2 immunoprecipitated from mammalian cells by the use of anti-rictor antibody. The activation of mTORC1 is specific to Rheb, because other G-proteins such as KRas, RalA/B, and Cdc42 did not activate mTORC1. Both Rheb1 and Rheb2 activate mTORC1. In addition, the activation is dependent on the presence of bound GTP. We also find that the effector domain of Rheb is required for the mTORC1 activation. FKBP38, a recently proposed mediator of Rheb action, appears not to be involved in the Rheb-dependent activation of mTORC1 in vitro, because the preparation of mTORC1 that is devoid of FKBP38 is still activated by Rheb. The addition of Rheb results in a significant increase of binding of the substrate protein 4E-BP1 to mTORC1. PRAS40, a TOR signaling (TOS) motif-containing protein that competes with the binding of 4EBP1 to mTORC1, inhibits Rheb-induced activation of mTORC1. A preparation of mTORC1 that is devoid of raptor is not activated by Rheb. Rheb does not induce autophosphorylation of mTOR. These results suggest that Rheb induces alteration in the binding of 4E-BP1 with mTORC1 to regulate mTORC1 activation.

Curcumin disrupts the Mammalian target of rapamycin-raptor complex

Curcumin (diferuloylmethane), a polyphenol natural product of the plant Curcuma longa, is undergoing early clinical trials as a novel anticancer agent. However, the anticancer mechanism of curcumin remains to be elucidated. Recently, we have shown that curcumin inhibits phosphorylation of p70 S6 kinase 1 (S6K1) and eukaryotic initiation factor 4E (eIF4E) binding protein 1 (4E-BP1), two downstream effector molecules of the mammalian target of rapamycin complex 1 (mTORC1) in numerous cancer cell lines. This study was designed to elucidate the underlying mechanism. We observed that curcumin inhibited mTORC1 signaling not by inhibition of the upstream kinases, such as insulin-like growth factor 1 receptor (IGF-IR) and phosphoinositide-dependent kinase 1 (PDK1). Further, we found that curcumin inhibited mTORC1 signaling independently of protein phosphatase 2A (PP2A) or AMP-activated protein kinase AMPK-tuberous sclerosis complex (TSC). This is evidenced by the findings that curcumin was able to inhibit phosphorylation of S6K1 and 4E-BP1 in the cells pretreated with PP2A inhibitor (okadaic acid) or AMPK inhibitor (compound C), or in the cells expressing dominant-negative (dn) PP2A, shRNA to PP2A-A subunit, or dn-AMPKalpha. Curcumin did not alter the TSC1/2 interaction. Knockout of TSC2 did not affect curcumin inhibition of mTOR signaling. Finally, we identified that curcumin was able to dissociate raptor from mTOR, leading to inhibition of mTORC1 activity. Therefore, our data indicate that curcumin may represent a new class of mTOR inhibitor.

Inhibition of Mammalian target of rapamycin by rapamycin causes the regression of carcinogen-induced skin tumor lesions

PURPOSE

The activation of Akt/mammalian target of rapamycin (mTOR) pathway represents a frequent event in squamous cell carcinoma (SCC) progression, thus raising the possibility of using specific mTOR inhibitors for the treatment of SCC patients. In this regard, blockade of mTOR with rapamycin prevents the growth of human head and neck SCC cells when xenotransplanted into immunodeficient mice. However, therapeutic responses in xenograft tumors are not always predictive of clinical anticancer activity.

EXPERIMENTAL DESIGN

As genetically defined and chemically induced animal cancer models often reflect better the complexity of the clinical setting, we used here a two-step chemical carcinogenesis model to explore the effectiveness of rapamycin for the treatment of skin SCC.

RESULTS

Rapamycin exerted a remarkable anticancer activity in this chemically induced cancer model, decreasing the tumor burden of mice harboring early and advanced tumor lesions, and even recurrent skin SCCs. Immunohistochemical studies on tumor biopsies and clustering analysis revealed that rapamycin causes the rapid decrease in the phosphorylation status of mTOR targets followed by the apoptotic death of cancer cells and the reduction in the growth and metabolic activity of the surviving ones, concomitant with a decrease in the population of cancer cells expressing mutant p53. This approach enabled investigating the relationship among molecular changes caused by mTOR inhibition, thus helping identify relevant biomarkers for monitoring the effectiveness of mTOR inhibition in the clinical setting.

CONCLUSIONS

Together, these findings provide a strong rationale for the early evaluation of mTOR inhibitors as a molecular targeted approach to treat SCC.

An in vivo and in vitro assessment of TOR signaling cascade in rainbow trout (Oncorhynchus mykiss)

In mammals, feeding promotes protein accretion in skeletal muscle through a stimulation of the insulin- and amino acid- sensitive mammalian target of rapamycin (mTOR) signaling pathway, leading to the induction of mRNA translation. The purpose of the present study was to characterize both in vivo and in vitro the activation of several major kinases involved in the mTOR pathway in the muscle of the carnivorous rainbow trout. Our results showed that meal feeding enhanced the phosphorylation of the target of rapamycin (TOR), PKB, p70 S6 kinase, and eIF4E-binding protein-1, suggesting that the mechanisms involved in the regulation of mRNA translation are well conserved between lower and higher vertebrates. Our in vitro studies on primary culture of trout muscle cells indicate that insulin and amino acids regulate TOR signaling and thus may be involved in meal feeding effect in this species as in mammals. In conclusion, we report here for the first time in a fish species, the existence and the nutritional regulation of several major kinases involved in the TOR pathway, opening a new area of research on the molecular bases of amino acid utilization in teleosts.

Evidence of placental translation inhibition and endoplasmic reticulum stress in the etiology of human intrauterine growth restriction

Unexplained intrauterine growth restriction of the fetus (IUGR) results from impaired placental development, frequently associated with maternal malperfusion. Some cases are complicated further by preeclampsia (PE+IUGR). Here, we provide the first evidence that placental protein synthesis inhibition and endoplasmic reticulum (ER) stress play key roles in IUGR pathophysiology. Increased phosphorylation of eukaryotic initiation factor 2alpha suggests suppression of translation initiation in IUGR placentas, with a further increase in PE+IUGR cases. Consequently, AKT levels were reduced at the protein, but not mRNA, level. Additionally, levels of other proteins in the AKT-mammalian target of rapamycin pathway were decreased, and there was associated dephosphorylation of 4E-binding protein 1 and activation of glycogen synthase kinase 3beta. Cyclin D1 and the eukaryotic initiation factor 2B epsilon subunit were also down-regulated, providing additional evidence for this placental phenotype. The central role of AKT signaling in placental growth regulation was confirmed in Akt1 null mice, which display IUGR. In addition, we demonstrated ultrastructural and molecular evidence of ER stress in human IUGR and PE+IUGR placentas, providing a potential mechanism for eukaryotic initiation factor 2alpha phosphorylation. In confirmation, induction of low-grade ER stress in trophoblast-like cell lines reduced cellular proliferation. PE+IUGR placentas showed elevated ER stress with the additional expression of the pro-apoptotic protein C/EBP-homologous protein/growth arrest and DNA damage 153. These findings may account for the increased microparticulate placental debris in the maternal circulation of these cases, leading to endothelial cell activation and impairing placental development.

Nitric oxide mediates NMDA-induced persistent inhibition of protein synthesis through dephosphorylation of eukaryotic initiation factor 4E-binding protein 1 and eukaryotic initiation factor 4G proteolysis

Cerebral ischaemia causes long-lasting protein synthesis inhibition that is believed to contribute to brain damage. Energy depletion promotes translation inhibition during ischaemia, and the phosphorylation of eIF (eukaryotic initiation factor) 2α is involved in the translation inhibition induced by early ischaemia/reperfusion. However, the molecular mechanisms underlying prolonged translation down-regulation remain elusive. NMDA (N-methyl-D-aspartate) excitotoxicity is also involved in ischaemic damage, as exposure to NMDA impairs translation and promotes the synthesis of NO (nitric oxide), which can also inhibit translation. In the present study, we investigated whether NO was involved in NMDA-induced protein synthesis inhibition in neurons and studied the underlying molecular mechanisms. NMDA and the NO donor DEA/NO (diethylamine–nitric oxide sodium complex) both inhibited protein synthesis and this effect persisted after a 30 min exposure. Treatments with NMDA or NO promoted calpain-dependent eIF4G cleavage and 4E-BP1 (eIF4E-binding protein 1) dephosphorylation and also abolished the formation of eIF4E–eIF4G complexes; however, they did not induce eIF2α phosphorylation. Although NOS (NO synthase) inhibitors did not prevent protein synthesis inhibition during 30 min of NMDA exposure, they did abrogate the persistent inhibition of translation observed after NMDA removal. NOS inhibitors also prevented NMDA-induced eIF4G degradation, 4E-BP1 dephosphorylation, decreased eIF4E–eIF4G-binding and cell death. Although the calpain inhibitor calpeptin blocked NMDA-induced eIF4G degradation, it did not prevent 4E-BP1 dephosphorylation, which precludes eIF4E availability, and thus translation inhibition was maintained. The present study suggests that eIF4G integrity and hyperphosphorylated 4E-BP1 are needed to ensure appropriate translation in neurons. In conclusion, our data show that NO mediates NMDA-induced persistent translation inhibition and suggest that deficient eIF4F activity contributes to this process.

Analysis of the regulatory motifs in eukaryotic initiation factor 4E-binding protein 1

Mammalian target of rapamycin complex 1 (mTORC1) phosphorylates proteins such as eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) and the S6 kinases. These substrates contain short sequences, termed TOR signalling (TOS) motifs, which interact with the mTORC1 component raptor. Phosphorylation of 4E-BP1 requires an additional feature, termed the RAIP motif (Arg-Ala-Ile-Pro). We have analysed the interaction of 4E-BP1 with raptor and the amino acid residues required for functional RAIP and TOS motifs, as assessed by raptor binding and the phosphorylation of 4E-BP1 in human cells. Binding of 4E-BP1 to raptor strongly depends on an intact TOS motif, but the RAIP motif and additional C-terminal features of 4E-BP1 also contribute to this interaction. Mutational analysis of 4E-BP1 reveals that isoleucine is a key feature of the RAIP motif, that proline is also very important and that there is greater tolerance for substitution of the first two residues. Within the TOS motif, the first position (phenylalanine in the known motifs) is most critical, whereas a wider range of residues function in other positions (although an uncharged aliphatic residue is preferred at position three). These data provide important information on the structural requirements for efficient signalling downstream of mTORC1.

Regulation of proline-rich Akt substrate of 40 kDa (PRAS40) function by mammalian target of rapamycin complex 1 (mTORC1)-mediated phosphorylation

The rapamycin-sensitive mammalian target of rapamycin (mTOR) complex 1 (mTORC1) contains mTOR, raptor, mLST8, and PRAS40 (proline-rich Akt substrate of 40 kDa). PRAS40 functions as a negative regulator when bound to mTORC1, and it dissociates from mTORC1 in response to insulin. PRAS40 has been demonstrated to be a substrate of mTORC1, and one phosphorylation site, Ser-183, has been identified. In this study, we used two-dimensional phosphopeptide mapping in conjunction with mutational analysis to show that in addition to Ser-183, mTORC1 also phosphorylates Ser-212 and Ser-221 in PRAS40 when assayed in vitro. Mutation of all three residues to Ala markedly reduces mTORC1-mediated phosphorylation of PRAS40 in vitro. All three sites were confirmed to be phosphorylated in vivo by [(32)P]orthophosphate labeling and peptide mapping. Phosphorylation of Ser-221 and Ser-183 but not Ser-212 is sensitive to rapamycin treatment. Furthermore, we demonstrate that mutation of Ser-221 to Ala reduces the interaction with 14-3-3 to the same extent as mutation of Thr-246, the Akt/protein kinase B-phosphorylated site. We also find that mutation of Ser-221 to Ala increases the inhibitory activity of PRAS40 toward mTORC1. We propose that after mTORC1 kinase activation by upstream regulators, PRAS40 is phosphorylated directly by mTOR, thus contributing to the relief of PRAS40-mediated substrate competition.

Dose- and schedule-dependent inhibition of the mammalian target of rapamycin pathway with everolimus: a phase I tumor pharmacodynamic study in patients with advanced solid tumors

PURPOSE

Everolimus is a selective mammalian target of rapamycin (mTOR) inhibitor with promising anticancer activity. In order to identify a rationally based dose and schedule for cancer treatment, we have conducted a tumor pharmacodynamic phase I study in patients with advanced solid tumors.

PATIENTS AND METHODS

Fifty-five patients were treated with everolimus in cohorts of 20, 50, and 70 mg weekly or 5 and 10 mg daily. Dose escalation depended on dose limiting toxicity (DLT) rate during the first 4-week period. Pre- and on-treatment steady-state tumor and skin biopsies were evaluated for total and phosphorylated (p) protein S6 kinase 1, eukaryotic initiation factor 4E (elF-4E) binding protein 1 (4E-BP1), eukaryotic initiation factor 4G (eIF-4G), AKT, and Ki-67 expression. Plasma trough levels of everolimus were determined on a weekly basis before dosing during the first 4 weeks.

RESULTS

We observed a dose- and schedule-dependent inhibition of the mTOR pathway with a near complete inhibition of pS6 and peIF-4G at 10 mg/d and >or= 50 mg/wk. In addition, pAKT was upregulated in 50% of the treated tumors. In the daily schedule, there was a correlation between everolimus plasma trough concentrations and inhibition of peIF4G and p4E-BP1. There was good concordance of mTOR pathway inhibition between skin and tumor. Clinical benefit was observed in four patients including one patient with advanced colorectal cancer achieving a partial response. DLTs occurred in five patients: one patient at 10 mg/d (grade 3 stomatitis) and four patients at 70 mg/wk (two with grade 3 stomatitis, one with grade 3 neutropenia, and one with grade 3 hyperglycemia).

CONCLUSION

Everolimus achieved mTOR signaling inhibition at doses below the DLT. A dosage of 10 mg/d or 50 mg/wk is recommended for further development.

A novel function of eIF2alpha kinases as inducers of the phosphoinositide-3 kinase signaling pathway

Phosphoinositide-3 kinase (PI3K) plays an important role in signal transduction in response to a wide range of cellular stimuli involved in cellular processes that promote cell proliferation and survival. Phosphorylation of the alpha subunit of the eukaryotic translation initiation factor eIF2 at Ser51 takes place in response to various types of environmental stress and is essential for regulation of translation initiation. Herein, we show that a conditionally active form of the eIF2alpha kinase PKR acts upstream of PI3K and turns on the Akt/PKB-FRAP/mTOR pathway leading to S6 and 4E-BP1 phosphorylation. Also, induction of PI3K signaling antagonizes the apoptotic and protein synthesis inhibitory effects of the conditionally active PKR. Furthermore, induction of the PI3K pathway is impaired in PKR(-/-) or PERK(-/-) mouse embryonic fibroblasts (MEFs) in response to various stimuli that activate each eIF2alpha kinase. Mechanistically, PI3K signaling activation is indirect and requires the inhibition of protein synthesis by eIF2alpha phosphorylation as demonstrated by the inactivation of endogenous eIF2alpha by small interfering RNA or utilization of MEFs bearing the eIF2alpha Ser51Ala mutation. Our data reveal a novel property of eIF2alpha kinases as activators of PI3K signaling and cell survival.

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.

Protein kinase C-mediated down-regulation of cyclin D1 involves activation of the translational repressor 4E-BP1 via a phosphoinositide 3-kinase/Akt-independent, protein phosphatase 2A-dependent mechanism in intestinal epithelial cells

We reported previously that protein kinase Calpha (PKCalpha), a negative regulator of cell growth in the intestinal epithelium, inhibits cyclin D1 translation by inducing hypophosphorylation/activation of the translational repressor 4E-BP1. The current study explores the molecular mechanisms underlying PKC/PKCalpha-induced activation of 4E-BP1 in IEC-18 nontransformed rat ileal crypt cells. PKC signaling is shown to promote dephosphorylation of Thr(45) and Ser(64) on 4E-BP1, residues directly involved in its association with eIF4E. Consistent with the known role of the phosphoinositide 3-kinase (PI3K)/Akt/mTOR pathway in regulation of 4E-BP1, PKC signaling transiently inhibited PI3K activity and Akt phosphorylation in IEC-18 cells. However, PKC/PKCalpha-induced activation of 4E-BP1 was not prevented by constitutively active mutants of PI3K or Akt, indicating that blockade of PI3K/Akt signaling is not the primary effector of 4E-BP1 activation. This idea is supported by the fact that PKC activation did not alter S6 kinase activity in these cells. Further analysis indicated that PKC-mediated 4E-BP1 hypophosphorylation is dependent on the activity of protein phosphatase 2A (PP2A). PKC signaling induced an approximately 2-fold increase in PP2A activity, and phosphatase inhibition blocked the effects of PKC agonists on 4E-BP1 phosphorylation and cyclin D1 expression. H(2)O(2) and ceramide, two naturally occurring PKCalpha agonists that promote growth arrest in intestinal cells, activate 4E-BP1 in PKC/PKCalpha-dependent manner, supporting the physiological significance of the findings. Together, our studies indicate that activation of PP2A is an important mechanism underlying PKC/PKCalpha-induced inhibition of cap-dependent translation and growth suppression in intestinal epithelial cells.

Parkin modulates gene expression in control and ceramide-treated PC12 cells

Mutations in the parkin gene cause autosomal-recessive early-onset parkinsonism as a result of the degeneration of mesencephalic dopaminergic neurons. In cell culture models, parkin expression has been shown to protect against cell death mediated by the sphingolipid ceramide. To determine whether the antiapoptotic effect of parkin involves changes in gene expression, we used Affymetrix oligonucleotide microarrays to analyse gene expression in stably transfected PC12 cells which conditionally overexpress parkin, that were treated or not with C2-ceramide. Overexpression of parkin and ceramide treatment both modulated gene expression. A number of the genes upregulated in the presence of ceramide, and modulated by parkin, were associated with apoptosis or cellular stress reactions. We validated the upregulation of four such genes (CHK, EIF4EBP1, GADD45A and PTPN-5) by real-time PCR after 3, 6, 9 and 12 h of ceramide treatment in cells that overexpressed parkin or not. All were upregulated 2 to 11-fold, 3 and 6 h after application of ceramide. Parkin overexpression reduced the upregulation of EIF4EBP1, GADD45A and PTPN-5, but only at 6 h. These results suggest that, in this assay, the cytoprotective effect of parkin might result not only from its E3-ligase activity, but also from direct or indirect modulation of gene expression in a time-dependent manner.

When translation meets transformation: the mTOR story

There is currently a high level of interest in signalling through the mammalian target of rapamycin (mTOR). This reflects both its key role in many cell functions and its involvement in disease states such as cancers. The best understood targets for mTOR signalling are proteins involved in controlling the translational machinery, including the ribosomal protein S6 kinases and proteins that regulate the initiation and elongation phases of translation. Indeed, there is compelling evidence that at least one of these targets of mTOR (eukaryotic initiation factor eIF4E) plays a key role in tumorigenesis. It is regulated through the mTOR-dependent phosphorylation of inhibitory proteins such as eIF4E-binding protein 1. Thus, targeting mTOR signalling may be an effective anticancer strategy, in at least a significant subset of tumours. Not all effects of mTOR are sensitive to the classical anti-mTOR drug rapamycin, and this compound also interferes with other processes besides eIF4E function. Developing new approaches to targeting mTOR for cancer therapy requires more detailed knowledge of signalling downstream of mTOR. Such advances are likely to come from further work to understand the regulation of mTOR targets such as components of the translational apparatus.