Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UNIPROT:P42345 (mTOR)
 26,049 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The multisubunit eukaryotic translation initiation factor (eIF) 4F recruits 40S ribosomal subunits to the 5' end of mRNA. The eIF4F subunit eIF4E interacts directly with the mRNA 5' cap structure. Assembly of the eIF4F complex is inhibited by a family of repressor polypeptides, the eIF4E-binding proteins (4E-BPs). Binding of the 4E-BPs to eIF4E is regulated by phosphorylation: Hypophosphorylated 4E-BP isoforms interact strongly with eIF4E, whereas hyperphosphorylated isoforms do not. 4E-BP1 is hypophosphorylated in quiescent cells, but is hyperphosphorylated on multiple sites following exposure to a variety of extracellular stimuli. The PI3-kinase/Akt pathway and the kinase FRAP/mTOR signal to 4E-BP1. FRAP/mTOR has been reported to phosphorylate 4E-BP1 directly in vitro. However, it is not known if FRAP/mTOR is responsible for the phosphorylation of all 4E-BP1 sites, nor which sites must be phosphorylated to release 4E-BP1 from eIF4E. To address these questions, a recombinant FRAP/mTOR protein and a FRAP/mTOR immunoprecipitate were utilized in in vitro kinase assays to phosphorylate 4E-BP1. Phosphopeptide mapping of the in vitro-labeled protein yielded two 4E-BP1 phosphopeptides that comigrated with phosphopeptides produced in vivo. Mass spectrometry analysis indicated that these peptides contain phosphorylated Thr-37 and Thr-46. Thr-37 and Thr-46 are efficiently phosphorylated in vitro by FRAP/mTOR when 4E-BP1 is bound to eIF4E. However, phosphorylation at these sites was not associated with a loss of eIF4E binding. Phosphorylated Thr-37 and Thr-46 are detected in all phosphorylated in vivo 4E-BP1 isoforms, including those that interact with eIF4E. Finally, mutational analysis demonstrated that phosphorylation of Thr-37/Thr-46 is required for subsequent phosphorylation of several carboxy-terminal serum-sensitive sites. Taken together, our results suggest that 4E-BP1 phosphorylation by FRAP/mTOR on Thr-37 and Thr-46 is a priming event for subsequent phosphorylation of the carboxy-terminal serum-sensitive sites.
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PMID:Regulation of 4E-BP1 phosphorylation: a novel two-step mechanism. 1036 59

The eukaryotic translation initiation factor 4G (eIF4G) proteins play a critical role in the recruitment of the translational machinery to mRNA. The eIF4Gs are phosphoproteins. However, the location of the phosphorylation sites, how phosphorylation of these proteins is modulated and the identity of the intracellular signaling pathways regulating eIF4G phosphorylation have not been established. In this report, two-dimensional phosphopeptide mapping demonstrates that the phosphorylation state of specific eIF4GI residues is altered by serum and mitogens. Phosphopeptides resolved by this method were mapped to the C-terminal one-third of the protein. Mass spectrometry and mutational analyses identified the serum-stimulated phosphorylation sites in this region as serines 1108, 1148 and 1192. Phosphoinositide-3-kinase (PI3K) inhibitors and rapamycin, an inhibitor of the kinase FRAP/mTOR (FKBP12-rapamycin-associated protein/mammalian target of rapamycin), prevent the serum-induced phosphorylation of these residues. Finally, the phosphorylation state of N-terminally truncated eIF4GI proteins acquires resistance to kinase inhibitor treatment. These data suggest that the kinases phosphorylating serines 1108, 1148 and 1192 are not directly downstream of PI3K and FRAP/mTOR, but that the accessibility of the C-terminus to kinases is modulated by this pathway(s).
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PMID:Serum-stimulated, rapamycin-sensitive phosphorylation sites in the eukaryotic translation initiation factor 4GI. 1065 41

Mammalian target of rapamycin (mTOR) controls initiation of translation through regulation of ribosomal p70S6 kinase (S6K1) and eukaryotic translation initiation factor-4E (eIF4E) binding protein (4E-BP). mTOR is considered to be located predominantly in cytosolic or membrane fractions and may shuttle between the cytoplasm and nucleus. In most previous studies a single cell line, E1A-immortalized human embryonic kidney cells (HEK293), has been used. Here we show that in human malignant cell lines, human fibroblasts, and murine myoblasts mTOR is predominantly nuclear. In contrast, mTOR is largely excluded from the nucleus in HEK293 cells. Hybrids between HEK293 and Rh30 rhabdomyosarcoma cells generated cells co-expressing markers unique to HEK293 (E1A) and Rh30 (MyoD). mTOR distribution was mainly nuclear with detectable levels in the cytoplasm. mTOR isolated from Rh30 nuclei phosphorylated recombinant GST-4E-BP1 (Thr-46) in vitro and thus has kinase activity. We next investigated the cellular distribution of mTOR substrates 4E-BP, S6K1, and eIF4E. 4E-BP was exclusively detected in cytoplasmic fractions in all cell lines. S6K1 was localized in the cytoplasm in colon carcinoma, HEK293 cells, and IMR90 fibroblasts. S6K1 was readily detected in all cellular fractions derived from rhabdomyosarcoma cells. eIF4E was detected in all fractions derived from rhabdomyosarcoma cells but was not detectable in nuclear fractions from colon carcinoma HEK293 or IMR90 cells.
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PMID:Predominant nuclear localization of mammalian target of rapamycin in normal and malignant cells in culture. 1200 Jul 55

Loss of functioning nephrons stimulates the growth of residual kidney tissue to augment work capacity and maintain normal renal function. This growth largely occurs by hypertrophy rather than from hyperplasia of the remaining nephrons. The signaling mechanisms that increase RNA and protein synthesis during compensatory renal hypertrophy are unknown. This study found that the remaining kidney hypertrophied 42% by 16 d after unilateral nephrectomy (UNX) in DBA/2 mice. Immunoblotting analysis revealed increased phosphorylation of the 40S ribosomal protein S6 (rpS6) and the eukaryotic translation initiation factor (eIF) 4E-binding protein 1 (4E-BP1), the two downstream effectors of the mammalian target of rapamycin (mTOR). The highly specific mTOR inhibitor rapamycin blocked UNX-increased phosphorylation of both rpS6 and 4E-BP1. UNX increased the content of not only 40S and 60S ribosomal subunits but also 80S monosomes and polysomes in the remaining kidney. Administration of rapamycin decreased UNX-induced polysome formation and shifted the polysome profile in the direction of monosomes and ribosomal subunits. Pretreatment of the mice with rapamycin inhibited UNX-induced hypertrophy. These studies demonstrate that activation of the mTOR signaling pathway in the remaining kidney after UNX plays an essential role in modulating RNA and protein synthesis during development of compensatory renal hypertrophy.
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PMID:Role of mammalian target of rapamycin signaling in compensatory renal hypertrophy. 1578 77

The rate of protein synthesis in quiescent peripheral blood T lymphocytes increases dramatically following mitogenic activation. The stimulation of translation is due to an increase in the rate of initiation caused by the regulation of initiation factor activities. Here, we focus on eIF3, a large multiprotein complex that plays a central role in the formation of the 40 S initiation complex. Using sucrose density gradient centrifugation to analyze ribosome complexes, we find that most eIF3 is not bound to 40 S ribosomal subunits in unactivated T lymphocytes but becomes ribosome-bound following activation. Immunoblot analyses of sucrose gradient fractions for individual eIF3 subunits show that the small eIF3j subunit is unassociated with the eIF3 complex in quiescent T lymphocytes, but upon activation joins the other eIF3 subunits and binds 40 S ribosomal subunits. Because eIF3j has been shown to be required for eIF3 binding to 40 S ribosomes in vitro, the results suggest that mitogenic stimulation of T lymphocytes leads to an activation of eIF3j, thereby enabling eIF3 to bind to the larger ribosome-free eIF3 subunit complex, and then to the 40 S ribosomes. The association of eIF3j with the other eIF3 subunits appears to be inhibited by rapamycin, suggesting a mechanism that lies downstream from the mammalian target of rapamycin kinase. This association requires ionomycin together with a phorbol ester, which also suggests that calcium signaling is involved. We conclude that the complex formation of eIF3 and its association with the ribosomes might contribute to increased translation rates during T lymphocyte activation.
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PMID:Changes in ribosomal binding activity of eIF3 correlate with increased translation rates during activation of T lymphocytes. 1594 46

Emerging knowledge on how the dysregulated function of signaling networks contributes to the malignant growth of squamous cell carcinoma of the head and neck (HNSCC) can now be exploited to identify novel mechanism-based anticancer treatments. In this regard, we have observed that persistent activation of the serine/threonine kinase Akt is a frequent event in HNSCC, and that blockade of its upstream kinase, 3'-phosphoinositide-dependent kinase 1, potently inhibits tumor cell growth. Akt promotes cell proliferation by its ability to coordinate mitogenic signaling with energy- and nutrient-sensing pathways that control protein synthesis through the atypical serine/threonine kinase, mammalian target of rapamycin (mTOR). This kinase, in turn, phosphorylates key eukaryotic translation regulators, including p70-S6 kinase and the eukaryotic translation initiation factor, 4E binding protein 1. Indeed, we show here that aberrant accumulation of the phosphorylated active form of S6, the most downstream target of the Akt-mTOR-p70-S6 kinase pathway, is a frequent event in clinical specimens from patients with HNSCC and their derived cell lines. Of interest, this enhanced level of the phosphorylated active form of S6 was rapidly reduced in HNSCC cell lines and HNSCC xenograft models at clinically relevant doses of rapamycin, which specifically inhibits mTOR. Furthermore, we observed that rapamycin displays a potent antitumor effect in vivo, as it inhibits DNA synthesis and induces the apoptotic death of HNSCC cells, ultimately resulting in tumor regression. These findings identify the Akt-mTOR pathway as a potential therapeutic target for HNSCC, and may provide the rationale for the early clinical evaluation of rapamycin and its analogues in patients with HNSCC.
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PMID:Mammalian target of rapamycin, a molecular target in squamous cell carcinomas of the head and neck. 1626 20

In the November 18 issue of Cell, discover an unidentified function for the eIF3 translation initiation factor as a scaffold for the dynamic associations of many preinitiation complex components, including the growth-regulating kinases mTOR and S6K1.
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PMID:eIF3: a connecTOR of S6K1 to the translation preinitiation complex. 1633 88

Insulin stimulates protein synthesis by increasing translation initiation. This response is mediated by mTOR and is believed to result from 4EBP1 phosphorylation, which allows eIF4E to bind eIF4G. Here, we present evidence that mTOR interacts directly with eIF3 and that mTOR controls the association of eIF3 and eIF4G. Activating mTOR signaling with insulin increased by as much as five-fold the amount of eIF4G bound to eIF3. This novel effect was blocked by rapamycin and other inhibitors of mTOR, and it required neither eIF4E binding to eIF4G nor eIF3 binding to the 40S ribosomal subunit. The increase in eIF4G associated with eIF3 occurred rapidly and at physiological concentrations of insulin. Moreover, the magnitude of the response was similar to the increase in eIF4E binding to eIF4G produced by insulin. Thus, increasing eIF4G association with eIF3 represents a potentially important mechanism by which insulin, as well as amino acids and growth factors that activate mTOR, stimulate translation.
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PMID:mTOR-dependent stimulation of the association of eIF4G and eIF3 by insulin. 1654 Nov 3

Molecular processes controlling mRNA translation are complex, multilayered, and their deregulation can lead to cancer pathogenesis. Eukaryotic initiation factor 3 (eIF3) is involved in the initiation process of protein translation and overexpression of its subunit eukaryotic translation initiation factor i (eIF3i) has been observed in carcinomas. Nevertheless, the potential role of eIF3i in carcinogenesis is poorly understood. Here, we show that in vitro overexpression of human eIF3i resulted in cell size increase, proliferation enhancement, cell-cycle progression, and anchorage-independent growth. Without external stimuli, eIF3i overexpressing cells arrested in G1/G0 phase, demonstrating the requirement of additional growth signals. Inhibition of the kinase mTOR, a key player in the integration of nutrition and growth signals into protein synthesis, with rapamycin reduced serine phosphorylation of eIF3i and resulted in a loss of anchorage-independent growth. Thus, eIF3i overexpression fosters the integration of growth signals by mTOR into the mRNA translation process, promoting protein synthesis and tumor growth.
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PMID:Carcinoma-associated eIF3i overexpression facilitates mTOR-dependent growth transformation. 1692 81

Matrix metalloproteinase (MMP)-7 is considered to play essential roles in cancer progression. We examined the efficacy of auraptene, a citrus coumarin derivative, for suppressing MMP-7 expression in the human colorectal adenocarcinoma cell line HT-29. Auraptene remarkably inhibited the production of proMMP-7 protein, without affecting its mRNA expression level. Rapamycin, an inhibitor of mammalian target of rapamycin (mTOR), showed similar results, suggesting that auraptene suppresses mTOR-dependent proMMP-7 translation. Interestingly, however, auraptene showed no effects on the activation of Akt/mTOR signaling, whereas the phosphorylation levels of 4E binding protein (4EBP)1 and eukaryotic translation initiation factor (eIF)4B were substantially decreased. In addition, auraptene remarkably dephosphorylated constitutively activated extracellular signal-regulated kinase (ERK)1/2. Transfection of ERK1/2 siRNA led to a significant reduction of proMMP-7 protein production as well as of the phosphorylation of eIF4B. These results demonstrate that auraptene targets the translation step for proMMP-7 protein synthesis by disrupting ERK1/2-mediated phosphorylation of 4EBP1 and eIF4B.
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PMID:Citrus auraptene targets translation of MMP-7 (matrilysin) via ERK1/2-dependent and mTOR-independent mechanism. 1697 34


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